Understanding Cisco IOS ACL Support

This chapter describes Cisco IOS ACL support on the Catalyst 6500 series switches:

•Cisco IOS ACL Configuration Guidelines and Restrictions

•Hardware and Software ACL Support

•Configuring IPv6 Address Compression

•Optimized ACL Logging with a PFC3

•Guidelines and Restrictions for Using Layer 4 Operators in ACLs

Note For complete information about configuring Cisco IOS ACLs, see the Cisco IOS Security Configuration Guide, Release 12.2, "Traffic Filtering and Firewalls," at this URL:

http://www.cisco.com/en/US/docs/ios-xml/ios/sec_data_acl/configuration/12-2sx/sec-data-acl-12-2sx-book.html

Tip For additional information about Cisco Catalyst 6500 Series Switches (including configuration examples and troubleshooting information), see the documents listed on this page:

http://www.cisco.com/en/US/products/hw/switches/ps708/tsd_products_support_series_home.html

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Cisco IOS ACL Configuration Guidelines and Restrictions

The following guidelines and restrictions apply to Cisco IOS ACL configurations:

•You can apply Cisco IOS ACLs directly to Layer 3 ports and to VLAN interfaces.

•You can apply VLAN ACLs (VACLs) to VLANs (refer to Chapter 35 "Configuring VLAN ACLs").

•Each type of ACL (IP, IPX, and MAC) filters only traffic of the corresponding type. A Cisco IOS MAC ACL never matches IP or IPX traffic.

•The PFC does not provide hardware support for Cisco IOS IPX ACLs. Cisco IOS IPX ACLs are supported in software on the MSFC.

•By default, the MSFC sends Internet Control Message Protocol (ICMP) unreachable messages when a packet is denied by an access group.

With the ip unreachables command enabled (which is the default), the supervisor engine drops most of the denied packets in hardware and sends only a small number of packets to the MSFC to be dropped (10 packets per second, maximum), which generates ICMP-unreachable messages.

To eliminate the load imposed on the MSFC CPU by the task of dropping denied packets and generating ICMP-unreachable messages, you can enter the no ip unreachables interface configuration command to disable ICMP unreachable messages, which allows all access group-denied packets to be dropped in hardware.

•ICMP unreachable messages are not sent if a packet is denied by a VACL.

•We strongly recommend that you use named ACLs (rather than numbered ACLs) as this conserves CPU usage when creating or modifying ACL configurations and during system restarts. When you create ACL entries (or modify existing ACL entries), the software performs a CPU-intensive operation called an ACL merge to load the ACL configurations into the PFC hardware. An ACL merge also occurs when the startup configuration is applied during a system restart.

With named ACLs, the ACL Merge is triggered only when the user exits the named-acl configuration mode. However with numbered ACLs, the ACL Merge is triggered for every ACE definition and results in a number of intermediate merges during ACL configuration.

Hardware and Software ACL Support

Access control lists (ACLs) can be processed in hardware by the Policy Feature Card (PFC), a Distributed Forwarding Card (DFC), or in software by the Multilayer Switch Feature Card (MSFC). The following behavior describes software and hardware handling of ACLs:

•ACL flows that match a "deny" statement in standard and extended ACLs (input and output) are dropped in hardware if "ip unreachables" is disabled.

•ACL flows that match a "permit" statement in standard and extended ACLs (input and output) are processed in hardware.

•VLAN ACL (VACL) flows are processed in hardware. If a field specified in the VACL match clause statement is not supported by hardware processing the field is ignored (for example the log keyword in and ACL used for the match clause statement), or the whole configuration is rejected (for example, a VACL containing IPX ACL parameters). VACL logging is processed in software, using the action clause.

•VACL logging is processed in software.

•Dynamic ACL flows are processed in hardware.

•Idle timeout is processed in software.

Note Idle timeout is not configurable. Catalyst 6500 series switches do not support the access-enable host timeout command.

•Except on MPLS interfaces, reflexive ACL flows are processed in hardware after the first packet in a session is processed in software on the RP.

•IP accounting for an ACL access violation on a given port is supported by forwarding all denied packets for that port to the MSFC for software processing without impacting other flows.

•The PFC does not provide hardware support for Cisco IOS IPX ACLs. Cisco IOS IPX ACLs are supported in software on the MSFC.

•Extended name-based MAC address ACLs are supported in hardware.

•The following ACL types are processed in software:

–Internetwork Packet Exchange (IPX) access lists

–Standard XNS access list

–Extended XNS access list

–DECnet access list

–Extended MAC address access list

–Protocol type-code access list

Note IP packets with a header length of less than five will not be access controlled.

•Unless you configure optimized ACL logging (OAL), flows that require logging are processed in software without impacting nonlogged flow processing in hardware (see the "Optimized ACL Logging with a PFC3" section).

