Chapter 4. Troubleshooting Routing Technologies

Network Layer Connectivity

Routers use three tables to make routing decisions: the routing table, ARP table, and CEF mappings.

The routing table is visible using show ip route. Each entry in the routing table has an output interface or next hop. Packets are routed per the routing table, matching the longest prefix match. If two identical routes are available, then routing is determined by IGP metrics.

When a determination of the next hop has been made, the router must turn this information into a destination Layer 2 address. For this purpose, mapping tables are maintained that match Layer 2 and Layer 3 addresses. The ARP table (show ip arp) is an example of this.

Cisco Express Forwarding (CEF) ...

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