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Forum Replies

  1. Hi Michael,

    IST0 is called IST (Internal Spanning-Tree) and these are the VLANs that haven’t been mapped to another instance.

    PVST uses 1 STP for each VLAN, MST uses 1 STP for multiple VLANs so we have a mismatch here. To make the two compatible, here’s what happens:

    - Within the MST region we run multiple STPs (one for each instance).
    - On the border links (switches that connect to PVST) our MST switch will detect PVST BPDUs (or another MST region) and marks these interfaces as “boundary”.
    - The MST switch won’t send any BPDUs from the instances that we config

    ... Continue reading in our forum

  2. Hi Ryan,
    Interaction between MST and PVST/Rapid STP can be quite confusing. There are a few key points that are helpful to keep in mind.

    1. MST tries to hide the details of what is going on inside a region to any STP topology outside of that region
    2. MST accomplishes this by using “Virtual Bridge” in that the entire MST region is present as one giant switch to an outside STP topology
    3. MST uses “Instance 0” for the mechanics of this virtual bridge interaction with an external STP domain
    4. Every MST region MUST have an Instance 0–MST does this automatically
    5. MST uses th
    ... Continue reading in our forum

  3. Hey Patrick,
    As you know, putting VLANs into Instance 0 is the default behavior of MST. Although there is not any serious issue in doing this, it is generally considered best practice to define explicitly separate instances for vlans. There are at least two reasons for this:

    1. Allowing numerous vlans to remain as part of the default instance potentially loses some of the benefits of MST - namely ensuring that blocked links in one instance will be utilized by another.

    2. Avoiding fate sharing. Let’s say you have traffic that is extremely time sensitive–it could

    ... Continue reading in our forum

  4. I work on Brocade mostly and some Cisco. So on Brocade they actually do have a RSTP. I got confused because I was looking for it on my VIRL switches version 15 and could not find it.

    I have to say reading the forum post on some of these topics is just as educational as the actual lesson. I have the CCNP INE series I use along with this and of course the howtonetwork CCNP Switching/Routing/Troubleshooting/101 labs books you recommended.

    between those three resources GNS3, cisco VIRL, and working in Global environment as well I feel I am really learning. My

    ... Continue reading in our forum

  5. Andrew,

    (P.S anyone reading this might want to read from bottom up I figured it out but don’t know for sure why it works the way it does)

    I was going through Cisco CCNP Switch Simplified and a lab lesson did not work for me. Its the very second lab at the end of the book



    Basically they setup MST on DLS1 and DLS2 and then have change configuration on IST 0

    ... Continue reading in our forum

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