Information Technology Reference
In-Depth Information
Figure23-13Root Bridge Location Is Important
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This example is obvious, but it is the same kind of reasoning that is needed in more complex topologies.
Important note:
For each VLAN, hard-code the root bridge and the backup root bridge by reducing the
value of the STP priority parameter (or using the
set spantree root
macro).
Know Where Redundancy Is
Plan the way that your redundant links are organized. Here again, forget about the plug-and-play feature
of the STP. Decide which ports will be blocking by turning the cost parameter of the STP. Hopefully,
this is usually not necessary if you have a hierarchical design and a well-located root bridge.
For each VLAN, know which ports should be blocking in the stable network. Have a
network diagram that clearly shows each physical loop in the network and which blocked
ports break the loops.
Note
In case of accidental bridging loops, knowing exactly where the redundant links are helps you identify
the loop and its cause. Knowing where the blocked ports should be also help you to find where the error
is coming from (by simple comparison).
Minimize the Number of Blocked Ports
The only critical action taken by STP is blocking ports. A single blocking port transitioning to
forwarding by error can meltdown a big part of the network. A good way to limit the risk implied by the
use of the STP is to reduce the number of blocked ports as much as possible.
Prune VLANs That Are Not Used
You don't need more than two redundant links between two nodes in a bridged network. However, a
configuration like that shown in Figure 23-14 frequently appears.
