Deploying Cisco Express Forwarding – CEF-Based Multilayer Switching
How to troubleshoot Layer 3 connectivity in a CEF-based multilayer switch

The following items will be covered in this section.

  • Troubleshoot Host Connectivity Using CEF
  • Displaying Detailed Adjacency Information
  • Debugging CEF Operations

Troubleshoot Host Connectivity Using CEF
The CEF tables stored in hardware are populated from information gathered by the route processor. To properly troubleshoot CEF operations, first ensure that the normal Layer 3 operations on the route processor are functioning properly so that the CEF tables will be populated with accurate and complete information. Next, verify that information from the route processor has properly populated the FIB and adjacency table used by CEF to perform Layer switching of packets.

The steps below will verify if packet transfer between these hosts is occurring using CEF:

  • Host 1 in VLAN10 with an IP address of 192.168.10.10
  • Host 2 in VLAN150 with an IP address of 192.168.150.3

1. Verify CEF Operations
Verify that CEF is operational at global or interface level using these commands:

show ip cef summary
show ip cef vlan 10
NOTE:

CEF cannot be turned off on most Catalyst platforms. If CEF is not operational, it is likely the result of the Catalyst disabling the feature. This may be due to a software, feature or hardware incompatibility or due to inadequate memory to support large FIB and adjacency table.

2. Verify the Configuration
If CEF is not operational, display the running configuration to determine if any switching functions have been configured that might disable CEF operations.

If CEF is operational, display the running configuration to verify the IP configuration of the Layer 3 interfaces used for the hosts to communicate. The IP addresses should be appropriate for the subnet and the interfaces should not be shut down. This is a sample of the configuration output expected for the VLANs associated with the host communication. On this router VLAN 199 is the transit path that will be traversed to arrive at subnet 192.168.150.0:

Switch#show running-config

3. Verify Population of the Routing Table on the Route Processor
The routing protocols and route processor must populate the routing table accurately before those routing table entries can be of use as they are transferred to the FIB to facilitate Layer 3 switching. Verify the routing table by referring to a network diagram, knowing what routes should appear in the routing table, and then execute the show ip route command. In the case of troubleshooting connectivity to the specific network of the destination host, (192.168.150.3/24) use this command:

Switch#show ip route | include 192.168.150.0

The network is accessible via next hop address 192.168.199.3, therefore the ARP entry by which to access 192.168.150.3 should be the MAC address of resolved for 192.168.199.3.

4. Verify an ARP entry on the Route Processor
Verify that there is an ARP entry for the next hop IP address before checking if that entry is represented in the adjacency table:

Switch#show ip arp 192.168.199.3

5. Verify the CEF FIB Table Entry for the Route
Step 3 verified that a route to network 192.168.150.0 existed in the routing table. Now verify that a CEF FIB entry exists to that same destination to ensure that packets will be CEF switched, using the FIB, rather than process switched using the routing table:

Switch#show ip cef 192.168.150.0

This output verifies that there is a valid CEF entry for the destination network; packets can be CEF switched to the destination host.

6. Verify an Adjacency Table Entry for the Destination
Now verify that the FIB entry shown in Step 5 has an associated adjacency table entry using this command:

Switch#show adjacency detail | begin 192.168.199.3

The above output indicates that there is an adjacency for the next hop IP address. The destination MAC address (003071506800) is the MAC address in the ARP table as displayed in Step 4, above.

The counters (0 packets, 0 bytes) are almost always 0, as packets are switched in hardware and, as such, they never reach the route processor which is required to increment counters.

7. Verify CEF from the Supervisor Engine
The CEF FIB and adjacency table entries shown in the example can also be verified from the Supervisor Engine on modular switch platforms such as the 6500 series switches. This step is not necessary on fixed configuration switches such as the 3550.

To display a FIB entry for the specific network from the Supervisor Engine:

Console>(enable)show mls entry cef ip 192.168.150.0/24

To display a FIB entry for the specific network from the Supervisor Engine:

Console>(enable)show mls entry cef ip 192.168.150.0/24 adjacency