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Difference Between STP BPDU Guard and STP Root Guard

July 8th, 2009

BPDU guard and root guard are similar, but their impact is different. BPDU guard disables the port upon BPDU reception if PortFast is enabled on the port.

The disablement effectively denies devices behind such ports from participation in STP. You must manually reenable the port that is put into errdisable state or configure errdisable-timeout.

Root guard allows the device to participate in STP as long as the device does not try to become the root. If root guard blocks the port, subsequent recovery is automatic. Recovery occurs as soon as the offending device ceases to send superior BPDUs.

Does the Root Guard Help with the Two Roots Problem?

There can be a unidirectional link failure between two bridges in a network. Because of the failure, one bridge does not receive the BPDUs from the root bridge. With such a failure, the root switch receives frames that other switches send, but the other switches do not receive the BPDUs that the root switch sends. This can lead to an STP loop. Because the other switches do not receive any BPDUs from the root, these switches believe that they are the root and start to send BPDUs.

When the real root bridge starts to receive BPDUs, the root discards the BPDUs because they are not superior. The root bridge does not change. Therefore, root guard does not help to resolve this issue. The UniDirectional Link Detection (UDLD) and loop guard features address this issue.

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