Which of the following statements best describes the purpose of ARP with respect to CEF?
- ARP is used to build the FIB.
- ARP is used to reindex the routing table.
- ARP is used to build the adjacency table.
- ARP is used to decrease the amount of time spent searching for an entry within a routing table.
Address Resolution Protocol (ARP) is used by Cisco Express Forwarding (CEF) to build the adjacency table. CEF is the switching method used by Catalyst switches. Unlike traditional multilayer switching (MLS), which merely caches Layer 3 information received when traffic passes through a switch, CEF attempts to optimize the routing process by reindexing the routing table and then building an adjacency table based on the routing table information. The type of MLS performed by CEF is called topology-based switching; traditional MLS is known as route caching, demand-based switching, and flow-based switching.
The routing table is reindexed by using a binary search method. The reindexed routing table is called the forwarding information base (FIB). Reindexing the routing table reduces the amount of time spent searching for an entry within a routing table.
After the FIB is created, an adjacency table is created to map the appropriate Layer 2 next-hop address or addresses to each FIB entry. ARP is used to retrieve the Layer 2 address information. If multiple Layer 2 next-hop addresses are available for an entry in the FIB, then CEF can employ load balancing for packets headed to that destination.
The final result is a single database of routing information (FIB) is built for the switching hardware.
Two extremely useful commands for verifying CEF are:
- show ip cef network address – displays entries in the forwarding information base (FIB)
- show adjacency detail | begin adjacency address – shows information about a specific adjacency in the adjacency table
Both commands are shown below with explanations.
SwitchA# show ip cef 192.168.6.0 192.168.6.0/24, version 302, cached adjacency 192.168.166.5, 0 packets, 0 bytes Via 192.168.166.5, VLAN 185, 0 dependencies Next-hop 192.168.166.5, VLAN 185 Valid cached adjacency
Above it can be determined that there is a valid CEF entry for the destination network 192.168.6.0 and that there is a valid cached adjacency to the 192.168.166.5 next hop IP address.
In the command output below, it can be determined that 005565946856 is the MAC address of the 192.168.166.5 next-hop address:
SwitchA# show adjacency detail | begin 192.168.166.5
IP VLAN 185 192.168.166.5(6) 0 packets, 0 bytes
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