Debug vlan packets

The debug vlan packet command displays messages about virtual LAN (VLAN) packets that the router receives (off the trunk connection) but is not configured to support (see Example 9-6). In other words, if an ISL packet with a VLAN ID of 6 is received, but none of the subinterfaces of the input interface are configured for VLAN 6, the router cannot process the encapsulated frame and the debug process displays a message indicating what has just happened. As mentioned earlier, you may use the show vlans command to see the list of all the VLANs configured on your router.

Example 9-6 A Sample Output of the debug vlan packets Command D_BackR#debug vlan packets

Virtual LAN packet information debugging is on D_BackR#

05:20:45: vLAN: Received ISL encapsulated UNKNOWN packet bearing colour ID 100 on interface FastEthernet0.100 which is not configured to route or bridge this packet type.

05:20:45: vLAN: Received ISL encapsulated UNKNOWN packet bearing colour ID 3 on interface FastEthernet0.3 which is not configured to route or bridge this packet type.

05:20:45: vLAN: ISL packet received bearing colour ID 4 on FastEthernet0 which has no subinterface configured to route or bridge ID 4.

05:20:45: vLAN: ISL packet received bearing colour ID 5 on FastEthernet0 which has no subinterface configured to route or bridge ID 5.

05:20:45: vLAN: ISL packet received bearing colour ID 6 on FastEthernet0 which has no subinterface configured to route or bridge ID 6.

05:20:45: vLAN: ISL packet received bearing colour ID 7 on FastEthernet0 which has no subinterface configured to route or bridge ID 7.

05:20:45: vLAN: Received ISL encapsulated UNKNOWN packet bearing colour ID 100 on interface FastEthernet0.100 which is not configured to route or bridge this packet type.

The first entry in the output of Example 9-6 notifies you that an ISL frame was received from the fastethernet 0.100 subinterface. That ISL frame's VLAN ID was 100, but it encapsulated a frame that in turn encapsulated a packet that the fastethernet 0.100 subinterface is not configured to route or bridge.

The third entry in the output of Example 9-6 tells you that an ISL frame was received from the fastethernet 0 interface. The ISL frame's VLAN ID was 4, and the fastethernet 0 interface does not have a subinterface to handle this VLAN's frames. In scenarios like this, it is usually wise to configure the device on the other side of the trunk not to send certain VLANs' frames in this direction. On the Catalyst 5000 switch, you may use the clear trunk command to take a VLAN out of a trunk port.

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