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Basic IP Connectivity and Troubleshooting in Cisco Express Forwarding

Chapter Description

This chapter presents the general troubleshooting used on Cisco IOS routers and switches as a first step in troubleshooting IP connectivity problems. It helps you verify whether CEF is the root cause of a particular IP connectivity problem.

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Troubleshooting Commands Reference

Table 4-6 reviews the most important commands discussed and illustrated in this chapter.

Table 4-6. Important CEF Troubleshooting Commands

Command

Description

debug ip cef

Enables debugging with IP CEF from the Cisco IOS CLI. Because this debug is intrusive, the recommended procedure is to use this debug option with optional parameters including ACLs to limit output. Furthermore, you should use this debug command under the supervision of a Cisco TAC engineer.

show adjacency detail

Displays the IP CEF Layer 2 rewrite information and statistical information from the perspective of the software-switching path.

show arp

Displays the ARP table contents. This command is the first step in troubleshooting adjacency issues because the adjacency table is built from the ARP table.

show ip cef

Displays the IP CEF information from the software-switching path perspective. This command displays the prefix, next hop, and next-hop interface (outgoing interface) for verification. This command does not clearly illustrate the information contained through the hardware-switching path.

show ip route

Displays the IP routing table. This command is the first step in troubleshooting IP routing or IP CEF issues because the CEF tables are built from the IP routing table from a high-level perspective.

The following checklist reviews the important points of troubleshooting a software-based CEF issue:

  • Never assume that you are experiencing a Cisco IOS CEF issue; always troubleshoot from Layer 1 (physical layer) up.
  • In terms of troubleshooting CEF, verify the IP routing table and ARP table as a first step, because the CEF tables are built from those tables.
  • In Cisco IOS, use the show interface command to verify physical layer connectivity as the initial step in troubleshooting any IP connectivity issue.
  • Using the ICMP echo command ping from the Cisco IOS CLI always initiates echoes using the software-switching path.
  • After verifying the ARP and IP routing table in Cisco IOS, verify the software-switching path for CEF by verifying the rewrite and next-hop information from the show ip cef detail and show adjacency commands.
  • In controlled environments on select platforms that do not support dCEF, disabling CEF is an available troubleshooting option.
  • Most mid- to high-end routers and all Catalyst switches do not support disabling CEF because CEF is required for normal operation.
  • To determine why CEF is dropping ingress packets, use the show cef drop command.
  • When troubleshooting CEF punt adjacencies, use the show cef not-cef-switched command to determine which CEF drop category the drops fall into.