Understanding Access Lists

Access lists perform packet filtering to control which packets move through the network and where. Such controls help to limit network traffic and restrict the access of users and devices to the network. Access lists have many uses, and therefore many commands accept a reference to an access list in their command syntax. Access lists can be used to do the following:

An access control list (ACL) consists of one or more access control entries (ACE) that collectively define the network traffic profile. This profile can then be referenced by Cisco IOS XR software features such as traffic filtering, route filtering, QoS classification, and access control.

Traditional ACLs don’t support compression. Object-group ACLs use compression to accommodate the large number of ACEs.

Purpose of IP Access Lists

  • Filter incoming or outgoing packets on an interface.

  • Filter packets for mirroring.

  • Redirect traffic as required.

  • Restrict the contents of routing updates.

  • Limit debug output based on an address or protocol.

  • Control vty access.

  • Identify or classify traffic for advanced features, such as congestion avoidance, congestion management, and priority and custom queueing.

How an IP Access List Works

An access list is a sequential list consisting of permit and deny statements that apply to IP addresses and possibly upper-layer IP protocols. The access list has a name by which it is referenced. Many software commands accept an access list as part of their syntax.

An access list can be configured and named, but it is not in effect until the access list is referenced by a command that accepts an access list. Multiple commands can reference the same access list. An access list can control traffic arriving at the router or leaving the router, but not traffic originating at the router.

Source address and destination addresses are two of the most typical fields in an IP packet on which to base an access list. Specify source addresses to control packets from certain networking devices or hosts. Specify destination addresses to control packets being sent to certain networking devices or hosts.

You can also filter packets on the basis of transport layer information, such as whether the packet is a TCP, UDP, ICMP, or IGMP packet.

ACL Workflow

The following image illustrates the workflow of an ACL.

IP Access List Process and Rules

Use the following process and rules when configuring an IP access list:

  • The software tests the source or destination address or the protocol of each packet being filtered against the conditions in the access list, one condition (permit or deny statement) at a time.

  • If a packet does not match an access list statement, the packet is then tested against the next statement in the list.

  • If a packet and an access list statement match, the remaining statements in the list are skipped and the packet is permitted or denied as specified in the matched statement. The first entry that the packet matches determines whether the software permits or denies the packet. That is, after the first match, no subsequent entries are considered.

  • If the access list denies the address or protocol, the software discards the packet and returns an Internet Control Message Protocol (ICMP) Host Unreachable message. ICMP is configurable in the Cisco IOS XR software.

  • If no conditions match, the software drops the packet because each access list ends with an unwritten or implicit deny statement. That is, if the packet has not been permitted or denied by the time it was tested against each statement, it is denied.

  • The access list should contain at least one permit statement or else all packets are denied.

  • Because the software stops testing conditions after the first match, the order of the conditions is critical. The same permit or deny statements specified in a different order could result in a packet being passed under one circumstance and denied in another circumstance.

  • Only one access list per interface, per protocol, per direction is allowed.

  • Inbound access lists process packets arriving at the router. Incoming packets are processed before being routed to an outbound interface. An inbound access list is efficient because it saves the overhead of routing lookups if the packet is to be discarded because it is denied by the filtering tests. If the packet is permitted by the tests, it is then processed for routing. For inbound lists, permit means continue to process the packet after receiving it on an inbound interface; deny means to discard the packet.

  • Outbound access lists process packets before they leave the router. Incoming packets are routed to the outbound interface and then processed through the outbound access list. For outbound lists, permit means send it to the output buffer; deny means discard the packet.

  • An access list cannot be removed if that access list is being applied by an access group in use. To remove an access list, remove the access group that is referencing the access list and then remove the access list.

  • Before removing an interface, which is configured with an ACL that denies certain traffic, you must remove the ACL and commit your configuration. If this is not done, then some packets are leaked through the interface as soon as the no interface <interface-name> command is configured and committed.

  • An access list must exist before you can use the ipv4 access group command.

  • ACL-based Forwarding (ABF) is not supported in common ACLs.

  • Filtering of MPLS packets with the explicit-null or de-aggregation label is supported on the ingress direction.

  • If the Ternary Content-Addressable Memory (TCAM) utilization is high and large ACLs are modified, then an error may occur. During such instances, remove the ACL from the interface and reconfigure the ACL. Later, reapply the ACL to the interface.

  • You can configure an ACL name with a maximum of 64 characters.

  • You can configure an ACL name to comprise of only letters and numbers.

ACL Filtering by Wildcard Mask and Implicit Wildcard Mask

Address filtering uses wildcard masking to indicate whether the software checks or ignores corresponding IP address bits when comparing the address bits in an access-list entry to a packet being submitted to the access list. By carefully setting wildcard masks, an administrator can select a single or several IP addresses for permit or deny tests.

Wildcard masking for IP address bits uses the number 1 and the number 0 to specify how the software treats the corresponding IP address bits. A wildcard mask is sometimes referred to as an inverted mask, because a 1 and 0 mean the opposite of what they mean in a subnet (network) mask.

  • A wildcard mask bit 0 means check the corresponding bit value.

  • A wildcard mask bit 1 means ignore that corresponding bit value.

You do not have to supply a wildcard mask with a source or destination address in an access list statement. If you use the host keyword, the software assumes a wildcard mask of 0.0.0.0.

Unlike subnet masks, which require contiguous bits indicating network and subnet to be ones, wildcard masks allow noncontiguous bits in the mask.

You can also use CIDR format (/x) in place of wildcard bits. For example, the IPv4 address 1.2.3.4 0.255.255.255 corresponds to 1.2.3.4/8 and for IPv6 address 2001:db8:abcd:0012:0000:0000:0000:0000 corresponds to 2001:db8:abcd:0012::0/64.

Including Comments in Access Lists

You can include comments (remarks) about entries in any named IP access list using the remark access list configuration command. The remarks make the access list easier for the network administrator to understand and scan. Each remark line is limited to 255 characters.

The remark can go before or after a permit or deny statement. You should be consistent about where you put the remark so it is clear which remark describes which permit or deny statement. For example, it would be confusing to have some remarks before the associated permit or deny statements and some remarks after the associated statements. Remarks can be sequenced.

Remember to apply the access list to an interface or terminal line after the access list is created.

User-Defined TCAM Keys for IPv4 and IPv6

Access-lists on the Cisco NCS 540 Series Routers use a TCAM (internal and external) to perform the lookup and action resolution on each packet. The TCAM is a valuable and constrained resource in hardware, which must be shared by multiple features. Therefore, the space (key width) available for these key definitions is also constrained. A key definition specifies which qualifier and action fields are available to the ACL feature when performing the lookup. Not all available qualifier and action fields can be included in each key definition.

The key definitions are specific to a given ACL type, which can depend on the following attributes of the access-list:

  • Direction of attachment, whether ingress or egress

  • Protocol type (IPv4/IPv6/L2)

  • Compression level (0:uncompressed, 3:compressed)

Because the default key definitions are constrained (do not include all qualifier/action fields), User-Defined Key (UDK) definitions are supported for the following types:

  • Traditional Ingress IPv4 ACL (uncompressed)

  • Traditional Ingress IPv6 ACL (uncompressed)

The User-Defined TCAM Key (UDK) functionality provides the flexibility to define your own TCAM key for one of the three possible reasons (for ingress, traditional, IPv4/IPv6 ACL only):

  • To include qualifier fields which are not included in the default TCAM key

  • To change the ACL mode from shared to unique to support a greater number of unique ACLs, unique counters, etc.

  • To reduce the size of the TCAM key (number of banks consumed)

A UDK can be configured using the following command:

hw-module profile tcam format access-list [ipv4 | ipv6] qualifiers [location rack/slot/cpu0]

If you want to use common ACL when a UDK is configured, you can add the common-acl option to the UDK.

User-Defined Fields

A TCAM key consists of several qualifiers, where the set of qualifiers are used to filter packets for a given ACL. The User-Defined Field (UDF) allows you to define a custom qualifier by specifying the location and size of the field, using the following UDF command:


Note

  • Up to 8 UDFs can be defined system wide. Currently, UDFs are globally defined.

 
udf udf-name header [ inner | outer ] [ l2 | l3 | l4 ] offset byte-offset length no of bytes

The UDF can then be added to a UDK as follows.

hw-module profile tcam format access-list [ipv4 | ipv6] qualifiers [udf1 udf-name udf2 udf-name] [location rack/slot/cpu0]

IPv4 and IPv6 Key Formats for Traditional Ingress ACL

User-defined TCAM key (UDK) definition is supported for ingress, traditional (uncompressed) IPv4 and IPv6 ACLs.

The following table shows the qualifier fields that are supported in the IPv4 and IPv6 key formats. If the default TCAM key is set as Enabled, then the Qualifier field is enabled by default. If the default TCAM key is set as Disabled, then the Qualifier field must use a UDK.

