Information Technology Reference
In-Depth Information
Bridging loops are the worst-case scenario in a bridged network because they will potentially impact
every user. In case of emergency, the best way of recovering quickly connectivity is to disable manually
all the interfaces providing a redundant path in the network. Unfortunately, the cause of the bridging loop
will be very difficult to identify afterward, if you do so. Try the actions recommended in Table 20-5 first,
if possible.
Table20-5 Transparent Bridging: Looping and Broadcast Storms Occur
Possible Causes
Suggested Actions
No Spanning
Tree
implemented
Examine a topology map of your internetwork to check for
possible loops.
1.
Eliminate any loops that exist, or make sure that the
appropriate links are in backup mode.
2.
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.
3.
Implement a Spanning-Tree Algorithm to prevent loops.
4.
Spanning-Tree
Algorithm
mismatch
1.
Use the
show span
exec command on each bridge to
determine which Spanning-Tree Algorithm is being used.
2.
Make sure that all bridges are running the same
Spanning-Tree Algorithm (either DEC or IEEE).
1
Use of both
DEC and IEEE Spanning-Tree Algorithms may be needed in
the network for some very specific configuration (generally
involving IRB). If the mismatch in the Spanning-Tree
Protocol is not intended, you should reconfigure bridges as
appropriate so that all bridges use the same Spanning-Tree
Algorithm.
Note:
The DEC and IEEE Spanning-Tree Algorithms are
incompatible.
