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:
Step | Command | Remarks |
---|---|---|
1. Enter system view. | system-view | N/A |
2. Enter Layer 2 Ethernet or aggregate interface view. | interface interface-type interface-number | N/A |
3. Enable the root guard function. | stp root-protection | By default, root guard is disabled. |