Verify the adjacency table

Once the FIB table has been checked, the next step in troubleshooting Cisco Express Forwarding lies with the rewrite information in the adjacency table.

After verifying the CEF FIB table, the next step in troubleshooting CEF is to verify the adjacency table. The adjacency table contains the rewrite information that CEF uses to switch packets. Verifying that the rewrite information is correct is an important step in troubleshooting CEF operation.

The four commands that provide different hierarchical levels of information for the adjacency table are show adjacency, show adjacency summary, show adjacency detail, and show adjacency internal. Example 4-20 shows examples of these commands, respectively.

Example 4-20 Viewing Adjacency Table Details

Table 4-3 describes the most significant fields from the show adjacency commands in Example 4-20 for the purpose of troubleshooting. Verifying the information against the show ip route and show arp commands is necessary in verifying CEF consistency. If the values are not correct, disable CEF as a workaround and open a Cisco TAC case.

Table 4-3 show adjacency Command Field Descriptions

Field Description The value in parentheses, 23, refers to the number of times a FIB
entry points to an adjacency entry (refCount). Numerous system
entries are not shown in the output of the show ip cef command. In
the hardware used for this example, a minimum of five references per
IP address existed. As a result, four additional FIB routing entries
point to the IP address.
The first 12 characters, 0008A37FCB7C, are the MAC address
associated with the destination next-hop interface (destination MAC
address rewrite). The next 12 characters represent the MAC address
of the source interface of the packet (source MAC address rewrite).
The last four characters represent the well-known Ethertype value
0x0800 for IP for Advanced Research Projects Agency (ARPA)
encapsulation, the default among Ethernet vendors.
ARP 04:02:58 The ARP value indicates that the entry was learned through the ARP
process. The timestamp indicates the time remaining before the entry
times out. The default ARP timeout is 4 hours.
Fast adjacency disabled An FIB entry caches an adjacency for a next-hop interface when not
doing load sharing over multiple active paths. A fast adjacency
increases the switching speed of packets.

In Figure 4-3 and Example 4-5, the destination MAC address rewrite information from the show adjacency detail command in Example 4-20, 0008A37FCB7C, must match the MAC address from the show arp command. Otherwise, if the MAC address did not match, an inconsistency issue exists between the ARP table and adjacency table that needs to be investigated with the Cisco TAC.

Learn how to troubleshoot Cisco's Express Forwarding network switching technology in "Basic IP Connectivity and CEF Troubleshooting," Chapter 4 from the book Cisco Express Forwarding by Nakia Stringfield, Russ White and Stacia McKee.

Basic IP Connectivity and CEF Troubleshooting

  Accurately describe the problem
  Scope the network topology
  Review the OSI model for troubleshooting
  Verify the ARP table
  Verify the IP routing table
  Verify the CEF FIB table
  Verify the adjacency table
  Conduct hardware-specific troubleshooting

Reproduced from the book Cisco Express Forwarding. Copyright 2007, Cisco Systems, Inc. Reproduced by permission of Pearson Education, Inc., 800 East 96th Street, Indianapolis, IN 46240. Written permission from Pearson Education, Inc. is required for all other uses.

Dig Deeper on Campus area networks and services