Troubleshooting guide

12-11
ATM and Layer 3 Switch Router Troubleshooting Guide
OL-1969-01
Chapter 12 Troubleshooting Layer 2 Interfaces
Troubleshooting Layer 2 Switching
Packets are Switched but are Not Appearing on the Wire
If you are sure the packets are being switched but the connection does not appear on the wire, try the
following:
• Use a Sniffer to see whether they are actually being sent on the wire.
• If the packets are not seen by Sniffer, then identify the output interface.
• Use the show controller {FastEthernet | GigabitEthernet} card/subcard/port command and
check the following:
– The MTx and SRx registers for doubts in transmission
– The MRx and STx registers for doubts in reception
Layer 2 CAM Display
You might need to check for a specific MAC address on an interface. If so, use the show epc patricia
interface {FastEthernet | GigabitEthernet} card/subcard/port mac command. The following is an
example with a description of some useful MAC addresses:
Switch# show epc patricia interface fastethernet 0/0/0 mac
1# MAC addr:0000.0000.0000 VC:0 Entry:
2# MAC addr:0900.2b01.0001 MyMAC VC:4 Entry:
3# MAC addr:0180.c200.0000 MyMAC VC:4 Entry:
4# MAC addr:0100.0ccc.cccc MyMAC VC:4 Entry:
5# MAC addr:0010.073d.8207 HsrpMAC VC:4 Entry:
6# MAC addr:0008.e0bc.4190 MyMAC VC:4 Entry:
Total number of MAC entries: 6
In this example check the following:
• MAC address 3# and 4# are spanning tree BPDU addresses
• MAC address 6# is the interface fastEthernet 0/0/0 Mac-address (IRB only)
Check for Spanning Tree Loop
The spanning-tree algorithm in the IOS software is probably not the source of the spanning tree loop.
The spanning-tree loop probably exists because of a problem in the end-to-end connectivity. Try one of
the following tests to confirm you do not have a spanning-tree loop:
• Use the show epc patricia interface {FastEthernet | GigabitEthernet} card/subcard/port {mac |
vlan #} command to confirm the spanning tree multicast address is present in the Layer 2 CAM. See
the section “Layer 2 CAM Display” section on page 12-11.
If the spanning tree MAC address is not in the output, then spanning tree is the problem.
• Use show spanning-tree bridge-group bridge-group-number command and check for the string
“BPDU: sent 0, received 0” in the output. If both switch routers connected with back-to-back
interfaces “received 0,” then there is a physical layer connectivity problem between the switch
routers.
Refer to the Layer 3 Software Feature and Configuration Guide if any changes are necessary to the
configuration of the interface.