Content

| 5.2 |  | Cisco Express Forwarding |
| 5.2.5 |  | CEF adjacency table |

The adjacency table contents are fundamentally a function of the ARP process, whereby Layer 2 addresses are mapped to corresponding Layer 3 addresses. When the router issues an ARP request, a corresponding reply is received, and a host entry is added to the adjacency table to reflect this. In addition, the router can also glean next hop routers from routing updates and make entries in the adjacency table to reflect this. This lets the router build the next hop rewrite information necessary for Layer 3 packet forwarding. By having this data already stored in a table, CEF can perform highly efficient and consistent forwarding, because no discovery process is required. The command show ip cef is used to view the contents of the CEF adjacency table from the MSFC2. The command show ip cef summary gives a brief overview of the CEF process. It shows information such as the total number of adjacencies and routes.

The third table used by CEF is the NetFlow table. Because this table compiles network accounting data and does not play a role in the PDU header rewrite mechanism of CEF, it will not be discussed.