To view the details of the IPv4 or IPv6 CEF tables, use the following commands:

• show cef {ipv4 address | ipv6 address} hardware egress

Displays the IPv4 or IPv6 CEF table. The next hop and forwarding interface are displayed for each prefix. The output of the show cef command varies by location.

Router# show cef 203.0.1.2 hardware egress 
  203.0.1.2/32, version 0, internal 0x1020001 0x0 (ptr 0x8d7db7f0) [1], 0x0 (0x8daeedf0), 0x0 (0x0)
 Updated Nov 20 13:33:23.557
 local adjacency 203.0.1.2
 Prefix Len 32, traffic index 0, Adjacency-prefix, precedence n/a, priority 15
   via 203.0.1.2/32, HundredGigE0/0/0/9, 3 dependencies, weight 0, class 0 [flags 0x0]
    path-idx 0 NHID 0x0 [0x8cfc81a0 0x0]
    next hop 203.0.1.2/32
    local adjacency

• show cef {ipv4 | ipv6} summary

Router#show cef ipv4 summary
Fri Nov 20 13:50:45.239 UTC

Router ID is 216.1.1.1

IP CEF with switching (Table Version 0) for node0_RP0_CPU0

  Load balancing: L4
  Tableid 0xe0000000 (0x8cf5b368), Vrfid 0x60000000, Vrid 0x20000000, Flags 0x1019
  Vrfname default, Refcount 4129
  56 routes, 0 protected, 0 reresolve, 0 unresolved (0 old, 0 new), 7616 bytes
  13 rib, 0 lsd, 0:27 aib, 1 internal, 10 interface, 4 special, 1 default routes
  56 load sharing elements, 24304 bytes, 1 references
  1 shared load sharing elements, 432 bytes
  55 exclusive load sharing elements, 23872 bytes
  0 route delete cache elements
  13 local route bufs received, 1 remote route bufs received,  0 mix bufs received
  13 local routes, 0 remote routes
  13 total local route updates processed
  0 total remote route updates processed
  0 pkts pre-routed to cust card
  0 pkts pre-routed to rp card
  0 pkts received from core card
  0 CEF route update drops, 0 revisions of existing leaves
  0 CEF route update drops due to version mis-match
  Resolution Timer: 15s
  0 prefixes modified in place
  0 deleted stale prefixes
  0 prefixes with label imposition, 0 prefixes with label information
  0 LISP EID prefixes, 0 merged, via 0 rlocs
 28 next hops
  1 incomplete next hop

 0 PD backwalks on LDIs with backup path

• show cef { ipv4 address | ipv6 address } detail

Displays the details of the IPv4 or IPv6 CEF table.

Router#show cef 203.0.1.2 detail 
203.0.1.2/32, version 0, internal 0x1020001 0x0 (ptr 0x8d7db7f0) [1], 0x0 (0x8daeedf0), 0x0 (0x0)
 Updated Nov 20 13:33:23.556
 local adjacency 203.0.1.2
 Prefix Len 32, traffic index 0, Adjacency-prefix, precedence n/a, priority 15
  gateway array (0x8d84beb0) reference count 1, flags 0x0, source aib (10), 0 backups
                [2 type 3 flags 0x8401 (0x8d99a598) ext 0x0 (0x0)]
  LW-LDI[type=3, refc=1, ptr=0x8daeedf0, sh-ldi=0x8d99a598]
  gateway array update type-time 1 Nov 20 13:33:23.556
 LDI Update time Nov 20 13:33:23.556
 LW-LDI-TS Nov 20 13:33:23.556
   via 203.0.1.2/32, HundredGigE0/0/0/9, 3 dependencies, weight 0, class 0 [flags 0x0]
    path-idx 0 NHID 0x0 [0x8cfc81a0 0x0]
    next hop 203.0.1.2/32
    local adjacency
        Load distribution: 0 (refcount 2)

    Hash  OK  Interface                 Address
    0     Y   HundredGigE0/0/0/9        203.0.1.2
    

• show adjacency detail

Displays detailed adjacency information, including Layer 2 information for each interface. The output of the show adjacency command varies by location.

