System information

Troubleshooting Transparent Bridging Environments 20-429

Transparent Bridging: Looping and Broadcast Storms Occur

Possible Causes Suggested Actions

No spanning tree implemented

Step 1 Examine a topology map of your internetwork to check for possible

loops.

Step 2 Eliminate any loops that exist or make sure that the appropriate links

are in backup mode.

Step 3 If broadcast storms and packet loops persist, use the show interfaces

exec command to obtain input and output packet count statistics. If

these counters increment at an abnormally high rate (with respect to

your normal trafﬁc loads), a loop is probably still present in the

network.

Step 4 Implement a spanning-tree algorithm to prevent loops.

Spanning-tree algorithm mismatch

Step 1 Use the show span exec command on each bridge to determine

which spanning-tree algorithm is being used.

Step 2 Make sure that all bridges are running the same spanning-tree

algorithm (either DEC or IEEE)

. If both DEC and IEEE algorithms

are being used, reconﬁgure bridges as appropriate so that all bridges

use the same spanning-tree algorithm.

Note: The DEC and IEEE spanning-tree algorithms are

incompatible.

1 IEEE = Institute of Electrical and Electronic Engineers

Multiple bridging domains

incorrectly conﬁgured

Step 1 Use the show span exec command on bridges to ensure that all

domain group numbers match for given bridging domains.

Step 2 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.

Step 3 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.