Table 1. Qualifier Fields Supported in IPv4 and IPv6 Key Formats

Parameter

Default TCAM Key

IPv4

IPv6

Source Address

Enabled

Enabled

Destination Address

Enabled

Enabled

Source Port

Enabled

Enabled

Destination Port

Enabled

Enabled

Port Range

Enabled

Not supported

Protocol/Next Header

Enabled

Enabled

Fragment bit

Enabled

Not supported

Packet length

Disabled

Disabled

Precedence/DSCP

Disabled

Enabled

TCP Flags

Enabled

Enabled

TTL Match

Disabled

Disabled

Interface based

Disabled

Not supported

UDF 1-7

Disabled

Disabled

ACL ID

Enabled

Enabled

common ACL bit

Enabled by default for IPv4/IPv6 on shared mode.

Disabled by default for IPv4/IPv6 on unique mode.

Enabled by default for IPv4/IPv6 on shared mode.

Disabled by default for IPv4/IPv6 on unique mode.

Interface-based (RIF)

Disabled

Disabled

The following table shows the action fields supported in the IPv4 and IPv6 key formats.


Note

You cannot configure QoS groups for ingress ACLs after a User-Defined TCAM Key (UDK) is configured because the command, permit ipv4 any any set qos-group, is not supported.

Table 2. Action Fields Supported in IPv4 and IPv6 Key Formats

Parameter

Default Action Field

IPv4

IPv6

Permit

Enabled

Enabled

Deny

Enabled

Enabled

Log

Enabled

Enabled

Capture

Enabled

Enabled

Stats Counter

Deny stats is always Enabled (permit stats has its own hw-module command)

Deny stats is always Enabled

TTL Set

Enabled

Enabled

To enable the monitoring of the packet count that is permitted based on the ACL rules, use the following configuration, and then reload the line card or router as required: 

/* Enable an egress ACL on on a hardware module profile. */
Router(config)# hw-module profile acl egress layer3 interface-based

Router(config)# commit
Router(config)# end
Router# reload location all
Wed Apr 5 23:05:46.193 UTC
Proceed with reload? [confirm]

To edit the ACL configuration, remove the hw-module configuration, edit the ACL configuration, and then enable the hw-module configuration again.

Configuring IPv4 ACLs

This section describes the basic configuration of IPv4 ingress and egress ACLs.

Notes and Restrictions for Configuring IPv4 Ingress ACLs

IPv4 ingress ACLs are characterized by the following behavior.

  • Ingress IPv4 ACLs are supported on all interfaces except management interfaces.

  • ACL-based Forwarding (ABF) is supported only in the ingress direction.

  • The total number of ingress ACLs allowed per NPU is 31 in shared ACL mode and more than 31 in unique ACL mode.

  • The following line card limits apply for traditional and hybrid ingress ACLs:

  • The number of attached ACEs allowed per line card is 4096.

Notes and Restrictions for Configuring IPv4 Egress ACLs

IPv4 egress ACLs are characterized by the following behavior.

  • Egress IPv4 ACLs are supported on main physical interfaces and bundle interfaces.


    Note

    Egress ACLs are not directly supported on sub-interfaces. However, If you configure an egress ACL on a main interface that has sub-interfaces, the ACL action is also applied to the sub-interface traffic. This egress ACL behavior holds true even if the sub-interfaces are configured after the ACL is applied to the main interface.

  • The total number of egress ACLs allowed per NPU is 255.

  • ACL is not supported on Management interface on egress direction.

  • Apart from the throughput limitation, router-generated traffic is not be affected by egress IPv4 ACLs.

  • The number of attached ACEs allowed per line card is 4096.

  • Filtering for egress IPv4 multicast traffic is not supported if H-QoS is configured on the router.

Configuring an Ingress IPv4 ACL on a Gigabit Ethernet Interface

Use the following configuration to configure an ingress IPv4 ACL on a GigE interface. 

/* Configure a GigE interface with an IPv4 address */
Router(config)# interface gigabitEthernet 0/0/0/0 
Router(config-if)# ipv4 address 10.1.1.1 255.255.255.0
Router(config-if)# no shut
Router(config-if)# commit
Thu Jan 25 10:07:54.700 IST
Router(config-if)# exit

/* Verify if the interface is up */
Router(config)# do show ipv4 interface brief
Thu Jan 25 10:08:49.087 IST

Interface                      IP-Address      Status          Protocol Vrf-Name
GigabitEthernet0/0/0/0         10.1.1.1        Up              Up       default 

/* Configure an IPv4 ingress ACL */
Router(config)# ipv4 access-list V4-ACL-INGRESS
Router(config-ipv4-acl)# 10 permit tcp 10.2.1.1 0.0.0.255 any
Router(config-ipv4-acl)# 20 deny udp any any
Router(config-ipv4-acl)# 30 permit ipv4 10.2.0.0 0.255.255.255 any
Router(config-ipv4-acl)# commit
Thu Jan 25 10:16:11.473 IST

/* Verify the ingress ACL creation */
Router(config)# do show access-lists ipv4 
Thu Jan 25 10:25:19.896 IST
...
ipv4 access-list V4-ACL-INGRESS
 10 permit tcp 10.2.1.0 0.0.0.255 any
 20 deny udp any any
 30 permit ipv4 10.0.0.0 0.255.255.255 any


/* Apply the ingress ACL to the GigE interface */
Router(config)# interface GigabitEthernet0/0/0/0
Router(config-if)# ipv4 access-group V4-ACL-INGRESS ingress
Router(config-if)# commit
Thu Jan 25 10:28:19.671 IST
Router(config-if)# exit

/* Verify if the ingress ACL has been successfully applied to the interface */
Router(config)# do show ipv4 interface 
Thu Jan 25 10:29:44.944 IST
GigabitEthernet0/0/0/0 is Up, ipv4 protocol is Up
  Vrf is default (vrfid 0x60000000)
  Internet address is 10.1.1.1/24
  MTU is 1514 (1500 is available to IP)
  Helper address is not set
  Directed broadcast forwarding is disabled
  Outgoing access list is not set
  Inbound  common access list is not set, access list is V4-ACL-INGRESS
  Proxy ARP is disabled
  ICMP redirects are never sent
  ICMP unreachables are always sent
  ICMP mask replies are never sent
  Table Id is 0xe0000000

You have successfully configured an IPv4 ingress ACL on a Gigabit Ethernet interface.

Configuring an Egress IPv4 ACL on a Gigabit Ethernet Interface

Use the following configuration to configure an egress IPv4 ACL on a GigE interface. 

/* Configure a GigE interface with an IPv4 address */
Router(config)# interface gigabitEthernet 0/0/0/0
Router(config-if)# ipv4 address 20.1.1.1 255.255.255.0
Router(config-if)# no shut 
Router(config-if)# commit
Thu Jan 25 10:08:38.767 IST
Router(config-if)# exit

/* Verify if the interface is up */
Router(config)# do show ipv4 interface brief
Thu Jan 25 10:08:49.087 IST

Interface                      IP-Address      Status          Protocol Vrf-Name
GigabitEthernet0/0/0/0         10.1.1.1        Up              Up       default 
GigabitEthernet0/0/0/0         20.1.1.1        Up              Up       default 

/* Configure an IPv4 egress ACL */
Router(config)# ipv4 access-list V4-ACL-EGRESS
Router(config-ipv4-acl)# 10 permit ipv4 10.2.0.0 0.255.255.255 20.2.0.0 0.255.255.255
Router(config-ipv4-acl)# 20 deny ipv4 any any
Router(config-ipv4-acl)# commit
Thu Jan 25 10:25:04.655 IST

/* Verify the egress ACL creation */
Router(config)# do show access-lists ipv4 
Thu Jan 25 10:25:19.896 IST
ipv4 access-list V4-ACL-EGRESS
 10 permit ipv4 10.0.0.0 0.255.255.255 20.0.0.0 0.255.255.255
 20 deny ipv4 any any
...

/* Apply the egress ACL to the GigE interface */
Router(config)# interface gigabitEthernet 0/0/0/1
Router(config-if)# ipv4 access-group V4-ACL-EGRESS egress
Router(config-if)# commit
Thu Jan 25 10:28:45.937 IST
Router(config-if)# exit

/* Verify if the egress ACL has been successfully applied to the interface */
Router(config)# do show ipv4 interface 
Thu Jan 25 10:29:44.944 IST
GigabitEthernet 0/0/0/1 is Up, ipv4 protocol is Up
  Vrf is default (vrfid 0x60000000)
  Internet address is 20.1.1.1/24
  MTU is 1514 (1500 is available to IP)
  Helper address is not set
  Directed broadcast forwarding is disabled
  Outgoing access list is V4-ACL-EGRESS
  Inbound  common access list is not set, access list is not set
  Proxy ARP is disabled
  ICMP redirects are never sent
  ICMP unreachables are always sent
  ICMP mask replies are never sent
  Table Id is 0xe0000000
...