Router#show adjacency detail

-------------------------------------------------------------------------------
0/RP0/CPU0
-------------------------------------------------------------------------------
Interface                   Address                  Version  Refcount Protocol
Hu0/9/0/0                  (interface)                   13        1(    0)
                           (interface entry)
                           mtu: 1500, flags 1 4


Hu0/9/0/1                  (interface)                   31        1(    0)
                           (interface entry)
                           mtu: 1500, flags 1 4

Configuration of Unicast IPv4 and IPv6 Reverse Path Forwarding (uRPF) enables a router to verify the reachability of the source address in packets being forwarded. Configuring uRPF, both strict and loose modes, helps to mitigate problems caused by the introduction of spoofed IP source addresses into a network. Configuration of uRPF discards IP packets that lack a verifiable IP source address after a reverse lookup in the CEF table.

When strict uRPF is enabled, the source address of the packet is checked in the FIB. If the packet is received on the same interface that would be used to forward the traffic to the source of the packet, the packet passes the check and is further processed. Otherwise, the packet is dropped. Configure strict uRPF only where there is natural or configured symmetry. Internal interfaces are likely to have a routing asymmetry, that is, multiple routes to the source of a packet. Therefore, you should not implement strict uRPF on interfaces that are internal to the network.

Implementation of strict mode uRPF requires maintenance of a uRPF interfaces list for the prefixes. The list contains only the interfaces configured with strict mode uRPF. The interfaces are provided by the prefix path. The uRPF interface list is shared among the prefixes wherever possible.

When loose uRPF is enabled, the source address of the packet is checked in the FIB. If the source address exists and matches a valid forwarding entry, the packet passes the check and is further processed. Otherwise, the packet is dropped.

Note

The behavior of strict uRPF varies slightly on the basis of platforms, the number of recursion levels, and the number of paths in Equal-Cost Multipath (ECMP) scenarios. A platform may switch to loose uRPF check for some or all prefixes, even though strict uRPF is configured. For example, if ECMP Path is eight or more, strict mode is converted to loose mode.

Loose and strict uRPF supports two options: allow self-ping and allow default. The allow self-ping option allows the source of the packet to ping itself. The allow default option allows the lookup result to match a default routing entry. When the allow default option is enabled with the strict mode of the uRPF, the packet is processed further only if it arrives through the default interface.

Restrictions

Consider the following restrictions when you configure uRPF:

• Global configuration followed by cold reload is required to enable or disable uRPF on the router.

• Configuration of uRPF per interface enables or disables uRPF mode in the hardware of the corresponding interface.

• Configuration of uRPF reduces the route scale to half. Half of the routing table is used for destination lookup and the other half is used for source lookup of received packets.

• Unicast RPF allows packets with 0.0.0.0 source addresses and 255.255.255.255 destination addresses to pass so that Bootstrap Protocol and Dynamic Host Configuration Protocol (DHCP) functions properly.

• Unicast RPF allows packets whose destination IP address is not a unicast address.

• The behavior of strict uRPF varies slightly on the basis of platforms, the number of recursion levels, and the number of paths in Equal-Cost Multipath (ECMP) scenarios. A platform may switch to loose uRPF check for some or all prefixes, even though strict uRPF is configured.

• The allow self-ping option is the default option in uRPF configuration for both strict and loose mode. You cannot disable the allow-self-ping option. The allow-default option needs to be configured specifically per interface.

In Figure 1, uRPF is enabled on Router R2 and R2 has the following FIB table entries:

• 10.1.1.0/24 [110/3] via 10.25.24.1, 2d08h, HundredGigE0/0/0/24

• 12.1.1.0/24 [110/3] via 10.25.24.1, 2d08h, HundredGigE0/0/0/25

• 14.0.5.0/24 [110/3] via 20.0.0.2, 2d08h, HundredGigE0/0/0/1

R2 allows the packet from Host 1 to be routed because Host 1 subnet is available in R2's FIB table. However, R2 does not allow Host 3 to be routed because Host 3 subnet is not available in R2's FIB table. If strict uRPF is enabled on R2, then the source address 10.1.1.1/24 should be reachable through the same interface from which it is received. If loose uRPF is enabled on R2, then it is not mandatory that the source address 10.1.1.1/24 be reachable through the same interface from which it is received. The only criteria for a packet to be forwarded is that the host address should be present in R2's FIB table.

