Adjacency Information

Switch#show adjacency gigabitethernet 9/5 detail Protocol Interface Address

IP GigabitEthernet9/5 06(11)

504 packets, 6110 bytes 00605C865B82 000164F83FA50800 ARP 03:49:31

Switch#show adjacency [{{type mod/port} | {port-channel number}} | detail | internal | summary]

Each time an adjacency entry is created, a Layer 2 data link layer header for that adjacent node is precomputed and stored in the adjacency table. This information is subsequently used for encapsulation during CEF switching of packets.

Output from the command show adjacency detail displays the content of the information to be used during this Layer 2 encapsulation. Verify that the header information is displayed as would be expected during Layer 2 operations, not using precomputed encapsulation from the adjacency table. Adjacency statistics are updated approximately every 60 seconds.

Also, the show cef drops command will display an indication of packets that are being dropped due to adjacencies that are either incomplete or nonexistent. There are two known reasons for incomplete or nonexistent adjacencies.

■ The router cannot use ARP successfully for the next-hop interface.

■ After a clear ip arp or a clear adjacency command, the router marks the adjacency as incomplete, and then it fails to clear the entry.

The symptoms of an incomplete adjacency include random packet drops during a ping test. Use the debug ip cef command to view CEF drops caused by an incomplete adjacency.

4-50 Building Cisco Multilayer Switched Networks (BCMSN) v3.0 © 2006 Cisco Systems, Inc.

Debugging CEF Operations

The debug facility can be used to display detailed information on CEF operations.

Debugging CEF Operations

Switch#debug ip cef {drops | access-list | receive | events | prefix-ipc | table}

• Displays debug information for CEF

Switch#debug ip cef {ipc | interface-ipc}


• Displays debug information related to IPC in CEF

Switch#ping ip


• Performs an extended ping

--— — "J

Use the debug ip cef arguments to limit the debug output, thereby reducing the overhead of the debug command and providing focus on a specific CEF operation.

debug ip cef {drops [access-list] | receive [access-list] | events [access-list] | prefix-ipc [access-list] | table [access-list]}

Adding an argument to the debug command limits the debug output as follows:

■ drops: Records dropped packets

■ access-list (optional): Controls collection of debugging information from specified lists

■ receive: Records packets that are not switched using information from the FIB table but that are received and sent to the next switching layer

■ events: Records general CEF events

■ prefix-ipc: Records updates related to IP prefix information, including these updates:

— Debugging of IP routing updates in a line card

— Reloading of a line card with a new table

— Notification that adding a route update from the route processor to the line card exceeds the maximum number of routes

— Control messages related to FIB table prefixes

■ table: Produces a table showing events related to the FIB table. Possible types of events include these types:

— Routing updates that populate the FIB table

— Flushing of the FIB table

— Adding or removing of entries to the FIB table

— Table reloading process

© 2006 Cisco Systems, Inc. Implementing Inter-VLAN Routing 4-51

Troubleshooting Layer 3 CEF-Based MLS

This topic describes the procedure to troubleshoot problems with CEF-based MLS.

How to Troubleshoot CEF

The CEF tables stored in hardware are populated from information that is 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 3 switching of packets.

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

■ Host 1 in VLAN10 with an IP address of

■ Host 2 in VLAN150 with an IP address of Step 1 Verify CEF.

Verify that CEF is operational at global or interface level using these commands: show ip cef summary show ip cef vlan 10

Note On most Cisco Catalyst platforms, CEF cannot be turned off. If CEF is not operational, it is likely that the Cisco Catalyst has disabled the feature. This may be due to a software, feature, or hardware incompatibility or due to inadequate memory to support a large FIB and adjacency table.

Step 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.

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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, VLAN199 is the transit path that will be traversed to arrive at subnet

Switch# show running-config interface VLAN 10

description Source VLAN

ip address

interface VLAN 199

description Transit VLAN

ip address

Step 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 (, use this command:

Switch# show ip route | include

O [110/2] via, 00:13:00, VLAN 199

Step 4 The network is accessible via next-hop address; therefore, the ARP entry by which to access should be the MAC address resolved for

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

Protocol Address Age Hardware Addr Type Interface

Internet 176 0030.7150.6800 ARPA VLAN 199

Step 5 Verify the CEF FIB table entry for the route.

Step 3 verified that a route to network 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, version 298, cached adjacency 0 packets, 0 bytes via, VLAN 199, 0 dependencies next-hop, VLAN 199 valid cached adjacency

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

© 2006 Cisco Systems, Inc. Implementing Inter-VLAN Routing 4-53

Step 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 by using this command:

Switch# show adjacency detail | begin

IP VLAN 199 0 packets, 0 bytes 003071506800

The preceding 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.

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

Step 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 an FIB entry for the specific network from the supervisor engine: Console> (enable) show mls entry cef ip

Mod FIB-Type Destination-IP Destination-Mask NextHop-IP Weight

15 resolved 1 To display an FIB entry for the specific network from the supervisor engine:

Console> (enable) show mls entry cef ip adjacency


Destination-IP : Destination-Mask : FIB-Type : resolved

AdjType NextHop-IP NextHop-Mac VLAN Encp TX-Packets connect 00-30-71-50-68-00 199 ARPA 0

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