Determining the Root

Determining the Root
Determining the root device is the most important decision that you make when configuring STP
on your network. If you place the root in the wrong place, 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. Choosing the root is typically the best thing to do, but if that root goes down
for maintenance, spanning tree will select a new root—and because all other switches have the
same priority, it might be a switch you wouldn’t usually want to be the root bridge.
Because the root bridge should be close to the center of the network, the device will typically
be a switch that a lot of traffic passes through such as a distribution layer switch, a core layer
switch, or one that does routing or multilayer switching. An access layer switch would not usually
be chosen.
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 several different factors 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,
which is the sum of all the port costs, is then determined. The STP first looks at the path cost
to calculate 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 used 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).