network engineer

[sajeer9745]

suppose we have three switches...each of them having vlan(vlan5)..the switches are connected directly...in this scenario which switch will be the root switch and wat are the criteria to select the root switch?

[Scott Kiewert · Accepted solution]

Hello,

More information is needed to be able to determine which switch would be the root.  However, if all switches are configured using the default spanning-tree settings, the switch with the lowest MAC address will be elected as the root.

http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3750x_3560x/software/release/12-2_55_se/configuration/guide/3750xscg/swstp.html#26965

[amagana · Accepted solution]

Sajeer,

The root switch will be the switch with the lowest bridge ID (BID).

Bridge ID = Bridge Priority + Extended System ID (5 in this case for VLAN 5) + MAC Address

The default bridge priority for all switches is 32769, thus, unless the bridge priority is modified the root bridge will be the switch with the lowest MAC Address. 

Example:

SW1: VLAN 5, MAC = 1111.1111.1111, BID = 32774.1111.1111.1111

SW2: VLAN 5, MAC = 2222.2222.2222, BID = 32774.2222.2222.2222

SW3: VLAN 5, MAC = 3333.3333.3333, BID = 32774.3333.3333.3333

SW1 would be the root bridge in this case.

If you wanted SW2 to be the root bridge, you can do so by lowering it's bridge priority:

spanning-tree vlan <#> priority <# in increments of 4096>
OR
spanning-tree vlan <#> root [primary|secondary]

Note: This command will not work if another Switch already has a bridge priority of zero (since that is the lowest possible priority).

For more information: 

http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3750x_3560x/software/release/12-2_55_se/configuration/guide/3750xscg/swstp.html#pgfId-1020470

-Alex