You have successfully configured an IPv4 egress ACL on a Gigabit Ethernet interface.

Configuring IPv6 ACLs

This section describes the steps to configure ingress and egress IPv6 ACLs over gigabit ethernet and bundle interfaces.

Notes and Restrictions for Configuring IPv6 Ingress ACLs

IPv6 ingress ACLs are characterized by the following behavior.

  • ACL-based Forwarding (ABF) is supported only in the ingress direction.

  • The total number of ingress ACLs allowed per NPU is 31 in shared ACL mode and more than 31 in unique ACL mode.

  • The following line card limits apply for traditional and hybrid ingress ACLs:

  • The number of attached ACEs allowed per line card is 2048 in the ingress direction.

  • Packet Length (using the pkt-length keyword) is not supported.

Notes and Restrictions for Configuring IPv6 Egress ACLs

IPv6 egress ACLs are characterized by the following behavior:

  • Configuring packet length is not supported on egress ACLs.

  • TCP flags are not supported on egress ACLs.

  • Egress ACLs are not supported on BVI interfaces and L2 interfaces.

  • Configuring qos-group is not supported on egress ACLs.

  • A throughput of 50% or less is supported on egress ACLs.

  • Apart from the throughput limitation, router-generated traffic is not be affected by egress IPv6 ACLs.

  • The total number of egress ACLs allowed per NPU is 255.

  • The total number of attached ACEs allowed per line card is 2048 in the egress direction.

  • Configuring dynamic TCAM key is not supported on egress ACLs.

  • Upto 160GB of total IPv6 egress ACL is supported per NPU because the Egress IPv6 ACLs take the recycle path.

  • Filtering for egress IPv6 multicast traffic is not supported if H-QoS is configured on the router.

Configuring an Ingress IPv6 ACL on a Gigabit Ethernet Interface

Use the following configuration to configure an ingress IPv6 ACL on a GigE interface. 

/* Configure a GigE interface with an IPv6 address */
Router(config)# interface gigabitEthernet 0/0/0/0 
Router(config-if)# ipv6 address 1001::1/64
Router(config-if)# no shut
Router(config-if)# commit
Thu Jan 25 10:07:54.700 IST
Router(config-if)# exit

/* Verify if the interface is up */
Router(config)# do show ipv6 interface brief
Thu Jan 25 12:38:35.742 IST
GigabitEthernet 0/0/0/0 [Up/Up]
    fe80::bd:b9ff:fea9:5606                       
    1001::1                                       
…

/* Configure an IPv6 ingress ACL */
Router(config)# ipv6 access-list V6-INGRESS-ACL
Router(config-ipv6-acl)# 10 permit ipv6 any any
Router(config-ipv6-acl)# 20 deny udp any any
Router(config-ipv6-acl)# commit
Thu Jan 25 11:31:24.488 IST
Router(config-ipv6-acl)# exit 


/* Verify the ingress ACL creation */
Router(config)# do show access-lists ipv6
Thu Jan 25 11:34:56.911 IST
ipv6 access-list V6-INGRESS-ACL
 10 permit ipv6 any any
 20 deny udp any any


/* Apply the ingress ACL to the GigE interface */
Router(config)# interface gigabitEthernet 0/0/0/0
Router(config-if)# ipv6 access-group V6-INGRESS-ACL ingress
Router(config-if)# commit
Thu Jan 25 11:32:55.194 IST
Router(config-if)# exit

/* Verify if the ingress ACL has been successfully applied to the interface */
Router(config)# do show ipv6 interface 
Thu Jan 25 11:34:08.028 IST
GigabitEthernet 0/0/0/0 is Up, ipv6 protocol is Up, Vrfid is default (0x60000000)
  IPv6 is enabled, link-local address is fe80::bd:b9ff:fea9:5606 
  Global unicast address(es):
    1001::1, subnet is 1001::/64 
  Joined group address(es): ff02::1:ff00:1 ff02::1:ffa9:5606 ff02::2
      ff02::1
  MTU is 1514 (1500 is available to IPv6)
  ICMP redirects are disabled
  ICMP unreachables are enabled
  ND DAD is enabled, number of DAD attempts 1
  ND reachable time is 0 milliseconds
  ND cache entry limit is 1000000000
  ND advertised retransmit interval is 0 milliseconds
  Hosts use stateless autoconfig for addresses.
  Outgoing access list is not set
  Inbound  common access list is not set, access list is V6-INGRESS-ACL
  Table Id is 0xe0800000
  Complete protocol adjacency: 0
  Complete glean adjacency: 0
  Incomplete protocol adjacency: 0
  Incomplete glean adjacency: 0
  Dropped protocol request: 0
  Dropped glean request: 0
…

You have successfully configured an IPv6 ingress ACL on a Gigabit Ethernet interface.

Configuring an Egress IPv6 ACL on a Gigabit Ethernet Interface

Use the following configuration to configure an egress IPv6 ACL on a GigE interface. 

/* Configure a GigE interface with an IPv6 address */
Router(config)# interface gigabitEthernet 0/0/0/1
Router(config-if)# ipv6 address 2001::1/64
Router(config-if)# no shut
Router(config-if)# commit
Thu Jan 25 11:41:25.778 IST
Router(config-if)# exit

/* Verify if the interface is up */
Router(config)# do show ipv6 interface brief
Thu Jan 25 12:38:35.742 IST
GigabitEthernet 0/0/0/0 [Up/Up]
    fe80::bd:b9ff:fea9:5606                       
    1001::1                                       
GigabitEthernet 0/0/0/1 [Up/Up]
    fe80::23:e9ff:fea8:a44e                       
    2001::1    

/* Configure an IPv6 egress ACL */
Router(config)# ipv6 access-list V6-EGRESS-ACL 
Router(config-ipv6-acl)# 10 permit ipv6 any any
Router(config-ipv6-acl)# 20 deny udp any any
Router(config-ipv6-acl)# commit
Thu Jan 25 11:44:03.969 IST
Router(config-ipv6-acl)# exit

/* Verify the egress ACL creation */
Router(config)# do show access-lists ipv6
Thu Jan 25 11:45:53.823 IST
ipv6 access-list V6-EGRESS-ACL
 10 permit ipv6 any any
 20 deny udp any any
…


/* Verify if the egress ACL has been successfully applied to the interface */
Router(config)# do show ipv6 interface    
Thu Jan 25 11:46:43.234 IST
…
GigabitEthernet 0/0/0/1 is Up, ipv6 protocol is Up, Vrfid is default (0x60000000)
  IPv6 is enabled, link-local address is fe80::23:e9ff:fea8:a44e 
  Global unicast address(es):
    2001::1, subnet is 2001::/64 
  Joined group address(es): ff02::1:ff00:1 ff02::1:ffa8:a44e ff02::2
      ff02::1
  MTU is 1514 (1500 is available to IPv6)
  ICMP redirects are disabled
  ICMP unreachables are enabled
  ND DAD is enabled, number of DAD attempts 1
  ND reachable time is 0 milliseconds
  ND cache entry limit is 1000000000
  ND advertised retransmit interval is 0 milliseconds
   Hosts use stateless autoconfig for addresses.
  Outgoing access list is V6-EGRESS-ACL
  Inbound  common access list is not set, access list is not set
  Table Id is 0xe0800000
  Complete protocol adjacency: 0
  Complete glean adjacency: 0
  Incomplete protocol adjacency: 0
  Incomplete glean adjacency: 0
  Dropped protocol request: 0
  Dropped glean request: 0
...

You have successfully configured an IPv6 egress ACL on a Gigabit Ethernet interface.

Configuring Ingress and Egress IPv6 ACLs on Bundle Interfaces

Use the following configuration to configure ingress and egress IPv6 ACLs on a bundle interface.