Configuring Unicast Reverse Path Forwarding (uRPF) enables a router to verify the reachability of the source address of packets being forwarded. If the source IP address is not valid, the packet is discarded. This capability can limit the appearance of spoofed addresses in a network.

To configure uRPF on a hardware module, use the following steps:

1. Configure a hardware module in the global configuration mode and enable uRPF.

2. Reboot the router.

3. Configure an interface.

4. Configure IPv4 or IPv6 uRPF through strict or loose mode under the interface.

/* Configure a hardware module in the global configuration mode and enable uRPF. */
Router# configure
Router(config)# hw-module urpf enable
Mon Sep 17 09:34:00.945 UTC
In order to activate/deactivate urpf, you must manually reboot the box.
Router(config)# commit


/* Reboot the router manually. */

/* Configure an interface. */
Router(config)# interface BVI1
Router(config-ipv6-acl)# description BVI INTERFACE
Router(config-ipv6-acl)# commit

/* Configure IPv4 or IPv6 uRPF through strict or loose mode under the interface. */
Router(config)# ipv4 address 11.1.1.1 255.255.255.0
Router(config-pifib-policer-global)# ipv4 verify unicast source reachable-via rx
Router(config-pifib-policer-global)# commit

/* Configure a hardware module in the global configuration mode and enable uRPF. */
Router# configure
Router(config)# hw-module urpf enable
Mon Sep 17 09:34:00.945 UTC
In order to activate/deactivate urpf, you must manually reboot the box.
Router(config)# commit


/* Reboot the router manually. */

/* Configure an interface. */
Router(config)# interface BVI1
Router(config-ipv6-acl)# description BVI INTERFACE
Router(config-ipv6-acl)# commit

/* Configure IPv4 or IPv6 uRPF through strict or loose mode under the interface. */
Router(config)# ipv4 address 11.1.1.1 255.255.255.0
Router(config-pifib-policer-global)# ipv4 verify unicast source reachable-via any
Router(config-pifib-policer-global)# commit

Router#show prm server  profile-status software
Software PROFILE STATUS VARIABLES
****************************************
BGP-3107 LB             : 0
LB Seed                 : 0x65c5208d
LB Seed Cfg             : NO
LB Sub-Sel-Offset Cfg   : NO
LB SubSel               : Hash Field B1
LB Offset               : 13
SR-PE Mode              : 0
Current URPF            : YES
Configured URPF         : YES
Current HW Profile      : SP-Profile
Configured HW Profile   : SP-Profile
****************************************

The system inherently supports the 7-tuple hash algorithm. Load balancing describes the functionality in a router that distributes packets across multiple links based on Layer 3 (network layer) and Layer 4 (transport layer) routing information. If the router discovers multiple paths to a destination, the routing table is updated with multiple entries for that destination.

Per-flow load balancing performs these functions:

• Incoming data traffic is evenly distributed over multiple equal-cost connections.

• Incoming data traffic is evenly distributed over multiple equal-cost connections member links within a bundle interface.

• Layer 2 bundle and Layer 3 (network layer) load balancing decisions are taken on IPv4, IPv6, and MPLS flows . If it is an IPv4 or an IPv6 payload, then a 7-tuple hashing is done. If it is an MPLS payload with three or less labels, then the hardware parses the payload underneath and identifies whether the payload packet has an IPv4 or an IPv6 header. If it is an IPv4 or IPv6 header, then a 4-tuple hashing is performed based on the IP source, IP destination, router ID, and label stack; otherwise, an MPLS label based hashing is performed. In case of MPLS label-based hashing, the top 4 labels are used in hash computation. However, for Cisco NC57 line cards, all the labels are used for MPLS label-based hashing..

• A 7-tuple hash algorithm provides more granular load balancing and used for load balancing over multiple equal-cost Layer 3 (network layer) paths. The Layer 3 (network layer) path is on a physical interface or on a bundle interface. In addition, load balancing over member links can occur within a Layer 2 bundle interface.

• The 7-tuple load-balance hash calculation contains:

  • Source IP address

  • Destination IP address

  • IP Protocol type

  • Router ID

  • Source port

  • Destination port

  • Input interface

Note	Cisco NC57 line cards support 6-tuple load-balance hash calculation because input interface is not considered as a parameter for load-balance hash calculations.

Load balancing decisions are taken based on a packet header, type of load balancing, type of scenario and platform specifics as follows:

• Packet header can contain one or many MAC, MPLS, IPv4 or IPv6 address, TCPor UDP headers, and so on.

• Load balancing can be done during ECMP or LAG (Bundle-Ether) forwarding.

• Scenarios can include IP forwarding, IP tunnel forwarding or decapsulation, MPLS forwarding or disaggregation, or Ethernet forwarding.

• The chipset type contains a packet’s fields. These fields are considered for load balancing.

The following tables includes detailed list of options, list of scenarios, and headers fields to specify how ECMP or LAG load balancing is done.

Note:

• For Jericho/Qumran-AX/Qumran-MX line cards, the fields superscripted with * are used for load balancing through LAG only. For the Jericho+ line cards, the fields superscripted with * are used for load balancing through ECMP and LAG both. For example, in an MPLS forwarding scenario for Jericho line cards, for an MPLS packet with three labels, with an IPv4 or IPv6 header, and with L4 (TCP or UDP) ECMP load balancing is done based on:

  • All label values in the MPLS label stack

  • Source and destination IPv4 or IPv6 addresses

  However, for the same MPLS packet, LAG load balancing is done based on:

  • All label values in the MPLS label stack

  • Source and destination IPv4 or IPv6 addresses

  • L4 source and destination ports

• Only the fields that are highlighted in bold font are used for load balancing hash calculations. For example, for IP forwarding for IPv4 or IPv6 header and L4 (TCP or UDP) header, ECMP or LAG load balancing is done based on:

  • Source and destination IPv4 or IPv6 addresses

  • L4 source and destination ports

Note	For MPLS packets, up to four labels are used for hash calculation.

Note	For MPLS packets, up to four labels are used for hash calculation.

Note	For MPLS packets with multiple labels, hash calculation is done based on the first five labels along other headers.

Note	For MPLS packets with one through three labels, only the first label is used for load balancing along with other headers.

Note	For MPLS packets with multiple labels, hash calculation is done based on first five labels along with other headers.

Routers forward packets using either route information from route table entries that you manually configure or the route information that is calculated using dynamic routing algorithms. Static routes, which define explicit paths between two routers, cannot be automatically updated; you must manually reconfigure static routes when network changes occur. Static routes use less bandwidth than dynamic routes. Use static routes where network traffic is predictable and where the network design is simple. You should not use static routes in large, constantly changing networks because static routes cannot react to network changes. Most networks use dynamic routes to communicate between routers but might have one or two static routes configured for special cases. Static routes are also useful for specifying a gateway of last resort (a default router to which all unroutable packets are sent).

Configuration Example

Create a static route between Router A and B over a HundredGigE interface. The destination IP address is 203.0.1.2/32 and the next hop address is 1.0.0.2.

Router(config)#router static address-family ipv4 unicast 
Router(config-static-afi)#203.0.1.2/32 HundredGigE 0/0/0/9 1.0.0.2
Router(config-static-afi)#commit

Router#show running-config router static address-family ipv4 unicast
router static
 address-family ipv4 unicast
 203.0.1.2/32 HundredGigE 0/0/0/9 1.0.0.2
 !
!

