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Chapter 5
Using Spanning Tree with VLANs
Set the port priorities.
Set the VLAN port priorities.
Change the STP timers.
Determining the Root
Determining the root device is the most important decision that you make
when configuring the STP protocol on your network. If you place the root in
the wrong place in your network, it will be difficult to scale the network, and
really, that is what you are trying to do: create a scalable layer 2 switched
internetwork.
However, by placing the root switch as close as possible to the center of
your network, more optimal and deterministic paths can be easily chosen.
You can choose the root bridge and secondary and backup bridges as well.
Secondary bridges are very important for network stability in case the root
bridge fails.
Because the root bridge should be close to the center of the network, the
device will typically be a distribution layer switch and not an access layer
switch.
After the root bridge has been chosen and configured, all the connected
switches must determine the best path to the root bridge. The STP uses sev-
eral different costs in determining the best path to the root bridge:
Port cost
Path cost
Port priority
When a BPDU is sent out a switch port, the BPDU is assigned a port cost.
The path cost is then determined by the sum of all of the port costs. The STP
will first look at the path cost to figure out the forwarding and blocking
ports. If the path costs are equal on two or more links to the root bridge, the
port ID is used to determine the root port. The port with the lowest port ID
is determined to be the forwarding port. You can change the port ID on a
switch by changing the port priority, but Cisco doesn't recommend this.
However, we'll show you how to do it later in this section (so you can have
some fun on a rainy Saturday).
Configuring the Root
After you choose the best switch to become your root bridge, you can use the
Cisco command-line interface (CLI) to configure the STP parameters in a
switched network.
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