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STP / RSTP designated verses blocked, confusion.

Discussion in 'Routing & Switching' started by Straylight, Sep 14, 2012.

  1. Straylight

    Straylight Bit Poster

    Hi guys.
    Excuse longwindedness but I need to ask rather than try it myself as can't use packet tracer. I'm blind and the screenreader software doesn't work with it. I have a lab setup but only 2 2950s for switches.

    So let's say we have 4 switches, arranged in a square.


    Let's say switch B is the root, has lowest BID. The switches are connected to each other with one trunk link between them. So
    A FA0/1 -- FA0/1 B

    C FA0/1-- FA0/1 D

    A FA0/2 -- FA0/2 C

    B FA0/2 -- FA0/2 D

    They're all fastethernet switches. As I understand it, all the ports on sswitch D will be designated and forwarding because it's the root bridge. A's FA0/1 will be a root port, as it's connected to the root bridge directly.


    What happens to switch A's FA0/2 port, linked to switch C? Is this:
    Blocked and designated?

    What about switch D's FA/02 port, that is directly linked to the root switch B? If I rememberr right, if the cost is equal STP / RSTP will choose a port with the lowest ID as rroot. Let's for argument sake, say that is still FA0/1 on switch A. So what state is the FA0/2 on switch D in?

    Thanks for any pointers. I've seen STP in action on the 2 switches when I've had redundant links between them but can't physically set up the above topology.
    Certifications: CCENT
    WIP: CCNA MS 70-680
  2. danielno8

    danielno8 Gigabyte Poster


    A's Fa0/2 port will be designated. In this topology, D's Fa0/2 port will be the root port, as it is directly connected to the root bridge. Its only other option would be the connection to switch C, however this will of course be a higher STP cost than the port directly connected to switch B. I can't remember thes exact mechanism, but on switch C, it will put either the connection to switch A, or the connection to switch D into the blocking state. All other non-root ports will be designated.

    At least i believe this will be the case, i can connect it up in packet tracer for you tonight and post up the relevant outputs from the show spanning tree commands if you would like.
    Certifications: CCENT, CCNA
  3. Straylight

    Straylight Bit Poster

    Thanks. That would seem to make sense. The material always says designated ports are ones with a connection to the root but not the least cost path. Having only used 2 switches before, I'd only seen blocked or forwarding ports on the non root switch. i.e. just with the redundant trunk link.

    If you get a chance to try it in Packet Tracer, I'd be grateful. No rush though.
    Certifications: CCENT
    WIP: CCNA MS 70-680

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