Router#show cef 203.0.1.2/32 hardware egress details location 0/0/CPU0
Wed Nov  6 10:09:23.548 UTC
111.0.0.1/32, version 221, attached, internal 0x1000041 0x0 (ptr 0x8b00ea80) [1], 0x0 (0x8afd9768), 0x0 (0x0)
Updated Nov  6 10:08:07.424
Prefix Len 32, traffic index 0, precedence n/a, priority 2
  gateway array (0x8ae4baf0) reference count 1, flags 0x0, source rib (7), 0 backups
                [2 type 3 flags 0x40008441 (0x8af020c0) ext 0x0 (0x0)]
  LW-LDI[type=3, refc=1, ptr=0x8afd9768, sh-ldi=0x8af020c0]
  gateway array update type-time 1 Nov  6 10:08:07.423
LDI Update time Nov  6 10:08:07.423
LW-LDI-TS Nov  6 10:08:07.424
   via tunnel-ip1, 0 dependencies, recursive [flags 0x8]
    path-idx 0 NHID 0x0 [0x8ae0d728 0x0]
    local adjacency
 
 

The BGP Attributes Download feature enables you to display the installed BGP attributes in CEF.

• The show cef bgp-attribute command displays the installed BGP attributes in CEF.

• The show cef bgp-attribute attribute-id command and the show cef bgp-attribute local-attribute-id command are used to view the specific BGP attributes by attribute ID and local attribute ID.

Router# show cef bgp-attribute
Router ID is 216.1.1.1

IP CEF with switching (Table Version 0) for node0_RP0_CPU0

  Load balancing: L4
  Tableid 0xe0000000 (0x8cf5b368), Vrfid 0x60000000, Vrid 0x20000000, Flags 0x1019
  Vrfname default, Refcount 4129
  56 routes, 0 protected, 0 reresolve, 0 unresolved (0 old, 0 new), 7616 bytes
  13 rib, 0 lsd, 0:27 aib, 1 internal, 10 interface, 4 special, 1 default routes
  56 load sharing elements, 24304 bytes, 1 references
  1 shared load sharing elements, 432 bytes
  55 exclusive load sharing elements, 23872 bytes
  0 route delete cache elements
  13 local route bufs received, 1 remote route bufs received,  0 mix bufs received
  13 local routes, 0 remote routes
  13 total local route updates processed
  0 total remote route updates processed
  0 pkts pre-routed to cust card
  0 pkts pre-routed to rp card
  0 pkts received from core card
  0 CEF route update drops, 0 revisions of existing leaves
  0 CEF route update drops due to version mis-match
  Resolution Timer: 15s
  0 prefixes modified in place
  0 deleted stale prefixes
  0 prefixes with label imposition, 0 prefixes with label information
  0 LISP EID prefixes, 0 merged, via 0 rlocs
 28 next hops
  1 incomplete next hop

 0 PD backwalks on LDIs with backup path


VRF: default
_____________
Table ID: 0xe0000000. Total number of entries: 0
OOR state: GREEN. Number of OOR attributes: 0

When CEF installs a route for which there is no layer 2 adjacency information, CEF creates an incomplete layer 3 next-hop and programs it on the hardware. Because of this incomplete programming, the first packet will be forwarded to the software forwarding path. The software forwarding in turn strips off the layer 2 header from the packet and forwards it to ARP (Address Resolution Protocol) or ND (Neighbor Discovery) in order to resolve the layer 2 adjacency information. In such a packet, if there is feature specific information present in the layer 2 header, the software forwarding path fails to strip off the layer 2 header completely and thus ARP or ND is unable to resolve the missing layer 2 adjacency information and thereby this results in traffic being dropped.

Proactive ARP and ND feature solves the above problem by ensuring that CEF proactively triggers ARP or ND in order to resolve the missing layer 2 adjacency information, retrying every 15 seconds until the next-hop information is resolved. Thus, when you configure a static route which has an incomplete next-hop information, this feature automatically triggers ARP or ND resolution.

/* Enter the configuration mode and configure Proactive ARP/ND */
Router# configure
Router(config)# cef proactive-arp-nd enable
Router(config)# commit

Show running-config
cef proactive-arp-nd enable
end