•The forwarding rate for software-processed flows is substantially less than for hardware-processed flows.

•When you enter the show ip access-list command, the match count displayed does not include packets processed in hardware.

•When you enter the show policy-map interface command, sometimes the counters that are displayed do not include all of the hardware switching platform counters.

Configuring IPv6 Address Compression

Access control lists (ACLs) are implemented in hardware in the Policy Feature Card (PFC), which uses the source or destination IP address and port number in the packet to index the ACL table. The index has a maximum address length of 128 bits.

The IP address field in an IPv6 packet is 128 bits, and the port field is 16 bits. To use full IPv6 addresses in the ACL hardware table, you can turn on compression of IPv6 addresses using the mls ipv6 acl compress address unicast command. This feature compresses the IPv6 address (including port) into 128 bits by removing 16 unused bits from the IPv6 address. Compressible address types can be compressed without losing any information. See Table 34-1 for details about the compression methods.

Release 12.2(17a)SX and later releases support the IPv6 compression command on the Supervisor Engine 720. The command is not supported on Supervisor Engine 2.

By default, the command is set for no compression.

Caution Do not enable the compression mode if you have noncompressible address types in your network. A list of compressible address types and the address compression method are listed in Table 34-1.

Table 34-1 Compressible Address Types and Methods 

Address Type
Compression Method

EUI-64 based on MAC address

This address is compressed by removing 16 bits from bit locations [39:24]. No information is lost when the hardware compresses these addresses.

Embedded IPv4 address

This address is compressed by removing the upper 16 bits. No information is lost when the hardware compresses these addresses.

Link Local

These addresses are compressed by removing the zeros in bits [95:80] and are identified using the same packet type as the embedded IPv4 address. No information is lost when the hardware compresses these addresses.

Others

If the IPv6 address does not fall into any of the above categories, it is classified as other. If the IPv6 address is classified as other, the following occurs:

•If the compress mode is on, the IPv6 address is compressed similarly to the EUI-64 compression method (removal of bits [39:24]) to allow for the Layer 4 port information to be used as part of the key used to look up the QoS TCAM, but Layer 3 information is lost.

•If the global compression mode is off, the entire 128 bits of the IPv6 address are used. The Layer 4 port information cannot be included in the key to look up the QoS TCAM because of the size constraints on the IPv6 lookup key.


To turn on the compression of IPv6 addresses, enter the mls ipv6 acl compress address unicast command. To turn off the compression of IPv6 addresses, enter the no form of this command.

This example shows how to turn on address compression for IPv6 addresses: 

Router(config)# mls ipv6 acl compress address unicast

Router(config)#

 
   
 
    
   
 This example shows how to turn off address compression for IPv6 addresses: 
 
    
   
 
    
Router(config)# no mls ipv6 acl compress address unicast

 
   
 
    
   
 
    
Router(config)#

 
   
 
     
    
    
   
 Optimized ACL Logging with a PFC3 
 
   
 
     
   
 
   
 
    
   
 Note Supervisor Engine 2 does not support optimized ACL logging (OAL). 
 
   
 
    
   
 Release 12.2(17d)SXB and later releases support OAL with a PFC3. These sections describe OAL: 
 
    
   
 •Understanding OAL 
 
    
   
 •OAL Guidelines and Restrictions 
 
    
   
 •Configuring OAL 
 
     
    
    
   
 Understanding OAL 
 
    
   
 Optimized ACL Logging (OAL) provides hardware support for ACL logging. Unless you configure OAL, packets that require logging are processed completely in software on the MSFC. OAL permits or drops packets in hardware on the PFC3 and uses an optimized routine to send information to the MSFC3 to generate the logging messages. 
 
     
    
    
   
 OAL Guidelines and Restrictions 
 
    
   
 The following guidelines and restrictions apply to OAL: 
 
    
   
 •OAL and VACL capture are incompatible. Do not configure both features on the switch. With OAL configured, use SPAN to capture traffic. 
 
    
   
 •OAL is supported only on the PFC3. 
 
    
   
 •OAL supports only IPv4 unicast packets. 
 
    
   
 •OAL supports VACL logging of permitted ingress traffic. 
 
    
   
 •OAL does not support port ACLs (PACLs). 
 
    
   
 •OAL does not provide hardware support for the following: 
 
    
   
 –Reflexive ACLs 
 
    
   
 –ACLs used to filter traffic for other features (for example, QoS) 
 
    
   
 –ACLs for unicast reverse path forwarding (uRPF) check exceptions 
 
    
   
 –Exception packets (for example, TTL failure and MTU failure) 
 
    
   
 –Packets with IP options 
 
    
   
 –Packets addressed at Layer 3 to the router 
 
    
   
 –Packets sent to the MSFC3 to generate ICMP unreachable messages 
 
    
   
 –Packets being processed by features not accelerated in hardware 
 
    
   
 •To provide OAL support for denied packets, enter the mls rate-limit unicast ip icmp unreachable acl-drop 0 command. 
 
