Tuesday, 19 May 2009

BDPU Guard Vs Filter

Spanning-tree BPDU Guard or BPDU Filter?

A question that has cropped up on more than one occasion is which of these options should be used? BPDU Guard or BPDU filter?

You may think it safest to use both, however that isn't the case.

BPDU Guard
The port is error disabled when a BPDU is received.

BPDU Filter
If the port receives BPDUs then portfast is disabled and it functions as a normal STP port.

You may think that BPDU filter is the more useful setting as it always results in a functioning port, however consider two switches connected together to form a bridging loop as shown here.

With BPDU guard configured on all four ports, the BPDUs are detected and ports are shut down.

With BPDU filter configured on all four ports, no BPDUs are transmitted. The switches are not aware of each other and happily forward traffic down all links, creating a bridging loop.

With both options enabled the same thing happens.

The important difference is that BPDU filter prevents transmission of BPDUs.

If you're looking at standard switchport setting then this is not desirable, so the configuration of choice is:

spanning-tree portfast
spanning-tree bpduguard enable

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