Root Bridge Selection - Cisco RJ-45-to-AUX Brochure

A switch's diameter is a unit of measurement between the root switch and child switches. The root bridge
counts as the first switch. Each subsequent child switch out from the root bridge is counted to yield the
diameter number. A parent switch brings you one switch closer to the root bridge, and a child switch takes
you one switch farther away from the root bridge.
Each root bridge can be configured with a diameter from a minimum of two switches to a maximum of seven
switches. By modifying the diameter, you will subsequently change the timer values that are advertised by the
root to reflect a more accurate network diameter. For example, a diameter of 2 yields a MaxAge of 10 seconds
and a FwdDelay of 7 seconds. Cisco recommends that you change the diameter to correctly reflect your
network rather than manually changing the timers.

Root Bridge Selection

One of the most important decisions that you make when configuring the STP protocol on your network is the
placement of the root bridge. In the spanning tree, the root bridge should be located as close as possible to the
center of the network. Certain commands can help the administrator determine which device will become the
root bridge. The proper placement of the root bridge(s) optimizes the paths that are chosen by the STP to
allow data traffic to flow through the network. It also provides deterministic paths for data to take.
In order to get the most optimal paths through the network, you must sometimes ignore the default root bridge
used by STP. This means you must manually configure the bridge that should be the root bridge, as well as
the secondary root bridge. The function of the secondary root bridge is to become the root bridge, should the
original root bridge fail.
Tip Typically, root bridges are Distribution layer switches, not Access layer switches. The root
bridge should never be a Core layer switch, because the Core layer's responsibility is to move
traffic as quickly as possible.
The Selection Process
The root bridge selection process begins as soon as the switch powers up. The root bridge is the reference
point in the network from which graph theory is used to calculate the cost of each link for each instance of a
spanning tree. Using these calculations, the switches must determine if loops exist in the network and the path
costs associated with each path through the network. The switch immediately assumes at startup that it gets to
be the root bridge, and it configures its bridge ID equal to the root ID in the BPDU. The bridge ID field of a
BPDU message is actually made up of two parts, as follows:
Bridge priority—A 2−byte value set by the switch. By default, the priority is set to 0x8000 or 32,768.
Media Access Control (MAC) address—The 6−byte MAC address of the switch or bridge.
These two fields of the bridge ID help an STP switch yield a value that can be compared with other switches'
bridge IDs to determine which switch will become the root bridge. The lower the bridge ID value, the higher
the chance of a root−bridge assignment. If more than one switch has the same low bridge priority value, the
bridge with the lowest MAC address then becomes the root bridge. Table 10.3 shows the bridge priority
values assigned by STP.
Table 10.3: The bridge priority values assigned by Spanning Tree Protocol.
Priority Assignment
Default bridge priority
Parent and Child Switches
Value
32,768
205