    
    
   
 •OAL and the mls verify ip length minimum command are incompatible. Do not configure both. 
 
     
    
    
   
 Configuring OAL 
 
    
   
 These sections describe how to configure OAL: 
 
    
   
 •Configuring OAL Global Parameters 
 
    
   
 •Configuring OAL on an Interface 
 
    
   
 •Displaying OAL Information 
 
    
   
 •Clearing Cached OAL Entries 
 
   
 
     
   
 
   
 
    
   
 Note•For complete syntax and usage information for the commands used in this section, refer to the Cisco IOS Master Command List, Release 12.2SX. 
 
    
   
 •To provide OAL support for denied packets, enter the mls rate-limit unicast ip icmp unreachable acl-drop 0 command. 
 
   
 
    
     
    
    
   
 Configuring OAL Global Parameters 
 
    
   
 To configure global OAL parameters, perform this task: 
 
    
   
 
 
   
 
    
 
      
      
      
 
        
        

          Command 
        
 		 
        
        

          Purpose 
        
 		 
      
 
      
 
       
Router(config)# logging ip access-list cache {{entries number_of_entries} | {interval seconds} | {rate-limit number_of_packets} | {threshold number_of_packets}} 
 
       
 Sets OAL global parameters. 
 		 
      
 
      
 
       
Router(config)# no logging ip access-list cache {entries | interval | rate-limit | threshold} 
 
       
 Reverts OAL global parameters to defaults. 
 		 
      
 
      
    
 
   
 
   
 
   
 
 
    
   
 When configuring OAL global parameters, note the following information: 
 
    
   
 •entries number_of_entries: 
 
    
   
 –Sets the maximum number of entries cached. 
 
    
   
 –Range: 0-1,048,576 (entered without commas). 
 
    
   
 –Default: 8192. 
 
    
   
 •interval seconds: 
 
    
   
 –Sets the maximum time interval before an entry is sent to be logged. Also if the entry is inactive for this duration it is removed from the cache. 
 
    
   
 –Range: 5-86,400 (1440 minutes or 24 hours, entered without commas). 
 
    
   
 –Default: 300 seconds (5 minutes). 
 
    
   
 •rate-limit number_of_packets: 
 
    
   
 –Sets the number of packets logged per second in software. 
 
    
   
 –Range: 10-1,000,000 (entered without commas). 
 
    
   
 –Default: 0 (rate limiting is off and all packets are logged). 
 
    
   
 •threshold number_of_packets: 
 
    
   
 –Sets the number of packet matches before an entry is logged. 
 
    
   
 –Range: 1-1,000,000 (entered without commas). 
 
    
   
 –Default: 0 (logging is not triggered by the number of packet matches). 
 
     
    
    
   
 Configuring OAL on an Interface 
 
    
   
 To configure OAL on an interface, perform this task: 
 
    
   
 
 
   
 
    
 
      
      
      
 
         
        
        

          Command 
        
 		 
        
        

          Purpose 
        
 		 
      
 
      
 
       
 Step 1  
 		 
       
Router(config)# interface {{type1  slot/port} 
 
       
 Specifies the interface to configure. 
 		 
      
 
      
 
       
 Step 2  
 		 
       
Router(config-if)# logging ip access-list cache in 
 
       
 Enables OAL for ingress traffic on the interface. 
 		 
      
 
      
 
       
Router(config-if)# no logging ip access-list cache 
 
       
 Disables OAL on the interface. 
 		 
      
 
      
 
       
 Step 3  
 		 
       
Router(config-if)# logging ip access-list cache out 
 
       
 Enables OAL for egress traffic on the interface. 
 		 
      
 
      
 
       
Router(config-if)# no logging ip access-list cache 
 
       
 Disables OAL on the interface. 
 		 
      
 
      
    
 
    
 
      
      
 
       
 1 type = any that supports Layer 3-switched traffic. 
 		 
      
 
      
    
 
   
 
   
 
 
     
    
    
   
 Displaying OAL Information 
 
    
   
 To display OAL information, perform this task: 
 
    
   
 
 
   
 
    
 
      
      
      
 
        
        

          Command 
        
 		 
        
        

          Purpose 
        
 		 
      
 
      
 
        
        
 
         
Router # show logging ip access-list cache 

 
        
 
       
 Displays OAL information. 
 		 
      
 
      
    
 
   
 
   
 
   
 
 
     
    
    
   
 Clearing Cached OAL Entries 
 
    
   
 To clear cached OAL entries, perform this task: 
 
    
   
 
 
   
 
    
 
      
      
      
 
        
        

          Command 
        
 		 
        
        

          Purpose 
        
 		 
      
 
      
 
        
        
 
         
Router # clear logging ip access-list cache 

 
        
 
       
 Clears cached OAL entries. 
 		 
