CEF Adjacency Table Entry

Previous post talks about the CEF FIB Table Entries which resides in the FIB table. As we know CEF is built of two tables one is FIB and another is Adjacency. We will talk more about the different kind of adjacencies available in the CEF table.

• Auto adjacencies actually contain a MAC header rewrite string, and can be used to forward traffic. These are either valid, meaning they are useable, or invalid, meaning the MAC header rewrite string isn’t available.

• Punt adjacencies indicate the packet should be queued to the next slower switching method (generally fast switching) because switching the packet requires a feature that isn’t supported in the CEF switching path. Generally, in newer code, punt adjacencies should be rare.

• The glean adjacency indicates the destination should be directly attached and reachable via a broadcast network, but there is no MAC header rewrite string available to forward traffic.

• Drop adjacencies indicate the packet cannot be CEF switched, but there is no alternate switching path to punt the packet.

• Discard adjacencies indicate the packet should be dropped because it is destined to a loopback address on the router. All the addresses that fall outside the loopback’s actual address (which will have a /32 receive FIB entry) will point to discard adjacencies.

• Any addresses which should be forwarded through a NULL interface on the router (NULL0), will point to the null adjacency.

• The FIB can cache a copy of the MAC header rewrite string directly in the FIB entry itself for faster lookups; load shared destinations cannot have their MAC header rewrite strings cached, so their adjacencies are marked uncached.

(Difference between Optimum, Fast and CEF Switching)

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