Spanning-Tree BPDUFilter

The spanning-tree BPDUfilter works similar to BPDUGuard as it allows you to block malicious BPDUs. The difference is that BPDUguard will put the interface that it receives the BPDU on in err-disable mode while BPDUfilter just “filters” it. In this lesson we’ll take a good look at how BPDUfilter works.

BPDUfilter can be configured globally or on the interface level and there’s a difference:

  • Global: if you enable BPDUfilter globally then any interface with portfast enabled will not send or receive any BPDUs. When you receive a BPDU on a portfast enabled interface then it will lose its portfast status, disables BPDU filtering and acts as a normal interface.
  • Interface: if you enable BPDUfilter on the interface it will ignore incoming BPDUs and it will not send any BPDUs. This is the equivalent of disabling spanning-tree.

You have to be careful when you enable BPDUfilter on interfaces. You can use it on interfaces in access mode that connect to computers but make sure you never configure it on interfaces connected to other switches; if you do you might end up with a loop.

Let’s use the following topology to demonstrate the BPDUfilter:

Spanning-Tree BPDU Guard Topology

I’m going to use SW2 and SW3 to demonstrate BPDUfilter:

SW2(config)#interface fa0/16
SW2(config-if)#spanning-tree portfast trunk
SW2(config-if)#spanning-tree bpdufilter enable

It will stop sending BPDUs and it will ignore whatever is received. Let’s enable a debug to see what it does:

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Forum Replies

  1. awesome explaination Renee .
    I really enjoy reading your topics and make me feel more confortable and confident when I learn it
    thanks

  2. Thanks Rene, The point here is usually you don’t know the end host capability, I saw cases loop can happen from a server with NIC bridging without STP capability.

    Just wanted to make sure Portfast + BPDUGuard can address this case

     

     

  3. Hello rosna

    By default, all ports on a switch, including those configured with portfast SENDBPDUs. (It is possible to disable BPDU sending on these ports using BPDU filtering.) Portfast essentially skips the listening and learning states to enter the forwarding state immediately but does not disable STP. It also applies the global BPDUGuard feature (if it is enabled) to all ports configured using portfast.

    In addition, as you mentioned, it won’t send any TC information on that port because by definition, there should be no switches connected to the specific

    ... Continue reading in our forum

  4. Good explanation !!
    I really enjoy reading your topics and make me feel more comfortable and confident.

  5. Hello Laz
    Thanks a lot.

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