Configuring Ingress and Egress IPv6 ACLs on Bundle Interfaces is not supported on the following Cisco NCS 540 variants:

  • N540-28Z4C-SYS-A

  • N540-28Z4C-SYS-D

  • N540X-16Z4G8Q2C-A

  • N540X-16Z4G8Q2C-D

  • N540X-16Z8Q2C-D

  • N540-12Z20G-SYS-A

  • N540-12Z20G-SYS-D

  • N540X-12Z16G-SYS-A

  • N540X-12Z16G-SYS-D

/* Configure a bundle interface with an IPv6 address */
Router(config)# interface Bundle-Ether 1
Router(config-if)# ipv6 address 3001::1/64
Router(config-if)# no shut
Router(config-if)# commit
Thu Jan 25 13:53:47.435 IST
Router(config-if)# exit 

/* Configure an IPv6 egress ACL */
Router(config)# ipv6 access-list V6-EGRESS-ACL-bundle interface
Router(config-ipv6-acl)# 100 permit tcp any any eq www
Router(config-ipv6-acl)# 110 permit tcp any any eq https
Router(config-ipv6-acl)# 120 permit tcp any any eq ssh
Router(config-ipv6-acl)# 130 permit udp any any eq snmp
Router(config-ipv6-acl)# commit 
Thu Jan 25 13:57:14.960 IST
Router(config-ipv6-acl)# exit

/* Configure an IPv6 ingress ACL to deny ingress traffic on the bundle interface */
Router(config)# ipv6 access-list V6-DENY-INGRESS-ACL
Router(config-ipv6-acl)# 10 deny ipv6 any any
Router(config-ipv6-acl)# commit
Thu Jan 25 13:59:23.198 IST
Router(config-ipv6-acl)# exit

/* Verify the egress and ingress ACL creation */
Router(config)# do show access-lists ipv6
Thu Jan 25 14:00:24.055 IST
ipv6 access-list V6-DENY-INGRESS-ACL
 10 deny ipv6 any any
ipv6 access-list V6-EGRESS-ACL-bundle
 100 permit tcp any any eq www
 110 permit tcp any any eq https
 120 permit tcp any any eq ssh
 130 permit udp any any eq snmp
...

/* Apply the egress and ingress ACLs to the bundle interface */
Router(config)# interface Bundle-Ether 1
Router(config-if)# ipv6 access-group V6-EGRESS-ACL-bundle egress
Router(config-if)# ipv6 access-group V6-DENY-INGRESS-ACL ingress
Router(config-if)# commit
Thu Jan 25 14:04:19.536 IST
Router(config-if)# exit

/* Verify if the ACLs have been successfully applied to the interface */
Router(config)# do show ipv6 interface    
Thu Jan 25 11:46:43.234 IST
…
Thu Jan 25 14:04:51.322 IST
Bundle-Ether1 is Down, ipv6 protocol is Down, Vrfid is default (0x60000000)
  IPv6 is enabled, link-local address is fe80::1:10ff:fe87:8d04 [TENTATIVE]
  Global unicast address(es):
    3001::1, subnet is 3001::/64 [TENTATIVE]
  Joined group address(es): ff02::2 ff02::1
  MTU is 1514 (1500 is available to IPv6)
  ICMP redirects are disabled
  ICMP unreachables are enabled
  ND DAD is enabled, number of DAD attempts 1
  ND reachable time is 0 milliseconds
  ND cache entry limit is 1000000000
  ND advertised retransmit interval is 0 milliseconds
  ND router advertisements are sent every 160 to 240 seconds
  ND router advertisements live for 1800 seconds
  Hosts use stateless autoconfig for addresses.
  Outgoing access list is V6-EGRESS-ACL-BI
  Inbound  common access list is not set, access list is V6-DENY-INGRESS-ACL
  Table Id is 0xe0800000
  Complete protocol adjacency: 0
  Complete glean adjacency: 0
  Incomplete protocol adjacency: 0
  Incomplete glean adjacency: 0
  Dropped protocol request: 0
  Dropped glean request: 0

You have successfully configured ingress and egress IPv6 ACLs on a bundle interface.

Configuring Chained ACLs

Chained ACLs also known as Multi-ACL enables you to apply more than one IPv4 or IPv6 (common and interface) ACL on an interface for packet filtering. This feature allows you to manage and configure different ACLs on an interface efficiently.

A typical ACL on the edge box for an ISP has two sets of ACEs:

  • Common ISP specific ACEs (ISP protected address block)

  • Customer/interface specific ACEs (Customer source address block)

The purpose of these address blocks is to deny access to ISP’s protected infrastructure networks and anti-spoofing protection by allowing only customer source address blocks. This results in the configuration of unique ACLs per interface. Most of the ACEs are common across all the ACLs on a router. Therefore, ACL provisioning and modification is very cumbersome. Any changes to the ACE impacts every customer interface. This process also wastes the HW/TCAM resources as the common ACEs are being replicated in all ACLs.

This feature provides Ternary Content Addressable Memory(TCAM)/HW scalability. You can configure more than one ACL on a single interface. Therefore, you can separate various types of ACLs for management and other reasons, yet apply both of them on the same interface, in a defined order.

Restrictions

The following restrictions apply while implementing Common ACLs:

  • Common ACLs are supported in only ingress direction and for L3 interfaces only.

  • Only one common IPv4 and IPv6 ACL is supported on each line card.

  • The common ACL option is not available for Ethernet Service (ES) ACLs.

  • Packets are filtered through the common ACL configuration before the interface ACL configuration.

  • You can edit common ACLs, but atomic replacement of the common ACLs is not supported.

  • You cannot configure a common ACL on the same line card on which a compressed ACL is configured.

  • The following line card limits apply for traditional and hybrid ingress ACLs:

    • A maximum of 127 different traditional ingress ACLs per LC in shared ACL mode.

    • A maximum of 255 different hybrid ingress ACLs per LC in shared ACL mode.

    • Unique ACL mode allows you to configure more than 128 different traditional ingress ACLs per LC.

  • If you configure chained ACLs, the sequence of each ACL is reduced and the maximum sequence number that you can configure is 1 million.

  • The compress option is not supported for common ACLs.

  • Object-groups are not supported with common ACLs.

Configuration

You can use the following steps to configure chained ACLs:

  1. Enter the interface configuration mode, and then configure an interface.

  2. Configure a common acl and an interface acl, for example common-1 and interface-1 on the interface.

Configuration Example

/* Enter the interface configuration mode, and then configure an interface. */
Router# configure
Router(config)# interface TenGigE 0/0/0/0

/* Configure a common acl and an interface acl, for example common-1 and interface-1 on the interface. */
Router(config-if)# ipv4 access-group common common-1 interface-1 ingress
Router(config-if)# commit

Associated Commands

Modifying ACLs

This section describes a sample configuration for modification of ACLs.

*/ Create an Access List*/
Router(config)#ipv4 access-list acl_1

*/Add entries (ACEs) to the ACL*/
Router(config-ipv4-acl)#10 permit ip host 10.3.3.3 host 172.16.5.34
Router(config-ipv4-acl)#20 permit icmp any any
Router(config-ipv4-acl)#30 permit tcp any host 10.3.3.3
Router(config-ipv4-acl)#end

*/Verify the entries of the ACL*/:
Router#show access-lists ipv4 acl_1
ipv4 access-list acl_1
10 permit ip host 10.3.3.3 host 172.16.5.34
20 permit icmp any any
30 permit tcp any host 10.3.3.3

*/Add new entries, one with a sequence number "15" and another without a sequence number to the ACL. Delete an entry with the sequence number "30":*/
Router(config)#ipv4 access-list acl_1
Router(config-ipv4-acl)# 15 permit 10.5.5.5 0.0.0.255
Router(config-ipv4-acl)# no 30
Router(config-ipv4-acl)# permit 10.4.4.4 0.0.0.255
Router(config-ipv4-acl)# commit


*/When an entry is added without a sequence number, it is automatically given a sequence number
that puts it at the end of the access list. Because the default increment is 10, the entry will have a sequence
number 10 higher than the last entry in the existing access list*/

*/Verify the entries of the ACL:*/
Router(config)#show access-lists ipv4 acl_1
ipv4 access-list acl_1
 10 permit ipv4 host 10.3.3.3 host 172.16.5.34

15 permit 10.5.5.5 0.0.0.255---*/newly added ACE (with the sequence number)*/
20 permit icmp any any
30 permit ipv4 10.4.4.0 0.0.0.255 any ---*/newly added ACE (without the sequence number)*/

*/The entry with the sequence number 30, that is, "30 permit tcp any host 10.3.3.3" is deleted from the ACL*/

You have successfully modified ACLs in operation.

Configuring ACL-based Forwarding

Converged networks carry voice, video and data. Users may need to route certain traffic through specific paths instead of using the paths computed by routing protocols. This is achieved by specifying the next-hop address in ACL configurations, so that the configured next-hop address from ACL is used for fowarding packet towards its destination instead of routing packet-based destination address lookup. This feature of using next-hop in ACL configurations for forwarding is called ACL Based Forwarding (ABF).

ACL-based forwarding enables you to choose service from multiple providers for broadcast TV over IP, IP telephony, data, and so on, which provides a cafeteria-like access to the Internet. Service providers can divert user traffic to various content providers.

Feature Highlights

  • ABF is only supported on ingress ACL.

  • ABF supports nexthop modifications. You can modify a nexthop, remove a nexthop, or make changes between existing nexthops.


