Why is this important?
Confirming the correct root bridge selection within each VLAN is important for increasing the stability of the spanning tree and the underlying VLAN(s).
If a low powered switch becomes the root bridge, it may not have sufficient memory or CPU to handle the task and the spanning tree becomes unusable.
The root bridge is selected by setting its bridge priority to a low value. The default value is 32768, out of a range from 0 to 65535.
If all switches in a single spanning tree have the same bridge priority, the switch with the lowest MAC address will become the root bridge.
Older switches typically have lower MAC addresses, but such a device may not have enough memory or CPU power to handle the task of being the root bridge.
I recently heard of a 900 server data center that was implemented with one big Layer 2 spanning tree and the default bridge priority was used on all switches. A few more ports were needed, so an old switch with a few ports was added to the network.
It became the root bridge but couldn’t handle the load, bringing down the entire data center.
Network best practices dictate setting a core switch’s bridge priority to a low value, say 8000. Select a backup root bridge and set its bridge priority to a slightly higher value, say 16000. It’s easier with something like ASUS RT. You can find an ASUS RT review here. These values are selected to stand out in a review of the device configurations.
Manual determination
The root switch can be manually determined by issuing the Cisco CLI command ‘show spanning-tree’ on the switches within each VLAN. The root switch will report ‘I am the root of the spanning tree’ whereas non-root switches will report something like ‘Current root has priority 32768, address 0002.b9fc.b700′.
The returned address is the MAC address of the root switch. Once the MAC address is determined, it can be correlated with the IP address of the desired switch.
VLAN Summary
Automatic determination
Use an automatic tool like NetMRI to correlate spanning tree data to identify the root bridge by its name or IP address. The VLAN Summaries view identifies the VLAN by number and name. You should use a different name for each unique VLAN, though reusing the VLAN number is acceptable.
The root bridge is identified for the VLAN as well as the list of switches in each VLAN.
Further reference:
Understanding and Configuring Spanning Tree Protocol (STP) on Catalyst Switches: http://www.cisco.com
