Information Technology Reference
In-Depth Information
Table20-5 Transparent Bridging: Looping and Broadcast Storms Occur (continued)
Possible Causes
Suggested Actions
Multiple
bridging domains
incorrectly
configured
Use the show span exec command on bridges to ensure that
all domain group numbers match for given bridging domains.
1.
If multiple domain groups are configured for the bridge,
ensure that all domain specifications are assigned correctly.
Use the bridge <group> domain <domain-number> global
configuration command to make any necessary changes.
2.
Make sure that no loops exist between bridging domains. An
interdomain bridging environment does not provide loop
prevention based on Spanning Tree. Each domain has its own
Spanning Tree, which is independent of the Spanning Tree in
another domain.
3.
Link error
(unidirectional
link), duplex
mismatch, high
level of error on a
port
Loops occur when a port that should block moves to the
forwarding state. A port needs to receive BPDUs from a neighbor
bridge to stay in a blocking state. Any error that lead to BPDUs
being lost can then be the cause of a bridging loop.
1.
Identify blocking ports from your network diagram.
2.
Check the status of the ports that should be blocking in your
bridged network, using the show interface and show bridge
exec commands.
3.
If you find a supposedly blocked port that is currently
forwarding (or about to forward, in learning or listening
state), you have found the real source of the problem. Check
where this port is receiving BPDUs. If not, there is probably
an issue on the link connected to this port (check then link
errors, duplex setting, and so on). If the port is still receiving
BPDUs, go to the bridge that you expect to be designated for
this LAN. From there, check all the links on the path toward
the root. You will find an issue on one of these links (provided
that your initial network diagram was correct).
1.
IEEE = Institute of Electrical and Electronic Engineers
Before Calling Cisco Systems' TAC Team
Try to collect as much information as you can on the topology of your network, when stable.
The minimal data to collect is this:
The physical topology of the network
The expected location of the root bridge (and the backup root bridge)
The location of blocked ports
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