    Note

    While defining an ACE rule, you must specify the VRF for all nexthops unless the nexthop is in the default VRF. This will ensure that the packets take the right path towards the nexthop.

  • VRF-aware ABF is supported for IPv4 and IPv6 with up to three next hops.

  • IPv4 ABF nexthops routed over GRE interfaces are supported.

  • IPv6 ABF ACL does not work on bundle interface when applied in ingress direction.

  • As ABF is ACL-based, packets that do not match an existing rule (ACE) in the ACL are subject to the default ACL rule (drop all). If the ACL is being used for ABF-redirect only (not for security), then include an explicit ACE rule at the end of the ACL (lowest user priority) to match and "permit" all traffic. This ensures that all traffic that does not match an ABF rule is permitted and forwarded as normal.

  • ABF is supported on permit rules only.

  • VRF-select (where only the VRF is configured for the nexthop) is not supported.

  • ABF default route is not supported.

  • Packets punted in the ingress direction from the NPU to the linecard CPU are not subjected to ABF treatment due to lack of ABF support in the slow path. These packets will be forwarded normally based on destination-address lookup by the software dataplane. Some examples of these types of packets are (but are not limited to) packets with IPv4 options, IPv6 extension headers, and packets destined for glean (unresolved/incomplete) adjacencies.

  • Packets destined to the local IP interface ("for-us" packets) are subjected to redirect if they match the rule containing the ABF action. This can be avoided by either designing the rule to be specific enough to avoid matching the “for-us” packets or placing an explicit permit ACE rule (with higher priority) into the ACL before the matching ABF rule.

Configuration Example

To configure ACL-based forwarding, use the following configuration example: 

/* Enter IPv4 access list configuration mode and configure an ACL: */
Router# configure
Router(config)# ipv4 access-list abf-acl

/* Set the conditions for the ACL and configure ABF: */
/* The next hop for this entry is specified. */
Router(config-ipv4-acl)# 10 permit ipv4 192.168.18.0 0.255.255.255 any nexthop1 ipv4 192.168.20.2
Router(config-ipv4-acl)# 15 permit ipv4 192.168.21.0 0.0.0.255 any
Router(config-ipv4-acl)# 20 permit ipv4 192.168.22.0 0.0.255.255 any nexthop1 ipv4 192.168.23.2
/* More than two nexthops */
Router(config-ipv4-acl)# 25 permit tcp any range 2000 3000 any range 4000 5000 nexthop1 ipv4 192.168.23.1 nexthop2 ipv4 192.168.24.1 nexthop3 ipv4 192.168.25.1

/* VRF support on ABF */
Router(config-ipv4-acl)# 30 permit tcp any eq www host 192.168.12.2 precedence immediate nexthop1 vrf vrf1_ipv4 ipv4 192.168.13.2 nexthop2 vrf vrf1_ipv4 ipv4 192.168.14.2

Router(config-ipv4-acl)# 35 permit ipv4 any any

Router(config-ipv4-acl)# commit

/* (Optional) Display ACL information: */
Router# show access-lists ipv4 abf-acl

Running Configuration 


ipv4 access-list abf-acl
10 permit ipv4 192.168.18.0 0.255.255.255 any nexthop1 192.168.20.2
15 permit ipv4 192.168.21.0 0.0.0.255 any
20 permit ipv4 192.168.22.0 0.0.255.255 any nexthop1 192.168.23.2
25 permit tcp any range 2000 3000 any range 4000 5000 nexthop1 ipv4 192.168.23.1 nexthop2 ipv4 192.168.24.1 nexthop3 ipv4 192.168.25.1
30 permit tcp any eq www host 192.168.12.2 precedence immediate nexthop1 vrf vrf1_ipv4 ipv4 192.168.13.2 nexthop2 vrf vrf1_ipv4 ipv4 192.168.14.2
35 permit ipv4 any any
commit
!



ipv4 access-list TEST
 10 permit ipv4 60.1.1.5 0.0.0.255 any nexthop1 vrf VRF1 nexthop2 vrf VRF2 nexthop3 vrf VRF3
!

Verification

Use the following command to verify the IP nexthop state in ABF to ensure that the expected nexthop is up: 

Router# show access-lists ipv4 abf nexthops client pfilter_ea location 0/0/CPU0
Wed Jan 24 14:18:58.667 UTC

ACL name : abf-acl
ACE seq. 		NH-1 		NH-2 		NH-3
	--------- --------------- --------------- ---------------
	10 	      192.168.13.2
	status          UP
	at status 	Not Present
	exist     	No
	vrf       	default
	track
	pd 	  	ctx		Present
	25 		192.168.14.2    192.168.11.1 	192.168.12.1
	status 		UP 		Down 		Down
	at status 	Not Present 	Not Present 	Not Present
	exist 		No 		Yes 		Yes
	vrf 		default 	default 	default
	track
	pd ctx 		Present 	Not present 	Not present
	30 		192.168.15.1 	192.168.12.7
	status 		Unknown 	Unknown
	at status 	Not Present 	Not Present
	exist 		No 		Yes
	vrf 		vrf1_ipv4 	vrf1_ipv4
	track
	pd ctx 		Not present 	Not present

Use the following command to verify if ABF is currently attached to any interfaces at any linecard: 

show access-lists usage pfilter location all

ACLs on Bridge Virtual Interfaces

Bridge Virtual Interfaces (BVIs) provide a bridge between the routing and bridging domains on a router. A BVI is configured with an IP address and operates as a regular routed interface. You can configure an ACL on a BVI to filter the traffic for the network that uses the interface.


Note

Do not delete an ACL attached to a BVI interface when the BVI interface is not part of a bridge domain. Later, if you add the BVI interface to the bridge domain then the traffic is dropped.

Increased TCAM Consumption with Configuring ACLs on BVIs

The consumption of TCAM resources is impacted in the following manner when ACLs are configured on BVIs.

  • For ingress ACLs, the TCAM entries for the same ACL are shared across interfaces on the same NPU.

  • For egress ACLs, the TCAM entries for the same ACL are unique for all interfaces. This leads to greater consumption of TCAM resources.

Restrictions for Configuring ACLs on BVIs

You must be aware of the following restrictions before proceeding to configure ACLs on BVIs.

  • When an egress ACL is enabled on a BVI through the hw-module command, no other interface types are supported for the ACL (non-BVI interfaces are not supported for the ACL in this mode).

Prerequisites for Configuring Egress ACLs on BVIs

By default, an egress ACL on a BVI is disabled, and ACL filtering does not take place even when the ACL is attached to the BVI. Hence, we use the hw-module command, which enables the ACL when the line cards are reloaded.


Note

IPv4 and IPv6 ingress ACLs do not require this configuration.

Configuration

The following section describes the procedure for configuring IPv4 ingress and egress ACLs on BVIs.

To configure IPv4 ingress and egress ACLs on a BVI, use the following procedure with sample configuration.

  1. Enter the Global Configuration mode, and configure an IPv4 ingress ACL.

    Router(config)# ipv4 access-list v4-acl-ingress
Router(config-ipv4-acl)# 10 permit tcp any 10.1.1.0/24 dscp cs6
Router(config-ipv4-acl)# 20 deny udp any any eq ssh
Router(config-ipv4-acl)# 30 permit ipv4 any any
Router(config-ipv4-acl)# commit
Router(config-ipv4-acl)# exit

  2. Configure an IPv4 egress ACL.

    Router(config)# ipv4 access-list v4-acl-egress
Router(config-ipv4-acl)# 10 deny ipv4 any any fragments log
Router(config-ipv4-acl)# 20 deny tcp any any ack
Router(config-ipv4-acl)# 30 permit ipv4 any any
Router(config-ipv4-acl)# commit
Router(config-ipv4-acl)# exit

  3. Configure the Gigabit Ethernet interface that must be mapped to the BVI, and enable it for Layer 2 transport.

    Router(config)# interface GigabitEthernet 0/0/0/0
Router(config-if)# l2transport 
Router(config-if-l2)# commit

  4. Attach the ingress and egress ACLs to the BVI.

    Router(config)# interface BVI1
Router(config-if)# ipv4 access-group v4-acl-ingress ingress
Router(config-if)# ipv4 access-group v4-acl-egress egress
Router(config-if)# commit
Router(config-if)# exit

  5. Configure the bridge domain with the Gigabit Ethernet interface and BVI.

    Router(config)# l2vpn
Router(config-l2vpn)# bridge group BG1
Router(config-l2vpn-bg)# bridge-domain B1
Router(config-l2vpn-bg-bd)# interface GigabitEthernet 0/0/0/0  
Router(config-l2vpn-bg-bd-ac)# routed interface BVI1                     
Router(config-l2vpn-bg-bd)# commit
Router(config-l2vpn-bg-bd)# exit
Router(config-l2vpn-bg)# exit
Router(config-l2vpn)# exit

  6. Confirm that your configuration has been successfully committed.

    Router(config)# show run
...
!
ipv4 access-list v4-acl-egress
 10 deny ipv4 any any fragments log
 20 deny tcp any any ack
 30 permit ipv4 any any
!
ipv4 access-list v4-acl-ingress
 10 permit tcp any 10.1.1.0/24 dscp cs6
 20 deny udp any any eq ssh
 30 permit ipv4 any any
!
interface GigabitEthernet0/0/0/0
 l2transport
 !
!
interface BVI1
ipv4 address 209.165.200.224/27
 ipv4 access-group v4-acl-ingress ingress
 ipv4 access-group v4-acl-egress egress
 
!
l2vpn
 bridge group BG1
  bridge-domain B1
   interface GigabitEthernet0/0/0/0
   !
   routed interface BVI1
  !
 !
!
end

You have successfully configured and enabled IPv4 ingress and egress ACL on a BVI.

Verification

This section explains how to verify the IPv4 ACL configuration.

In Executive Privilege mode, confirm that the ACLs are in operation.

Router# show access-lists interface bvi1
Tue May 9 10:01:25.732 EDT
Input ACL (common): GigabitEthernet 0/0/0/0 (interface): v4-acl-ingress
Output ACL: v4-acl-egress

Router# show access-lists summary
Tue May 9 10:02:01.167 EDT
ACL Summary:
Total ACLs configured: 2
Total ACEs configured: 6


Router# show access-lists ipv4 v4-acl-egress hardware egress location 0/0/CPU0  
ipv4 access-list v4-acl-egress
10 deny ipv4 any any fragments log (15214 matches)
20 deny tcp any any ack (15214 matches)
30 permit ipv4 any any (15214 matches)

The output clearly shows the configured ACLs, the total number of ACEs (three per ACL), and also the ACE matches in hardware.

Configuring ACLs with Fragment Control

The non-fragmented packets and the initial fragments of a packet were processed by IP extended access lists (if you apply this access list), but non-initial fragments were permitted, by default. However, now, the IP Extended Access Lists with Fragment Control feature allows more granularity of control over non-initial fragments of a packet. Using this feature, you can specify whether the system examines non-initial IP fragments of packets when applying an IP extended access list.

As non-initial fragments contain only Layer 3 information, these access-list entries containing only Layer 3 information, can now be applied to non-initial fragments also. The fragment has all the information the system requires to filter, so the access-list entry is applied to the fragments of a packet.

This feature adds the optional fragments keyword for IPv4 ACLs to the following IP access list commands: deny and permit . By specifying the fragments keyword in an access-list entry, that particular access-list entry applies only to non-initial fragments of packets; the fragment is either permitted or denied accordingly.

The behavior of access-list entries regarding the presence or absence of the fragments keyword can be summarized as follows:

If the Access-List Entry has...

Then...

...no fragments keyword and all of the access-list entry information matches

For an access-list entry containing only Layer 3 information:

  • The entry is applied to non-fragmented packets, initial fragments, and non-initial fragments.

For an access-list entry containing Layer 3 and Layer 4 information:

  • The entry is applied to non-fragmented packets and initial fragments.

    • If the entry matches and is a permit statement, the packet or fragment is permitted.

    • If the entry matches and is a deny statement, the packet or fragment is denied.

  • The entry is also applied to non-initial fragments in the following manner. Because non-initial fragments contain only Layer 3 information, only the Layer 3 portion of an access-list entry can be applied. If the Layer 3 portion of the access-list entry matches, and

    • If the entry is a permit statement, the non-initial fragment is permitted.

    • If the entry is a deny statement, the next access-list entry is processed.

    Note

     

    The deny statements are handled differently for non-initial fragments versus non-fragmented or initial fragments.

...the fragments keyword and all of the access-list entry information matches

The access-list entry is applied only to non-initial fragments.

Note

 

The fragments keyword cannot be configured for an access-list entry that contains any Layer 4 information.

You should not add the fragments keyword to every access-list entry, because the first fragment of the IP packet is considered a non-fragment and is treated independently of the subsequent fragments. Because an initial fragment will not match an access list permit or deny entry that contains the fragments keyword, the packet is compared to the next access list entry until it is either permitted or denied by an access list entry that does not contain the fragments keyword. Therefore, you may need two access list entries for every deny entry. The first deny entry of the pair will not include the fragments keyword, and applies to the initial fragment. The second deny entry of the pair will include the fragments keyword and applies to the subsequent fragments. In the cases where there are multiple deny access list entries for the same host but with different Layer 4 ports, a single deny access-list entry with the fragments keyword for that host is all that has to be added. Thus all the fragments of a packet are handled in the same manner by the access list.

Packet fragments of IP datagrams are considered individual packets and each fragment counts individually as a packet in access-list accounting and access-list violation counts.


Note

The fragments keyword cannot solve all cases involving access lists and IP fragments.


Note

Within the scope of ACL processing, Layer 3 information refers to fields located within the IPv4 header; for example, source, destination, protocol. Layer 4 information refers to other data contained beyond the IPv4 header; for example, source and destination ports for TCP or UDP, flags for TCP, type and code for ICMP.

Configuration

You can use the following configuration to configure the fragments keyword for an IPv4 access list: 

/* Configure an Access List */
Router# configure
Router(config)# ipv4 access-list IPv4_Fragments

/* Configure the fragments keyword for the IPv4 access list */
Router(config-ipv4-acl)# 10 permit ipv4 any any fragments
Router(config-ipv4-acl)# commit

Associated Commands

Associated Topics

Configuring an IPv4 ACL to Match on Fragment Type

Most DoS (Denial of Service) attacks work by flooding the network with fragmented packets. By filtering the incoming fragments of the packet in a network, an extra layer of protection can be added against such attacks.

You can configure an IPv4 ACL to match on the fragment type, and perform an appropriate action. You can use the following sample configuration with the different fragment options: 


/* Enter the global configuraton mode and configure an IPv4 access list */
Router# config
Router(config)# ipv4 access-list TEST
Router(config-ipv4-acl)# 10 permit tcp any any
 

/* Configure an ACE to match on the is-fragment flag (indicates a fragmented packet)
 and forward the packet to a next hop of 10.10.10.1  */
Router(config-ipv4-acl)# 30 permit udp any any fragment-type is-fragment nexthop1 ipv4 10.10.10.1

/* Configure an ACE to match on the first-fragment flag (indicates the first fragment of a fragmented packet)
 and forward the packet to a next hop of 20.20.20.1  */
Router(config-ipv4-acl)# 40 permit ospf any any fragment-type first-fragment nexthop1 ipv4 20.20.20.1


/* Configure an ACE to match on the last-fragment flag (indicates the last fragment of a fragmented packet)
 and forward the packet to a next hop of 30.30.30.1  */
Router(config-ipv4-acl)# 50 permit icmp any any fragment-type last-fragment nexthop1 ipv4 30.30.30.1
Router(config-ipv4-acl)# commit

Use Case: Configuring an IPv4 ACL to Match on the First Fragment and Last Fragment

This section describes an use case, where you configure an ACL to forward a fragment if it is the first fragment of the packet and discard a fragment if it is the last fragment of the packet.

In this configuration, the ACL checks the fragment offset value ( '0' for the first fragment). If the fragment is the first fragment of the packet, the packet is forwarded. If the fragment is the last fragment of the packet, it is dropped at the interface. 

/* Enter the global configuraton mode and configure an IPv4 access list */
Router# config
Thu Jan 11 11:56:27.221 IST
Router(config)# ipv4 access-list ACLFIRSTFRAG

/* Configure an ACE to match on the first fragment. 
If the fragment offset value equals 0, the fragment is forwarded to the 192.168.1.2 next hop */
Router(config-ipv4-acl)# 10 permit tcp any any fragment-type first-fragment nexthop1 ipv4 192.168.1.2

/* Configure an ACE to match on the last fragment, and drop the fragment at the interface. */
Router(config-ipv4-acl)# 20 deny tcp any any fragment-type last-fragment 
Router(config-ipv4-acl)# commit
Thu Jan 11 12:01:33.297 IST

/* Validate the configuration */
Router(config-ipv4-acl)# do show access-lists 
Thu Jan 11 12:05:23.646 IST
ipv4 access-list ACLFIRSTFRAG
 10 permit tcp any any fragment-type first-fragment nexthop1 ipv4 192.168.1.20
 20 deny tcp any any fragment-type last-fragment

You have successfully configured an IPv4 ACL to match on the fragment type.

Associated Commands

Configuring ACL Filtering by IP Packet Length

You can configure an access control list to filter packets by the packet length at an ingress interface. Depending on whether a packet matches the packet-length condition in a permit or deny statement, the packet is either processed or dropped respectively at the interface.

To configure packet length filtering in ACL, use the packet-length option in permit or deny command in IPv4 or IPv6 access-list configuration mode.

Restrictions

Packet length filtering feature in ACL is subjected to these restrictions:

  • Packet length filtering is supported only on ingress direction, for both traditional (non-compression) and hybrid (compression) ACLs.

  • IPv6 packet length filtering is supported only for hybrid ACLs; not for traditional ACLs.

  • Only quantized (value divisible by 16) packet length filtering is supported for traditional ACLs on IPv4.

  • Packet length filtering is not supported in the default TCAM key, but instead requires a User-Defined TCAM Key (UDK) that can be specified using the hw-module profile tcam format command as described in the configuration section.

Associated Commands

Configuring Simple IPv4 ACLs to Filter by Packet Length

To configure a simple ACL to filter by packet length in IPv4 networks, use the following steps.

  1. Enable packet length filtering in the global configuration mode by using the hw-module command. 

    
Router# config
Router(/config)# hw-module profile tcam format access-list ipv4 dst-addr dst-port proto packet-length frag-bit port-range

  2. Enter the global configuration mode and configure a simple IPv4 access list to filter packets by the packet length value.

    In this particular example, we configure a set of statements to process only those packets that match the specified packet length condition. All other packets are dropped when this ACL is applied to an ingress interface. 

    
Router# config
Router(config)# ipv4 access-list pktlen-v4
Router(config-ipv4-acl)# 10 permit tcp any any packet-length eq 1664
Router(config-ipv4-acl)# 20 permit udp any any packet-length range 1600 2000
Router(config-ipv4-acl)# 30 deny ipv4 any any

  3. Commit the ACL and exit the IPv4 ACL configuration mode. 

    
Router(config-ipv4-acl)# commit
Router(config-ipv4-acl)# end

  4. Apply the ACL to the required Ethernet interface. 

    
Router(config)# interface Te0/0/0/0
Router(config-if)# ipv4 access-group pktlen-v4 ingress

  5. Commit the configuration and exit the interface configuration mode. 

    
Router(config-if)# commit
Router(config-if)# end

  6. Verify your configuration. 

    Router# show access-lists pktlen-v4


ipv4 access-list pktlen-v4
10 permit ipv4 host 10.0.0.10 any packet-length lt 1008
20 permit ipv4 host 10.0.0.9 any packet-length gt 992

  7. Verify the ACL matches in hardware. 

    Router# show access-lists pktlen-v4 hardware ingress location 0/RP0/CPU0

ipv4 access-list pklen-v4
10 permit ipv4 host 10.0.0.10 any packet-length lt 1008
20 permit ipv4 host 10.0.0.9 any packet-length gt 992

You have successfully configured a simple IPv4 ACL to filter by packet length.

Configuring Scaled IPv6 ACLs to Filter by Packet Length

To configure a scaled ACL to filter by packet length in IPv6 networks, use the following steps.

  1. Enable packet length filtering in the global configuration mode by using the hw-module command. 

    
Router# config
Router(/config)# hw-module profile tcam format access-list ipv4 dst-addr dst-port proto packet-length frag-bit port-range

  2. Enter the global configuration mode and create an object group for configuring a scaled ACL. 

    
Router(config)# object-group  network ipv6 netobject2
Router(config-object-group-ipv6)# 2001::0/128
Router(config-object-group-ipv6)# commit

  3. From the global configuration mode, configure a scaled IPv6 access list to filter packets by the packet length value.

    In this particular example, we configure a statement to process only those packets that match the specified packet length condition. All other packets are dropped when this ACL is applied to an ingress interface. 

    
Router(config)# ipv6 access-list scaled_acl2
Router(config-ipv6-acl)# 10 permit ipv6 net-group netobject2 any packet-length eq 1000 
Router(config-ipv6-acl)# commit

  4. Commit the ACL and exit the IPv6 ACL configuration mode. 

    Router(config-ipv6-acl)# commit
Router(config-ipv6-acl)# end

  5. Apply the ACL to the required Gigabit Ethernet interface. 

    Router# config
Router(config)# interface Te/0/0/0/3
Router(config-if)# ipv6 access-group scaled_acl2 ingress

  6. Commit the configuration and exit the interface configuration mode. 

    Router(config-if)# commit
Router(config-if)# end

  7. Verify your configuration. 

    Router# show access-lists ipv6 scaled_acl2
ipv6 access-list scaled_acl2
10 permit ipv6 net-group netobject2 any packet-length eq 1000

  8. Verify the ACL matches in hardware. 

    Router# show access-lists ipv6 scaled_acl2 hardware ingress location 0/0/CPU0
ipv6 access-list scaled_acl2
10 permit ipv6 net-group netobject2 any packet-length eq 1000 (2000 hw matches)

You have successfully configured a scaled IPv6 ACL to filter by packet length.

Understanding Object-Group ACLs

You can use object-group ACLs to classify users, devices, or protocols into groups so you can have a group-level access control policy. Instead of specifying individual IP addresses, protocols, and port numbers in multiple ACEs, you can specify just the object group in a single ACL.

This feature is very beneficial in large scale networks which currently contain hundreds of ACLs. By using the object-group ACL feature, the number of ACEs per ACL are significantly reduced. Object-group ACLs are also more readable, and easier to manage than conventional ACLs. Using object-group ACLs instead of conventional ACLs optimizes the storage needed in TCAM.

Types of Object-Group ACLs

You can create two types of object-group ACLs on Cisco IOS XR:

  • Network object-group ACLs: Consist of groups of host IP Addresses and network IP addresses.

  • Port object-group ACLs: Consist of groups of ports and supporting Layer 3/Layer 4 protocols.

Compressing ACLs

Object-group ACLs use compression to accommodate the large number of ACEs. Compression is achieved by compressing the following three fields of an ACE:

  • Source IP prefix

  • Destination IP prefix

  • Source port number

From IOS XR Release 7.7.1 onwards, you can enable egress, which allows you to separate address prefixes and ports into network object groups and port object groups respectively.

There are only two compression levels in the access-group configuration for an ACL on an ingress interface:

  • Compress level 0: No compression is done on the ACE fields.

    In this mode, the object-group ACL behaves like a traditional ACL. Internal TCAM resources are utilized and there will be a huge impact on system resources and time taken for processing the ACL.

  • Compress level 3: All three fields (source IP, destination IP, and source port) in an ACE are compressed.

    In this mode, external TCAM is used for prefix lookup, and internal TCAM is used for ACE lookup. This mode supports 16-bit based packet length filtering and fragment offset filtering.


Note

Compress level 3 is not supported in the Cisco NCS 540 Series Routers because they do not have external TCAMs.

Configuring an Object-Group ACL

Before You Begin

You must be aware of the following information that apply to object-group ACLs:

  • You can configure ACLs that contain both conventional and object-group ACEs.

  • You can modify the objects in an object group dynamically without redefining the object group or the ACE that references the object group.

  • You can configure an object-group ACL multiple times with a source group, or a destination group, or both source and destination groups.

Restrictions

Configuring object-group ACLs involves the following restrictions:

  • Object-group ACLs can only be configured to an interface. They cannot be used or referenced by applications like SSH, SNMP, NTP.

  • To delete an object-group, you must first delete it from all ACLs.

  • You cannot configure object-group ACLs along with QoS policies.

  • Object-group ACLs are not supported in any policy based configuration.

  • Object-group is not supported in common ACLs.

  • Nested object-groups are not supported from Release 6.2.1.

  • Any inline ACE update to an object group ACL clears complete stats of the ACL.

Configuring TTL Matching and Rewriting for IPv6 ACLs

You can configure ACLs to match on the TTL value specified in the IPv6 header. You can specify the TTL match condition to be based on a single value, or multiple values. You can also rewrite the TTL value in the IPv6 header by using the set ttl command.


Note

A reboot of the line cards is required after entering the hw-module profile command to activate the command.

Limitations for using TTL matching and rewriting for IPv6 ACLs

Using TTL matching and rewriting for IPv6 ACLs is known to have the following limitations.

  • TTL matching is supported only for ingress ACLs.

  • ACL logging is not supported for ingress ACLs after a User-Defined TCAM Key (UDK) is configured with the enable-set-ttl option.

  • If a TTL rewrite is applied to the outer IPv6 header of an IP-in-IP header, then when the outer IPv6 header is decapsulated, (by GRE decapsulation) the TTL rewrite is also applied to the inner IPv6 header.

  • TTL matching is not supported in the default TCAM key, but instead requires a User-Defined TCAM Key (UDK) using the hw-module profile tcam format command as described in the Configuration section.

Configuration

Use the following steps to configure TTL matching and rewriting for IPv6 ACLs.

/* Enable TTL matching and rewriting in the global configuration mode by using the hw-module command */
Router(config)# hw-module profile tcam format access-list ipv6 dst-addr dst-port src-port next-hdr enable-set-ttl ttl-match

/* Configure an IPv6 ACL with the TTL parameters */
Router(config)# ipv6 access-list acl-v6
Router(config-ipv6-acl)# 10 deny tcp any any ttl eq 50
Router(config-ipv6-acl)# 20 permit tcp any any ttl lt 50 set ttl 255
Router(config-ipv6-acl)# 30 permit tcp any any ttl gt 50 set ttl 200
Router(config-ipv6-acl)# commit
Thu Nov  2 12:22:58.948 IST

/* Attach the IPv6 ACL to the GigE interface */
Router(config)# interface Te0/0/0/0
Router(config-if)# ipv6 address 2001:2:1::1/64
Router(config-if)# ipv6 access-group acl-v6 ingress
Router(config-if)# commit

Running Configuration

Validate your configuration by using the show run command. 

Router(config)# show run
Thu Nov  2 14:01:53.376 IST
Building configuration...
!! IOS XR Configuration 0.0.0
!! Last configuration change at Thu Nov  2 12:22:59 2017 by annseque
!hw-module profile tcam format access-list ipv6 dst-addr dst-port src-port next-hdr enable-set-ttl ttl-match
!
ipv6 access-list acl-v6
 10 deny tcp any any ttl eq 50
20 permit tcp any any ttl lt 50 set ttl 255
30 permit tcp any any ttl gt 50 set ttl 200
!
interface Te0/0/0/0
 ipv6 address 2001:2:1::1/64
 ipv6 access-group acl-v6 ingress
!

You have successfully configured TTL matching and rewriting for IPv6 ACLs.

Understanding IP Access List Logging Messages

Cisco IOS XR software can provide logging messages about packets permitted or denied by a standard IP access list. That is, any packet that matches the access list causes an informational logging message about the packet to be sent to the console. The level of messages logged to the console is controlled by the logging console command in global configuration mode.


Note

ACL logging isn’t supported for ingress MPLS packets with the explicit-null or deaggregation label.

Note

ACL logging isn’t supported for ingress MPLS packets

The first packet that triggers the access list causes an immediate logging message, and subsequent packets are collected over 5-minute intervals before they are displayed or logged.

However, you can use the { ipv4 | ipv6 } access-list log-update threshold command to set the number of packets that, when they match an access list (and are permitted or denied), cause the system to generate a log message. You might do this to receive log messages more frequently than at 5-minute intervals.


Caution

If you set the update-number argument to 1, a log message is sent right away, rather than caching it; every packet that matches an access list causes a log message. A setting of 1 isn’t recommended because the volume of log messages could overwhelm the system.

Even if you use the { ipv4 | ipv6} access-list log-update threshold command, the 5-minute timer remains in effect, so each cache is emptied at the end of 5 minutes, regardless of the number of messages in each cache. Regardless of when the log message is sent, the cache is flushed and the count reset to 0 for that message the same way it’s when a threshold isn’t specified.


Note

The logging facility might drop some logging message packets if there are too many to be handled or if more than one logging message is handled in 1 second. This behavior prevents the router from using excessive CPU cycles because of too many logging packets. Therefore, the logging facility shouldn’t be used as a billing tool or as an accurate source of the number of matches to an access list.

Enable Logging on ACE

This section shows you how to enable the ACE of an ACL to log informational messages when it matches incoming packets, using the optional keyword log . The router supports this feature only for IPv4 or IPv6 ingress ACLs. The logging message includes the access list number, whether the packet was permitted or denied, the source IP address of the packet, and the number of packets from that source permitted or denied in the prior 5-minute interval. 

Router#configure
Router(config)#ipv4 access-list test
Router(config-ipv4-acl)#10 permit udp 10.85.1.0 255.255.255.0 log
Router(config-ipv4-acl)#exit
Router(config)# interface FortyGigE0/0/0/22
Router(config-if)# ipv4 access-group test ingress
Router(config-if)# commit

Note

Set log-level to informational or higher with the logging console command, so that the router displays the ACL log-messages on the console. 

Router#configure
Router(config)#logging console informational
Router(config)# commit

For more information on log-levels, see section Syslog Message Severity Levels in the Implementing System Logging chapter of the System Monitoring Configuration Guide.

The following snippet shows a sample log message:

Router: ipv4_acl_mgr[350]: %ACL-IPV4_ACL-6-IPACCESSLOGP : access-list test (10) permit udp 
10.85.1.2(0) -> 10.0.0.1(0), 1 packet

Understanding Prefix Lists

Prefix lists are used in route maps and route filtering operations and can be used as an alternative to access lists in many Border Gateway Protocol (BGP) route filtering commands. A prefix is a portion of an IP address, starting from the far left bit of the far left octet. By specifying exactly how many bits of an address belong to a prefix, you can then use prefixes to aggregate addresses and perform some function on them, such as redistribution (filter routing updates).

BGP Filtering Using Prefix Lists

Prefix lists can be used as an alternative to access lists in many BGP route filtering commands. It is configured under the Global configurations of the BGP protocol. The advantages of using prefix lists are as follows:

  • Significant performance improvement in loading and route lookup of large lists.

  • Incremental updates are supported.

  • More user friendly CLI. The CLI for using access lists to filter BGP updates is difficult to understand and use because it uses the packet filtering format.

  • Greater flexibility.

Before using a prefix list in a command, you must set up a prefix list, and you may want to assign sequence numbers to the entries in the prefix list.

How the System Filters Traffic by Prefix List

Filtering by prefix list involves matching the prefixes of routes with those listed in the prefix list. When there is a match, the route is used. More specifically, whether a prefix is permitted or denied is based upon the following rules:

  • An empty prefix list permits all prefixes.

  • An implicit deny is assumed if a given prefix does not match any entries of a prefix list.

  • When multiple entries of a prefix list match a given prefix, the longest, most specific match is chosen.

Sequence numbers are generated automatically unless you disable this automatic generation. If you disable the automatic generation of sequence numbers, you must specify the sequence number for each entry using the sequence-number argument of the permit and deny commands in IPv4 prefix list configuration command. Use the no form of the permit or deny command with the sequence-number argument to remove a prefix-list entry.

The show commands include the sequence numbers in their output.

Configuring Prefix Lists

Configuration Example

Creates a prefix-list "pfx_2" with a remark "Deny all routes with a prefix of 10/8". This prefix-list denies all prefixes matching /24 in 128.0.0.0/8. 

Router#configure
Router(config)#ipv4 prefix-list pfx_2


Router(config-ipv4_pfx)#10 remark Deny all routes with a prefix of 10/8
Router(config-ipv4_pfx)#20 deny 128.0.0.0/8 eq 24
/* Repeat the above step as necessary. Use the no sequence-number command to delete an entry. */

Router(config-ipv4_pfx)#commit

Running Configuration 

Router#show running-config ipv4 prefix-list pfx_2  
ipv4 prefix-list pfx_2
 10 remark Deny all routes with a prefix of 10/8
 20 deny 128.0.0.0/8 eq 24
!

Verification

Verify that the permit and remark settings are according to the set configuration. 

Router# show prefix-list pfx_2
ipv4 prefix-list pfx_2
 10 remark Deny all routes with a prefix of 10/8
 20 deny 128.0.0.0/8 eq 24
RP/0/RP0/CPU0:ios#

Associated Commands

Sequencing Prefix List Entries and Revising the Prefix List

Configuration Example

Assigns sequence numbers to entries in a named prefix list and how to add or delete an entry to or from a prefix list. It is assumed a user wants to revise a prefix list. Resequencing a prefix list is optional.


Note

It is possible to resequence ACLs for prefix-list but not for security ACLs.

 
Router#config
Router(config)#ipv4 prefix-list cl_1 


Router(config)#10 permit 172.16.0.0 0.0.255.255
/* Repeat the above step as necessary adding statements by sequence number where you planned; use the no sequence-number command to delete an entry */

Router(config)#commit
end
Router#resequence prefix-list ipv4 cl_1 20 15
Running Configuration 
/*Before resequencing/*
Router#show running-config ipv4 prefix-list cl_1
ipv4 prefix-list cl_1
 10 permit 172.16.0.0/16
!
/* After resequencing using the resequence prefix-list ipv4 cl_1 20 15 command: */
Router#show running-config ipv4 prefix-list cl_1 
ipv4 prefix-list cl_1
 20 permit 172.16.0.0/16
!
Verification

Verify that the prefix list has been resequenced: 

Router#show prefix-list cl_1  
ipv4 prefix-list cl_1
 20 permit 172.16.0.0/16
Associated Commands