      
 
      
    
 
   
 
   
 
   
 
 
     
    
    
    
   
 Guidelines and Restrictions for Using Layer 4 Operators in ACLs 
 
    
   
 These sections describe guidelines and restrictions when configuring ACLs that include Layer 4 port operations: 
 
    
   
 •Determining Layer 4 Operation Usage 
 
    
   
 •Determining Logical Operation Unit Usage 
 
     
    
    
    
   
 Determining Layer 4 Operation Usage 
 
    
   
 You can specify these types of operations: 
 
    
   
 •gt (greater than) 
 
    
   
 •lt (less than) 
 
    
   
 •neq (not equal) 
 
    
   
 •eq (equal) 
 
    
   
 •range (inclusive range) 
 
    
   
 We recommend that you do not specify more than nine different operations on the same ACL. If you exceed this number, each new operation might cause the affected ACE to be translated into more than one ACE. 
 
    
   
 Use the following two guidelines to determine Layer 4 operation usage: 
 
    
   
 •Layer 4 operations are considered different if the operator or the operand differ. For example, in this ACL there are three different Layer 4 operations ("gt 10" and "gt 11" are considered two different Layer 4 operations): 
 
    
   
 
    
... gt 10 permit

 
   
 
    
   
 
    
... lt 9 deny

 
   
 
    
   
 
    
... gt 11 deny

 
   
 
   
 
     
   
 
   
 
    
   
 Note There is no limit to the use of "eq" operators as the "eq" operator does not use a logical operator unit (LOU) or a Layer 4 operation bit. See the "Determining Logical Operation Unit Usage" section for a description of LOUs. 
 
   
 
    
   
 •Layer 4 operations are considered different if the same operator/operand couple applies once to a source port and once to a destination port. For example, in this ACL there are two different Layer 4 operations because one ACE applies to the source port and one applies to the destination port. 
 
    
   
 
    
... Src gt 10 ...

 
   
 
    
   
 
    
... Dst gt 10

 
   
 
     
    
    
    
    
    
   
 Determining Logical Operation Unit Usage 
 
    
   
 Logical operation units (LOUs) are registers that store operator-operand couples. All ACLs use LOUs. There can be up to 32 LOUs; each LOU can store two different operator-operand couples with the exception of the range operator. LOU usage per Layer 4 operation is as follows: 
 
    
   
 •gt uses 1/2 LOU 
 
    
   
 •lt uses 1/2 LOU 
 
    
   
 •neq uses 1/2 LOU 
 
    
   
 •range uses 1 LOU 
 
    
   
 •eq does not require a LOU 
 
    
   
 For example, this ACL would use a single LOU to store two different operator-operand couples: 
 
    
   
 
    
... Src gt 10 ...

 
   
 
    
   
 
    
... Dst gt 10

 
   
 
    
   
 
    
 
   
 
    
   
 A more detailed example follows: 
 
    
   
 
    
ACL1

 
   
 
    
   
 
    
... (dst port) gt 10 permit

 
   
 
    
   
 
    
... (dst port) lt 9 deny

 
   
 
    
   
 
    
... (dst port) gt 11 deny

 
   
 
    
   
 
    
... (dst port) neq 6 permit

 
   
 
    
   
 
    
... (src port) neq 6 deny

 
   
 
    
   
 
    
... (dst port) gt 10 deny

 
   
 
    
   
 
    
 
   
 
    
   
 
    
ACL2

 
   
 
    
   
 
    
... (dst port) gt 20 deny

 
   
 
    
   
 
    
... (src port) lt 9 deny

 
   
 
    
   
 
    
... (src port) range 11 13 deny

 
   
 
    
   
 
    
... (dst port) neq 6 permit

 
   
 
    
   
 
    
 
   
 
    
   
 The Layer 4 operations and LOU usage is as follows: 
 
    
   
 •ACL1 Layer 4 operations: 5 
 
    
   
 •ACL2 Layer 4 operations: 4 
 
    
   
 •LOUs: 4 
 
    
   
 An explanation of the LOU usage follows: 
 
    
   
 •LOU 1 stores "gt 10" and "lt 9" 
 
    
   
 •LOU 2 stores "gt 11" and "neq 6" 
 
    
   
 •LOU 3 stores "gt 20" (with space for one more) 
 
    
   
 •LOU 4 stores "range 11 13" (range needs the entire LOU) 
 
   
 
     
   
 
   
 
    
   
 Tip For additional information about Cisco Catalyst 6500 Series Switches (including configuration examples and troubleshooting information), see the documents listed on this page: 
 
    
   
 http://www.cisco.com/en/US/products/hw/switches/ps708/tsd_products_support_series_home.html 
 
    
   
 Participate in the Technical Documentation Ideas forum