Operation of CEF
When a packet enters the router, the router strips off the Layer 2 information. The router looks up the destination IP address in the CEF table (FIB), and it makes a forwarding decision. The result of this forwarding decision points to one adjacency entry in the adjacency table. The information retrieved from the adjacency table is the Layer 2 rewrite string, which enables the router to put a new Layer 2 header onto the frame, just before switching the packet out onto the outgoing interface toward the next hop. Figure 6-3 shows CEF forwarding an IP packet and the role of both the Layer 3 lookup of the destination IP address and the Layer 2 rewrite on the frame.
Figure 6-3. Layer 3 Lookup and Layer 2 Rewrite When Forwarding ...
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