Spanning-Tree BPDUGuard

Spanning-tree BPDUguard is one of the features that helps you protect your spanning-tree topology. Let me give you an example:

spanning tree fake bpdu

In my topology above we have a perfectly working spanning tree topology. By default spanning tree will send and receive BPDUs on all interfaces. In our example we have a computer on the fa0/2 interface of SW2. Someone with curious hostile intentions could start a tool that generates BPDUs with a superior bridge ID. What’ll happen is that our switches will believe that the root bridge can now be reached through SW2 and we’ll have a spanning tree re-calculation. Doesn’t sound like a good idea right? Here’s what could go wrong:

spanning tree mitm

You could even do a man in the middle attack without anyone knowing. Imagine I connect my computer to two switches. If I become the root bridge all traffic from SW1 or SW3 towards SW2 will flow through me. I’ll run Wireshark and wait till the magic happens.

We can use BPDUGuard to prevent this from happening as it will block BPDUs:

spanning tree bpdu guard active

BPDUguard will ensure that when we receive a BPDU on an interface that the interface will go into err-disable mode.

Let’s take a look how to configure this…

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Forum Replies

  1. In the Portfast tutorial, it was mentioned that once portfast is enabled on an access port it won’t send topology change notification. Will it still send BPDUs?
    Is that why we use BUDUGuard to stop BPDUs on Accessport?

  2. 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

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

  4. Hello Laz
    Thanks a lot.

  5. dear Laz,

    this is very clear. i am amazed by your clear explanation, thank you very much for your patience.


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