Enabling root guard

The root bridge and secondary root bridge of a spanning tree should be located in the same MST region. Especially for the CIST, the root bridge and secondary root bridge are put in a high-bandwidth core region during network design. However, due to possible configuration errors or malicious attacks in the network, the legal root bridge might receive a configuration BPDU with a higher priority. Another device supersedes the current legal root bridge, causing an undesired change of the network topology. The traffic that should go over high-speed links is switched to low-speed links, resulting in network congestion.

To prevent this situation, MSTP provides the root guard function. If root guard is enabled on a port of a root bridge, this port plays the role of designated port on all MSTIs. After this port receives a configuration BPDU with a higher priority from an MSTI, it performs the following tasks:

• Immediately sets that port to the listening state in the MSTI.

• Does not forward the received configuration BPDU.

This is equivalent to disconnecting the link connected with this port in the MSTI. If the port receives no BPDUs with a higher priority within twice the forwarding delay, it reverts to its original state.

On a port, the loop guard function and the root guard function are mutually exclusive.

Configure root guard on a designated port.

To enable root guard: