Identifying A Data Loop; Avoiding Network Problems - IBM BladeCenter Installation And User Manual
4-port gb ethernet switch module
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network cannot be more than seven hops apart. Part of this diameter restriction is
the BPDU age field. As BPDUs are propagated from the root bridge to the leaves of
the spanning tree, each bridge increments the age field. When this field is beyond
the maximum age, the packet is discarded. For large diameter networks, STP
convergence can be very slow.
Identifying a data loop
Broadcast storms have a very similar effect on the network to data loops, but
broadcast storm controls in modern switches have (along with subnetting and other
network practices) have been very effective in controlling broadcast storms. The
best way to determine if a data loop exists is to capture traffic on a saturated link
and check if similar packets are seen multiple times.
Generally, if all the users of a given domain are unable to connect to the network at
the same time, a data loop is the cause. In this case, the port-utilization data in the
switch Telnet windows will give unusually high values.
The priority for most cases is to restore connectivity as soon as possible. The
simplest remedy is to manually disable all of the ports that provide redundant links.
Disabling the ports one at a time, and then checking for the restoration of a user's
connectivity will identify the link that is causing the problem, if sufficient time is
available. Connectivity will be restored immediately after disabling a data loop.
Avoiding network problems
To help your network operate more efficiently, you can avoid or minimize network
problems, as described in this section.
v Know where the root is located.
v Know which links are redundant.
v Minimize the number of ports in the blocking state.
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IBM BladeCenter 4-Port Gb Ethernet Switch Module: Installation and User's Guide
Although the STP can elect a root bridge, a well-designed network has an
identifiable root for each VLAN. Careful setup of the STP parameters results in
the selection of this best switch as the root for each VLAN. Redundant links can
then be built into the network. STP is well-suited to maintaining connectivity in
the event of a device failure or removal, but is poorly suited to designing
networks.
Organize the redundant links and tune the port cost parameter of STP to force
those ports into the blocking state.
For each VLAN, know which ports should be blocking in a stable network. A
network illustration that shows each physical loop in the network and which ports
break which loops is extremely helpful.
A single blocking port changing to the forwarding state at an inappropriate time
can cause a large part of a network to fail. Limiting the number of blocked ports
helps to limit the risk of an inappropriate change.
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Troubleshooting
