Root Bridge Election
For all switches in a network to agree on a loop-free topology, a common frame of reference must exist. This reference point is called the Root Bridge. The Root Bridge is chosen by an election process among all connected switches. Each switch has a unique Bridge ID that it uses to identify itself to other switches. The Bridge ID is an 8-byte value. 2 bytes of the Bridge ID is used for a Bridge Priority field, which is the priority or weight of a switch in relation to all other switches. The other 6 bytes of the Bridge ID is used for the MAC Address field, which can come from the Supervisor module, the backplane, or a pool of 1024 addresses that are assigned to every Supervisor or backplane depending on the switch model. This address is hardcoded, unique, and cannot be changed.
The election process begins with every switch sending out BPDUs with a Root Bridge ID equal to its own Bridge ID as well as a Sender Bridge ID. The latter is used to identify the source of the BPDU message. Received BPDU messages are analyzed for a lower Root Bridge ID value. If the BPDU message has a Root Bridge ID of the lower value than the switch's own Root Bridge ID, it replaces its own Root Bridge ID with the Root Bridge ID announced in the BPDU. If two Bridge Priority values are equal, then the lower MAC address takes preference. The switch is then nominates the new Root Bridge ID in its own BPDU messages although it will still identify itself as the Sender Bridge ID. Once the process has converged, all switches will agree on the Root Bridge until a new switch is added.
The Root Bridge election is based on the idea that one switch is chosen as a common reference point, and all other switches choose ports that are closest to the Root. The Root Bridge election is also based on the idea that the Root Bridge can become a central hub that interconnects other legs of the network. Therefore, the Root Bridge can be faced with heavy switching loads in its central location. If heavy loads of traffic are expected to pass through the Root Bridge, the slowest switch is not the ideal candidate. Furthermore, only one Root Bridge is elected. This is thus not fault tolerant. To overcome these problems, you should set a Root Bridge in a determined fashion, and set a secondary Root Bridge in case of primary Root Bridge failure. The Root Bridge and the secondary Root Bridge should be placed near the center of the network.
To configure a CLI-based Catalyst switch to become the Root Bridge, use the following command to modify the Bridge Priority value so that a switch can be given a lower Bridge ID value to win a Root Bridge election:
Switch (enable) set spantree priority bridge_priority [ vlan ]
Alternatively, you can use the following command:
Switch (enable) set spantree root [ secondary
] [ vlan_list ]
[ dia diameter ] [ hello hello_time ]
This command is a macro that executes several other commands. The result is a more direct and automatic way to force one switch to become the Root Bridge. Actual Bridge Priorities are not given in the command. Rather, the switch will modify STP values according to the current values in use within the active network. To configure an IOS-based Catalyst switch to become the Root Bridge, use the following command to modify the Bridge Priority value so that a switch can be given a lower Bridge ID value to win a Root Bridge election:
Switch (config)# spanning-tree [ vlan vlan_list ] priority bridge_priority