No Topology needed for this one, just wanted to jot some quick notes from a video I’m reviewing on the subject.

Before getting into what CEF and AT is all about, I feel its important this is required to be on and working for uRPF (Unicast Reverse Path Forwarding).

CEF is derived from the IP Route table, is also named FIB, and runs at the “Data Plane” while the IP Route table runs at the “Control Plane”.

One way two keep the two differentiated, is to remember the 3 letter acronym is the L3 information (CEF/FIB) and the L2 information has a 2 letter acronym (AT) Adjacency table.

“sh ip int …” to view CEF info for an interface, in fact adding the “ip” part to interface gives you basically the services statistics or information running on the interface.

“sh ip cef” to see the prefix-list for CEF

“sh adj” to view the adjacency table, “sh adj det” for more details

Like the CEF table is derived from the IP Route Table, the Adjacency Table is derived from ARP

If you see (incomplete) within the AT, you have an ARP issue somewhere in the network.

If you narrow down CEF to a single route, like “sh ip cef 172.12.123.1” and see the term invalid cached adjacency that is also an indication of a Layer 2 ARP issue.

Process Switching = Router looks at every packet to determine how to forward it.

Fast Switching = Router keeps a destination cache for packets, inspects the first packet that matches a destination, then allows the flow through without inspecting all packets.

Thats all I got for that, just a little nugget of straight to the point info, enjoy! 🙂