Spanning-tree Gig / Fa cost question

bndeulogy2bndeulogy2 Posts: 8Member ■□□□□□□□□□
Hello all!

I have been struggling for a while now with the STP election process. Every time I feel I have it 100% figured out something else pops up and confuses me.

Right now I am having an issue with links when they connect from an ethernet to a gigabit port. For instance, a cable from Gi0/0 to Fa0/0. I am trying to practice by with a lab where I change a priority on a switch, then attempt to figure out the final port roles. All switches are running PVST and show spanning-tree mode pvst under show run. Switch A is the root. I have attached a picture of the topology below.

My question is, why is it that Switch B shows a port cost of 4 for the Gi1/2 link, and a cost of 19 for the Gi1/1 port? I was under the impression that Switch A will send a 0 cost BPDU to Switch B. After adding the Gi1/1 port cost of 4, I thought the cost should be 4. (0+4). I'm at a complete loss icon_sad.gif I have attached a picture of my packet tracer with a show spanning-tree on Switch B.

Thanks so much in advance guys,
Kyle

(Edit to add another picture showing all link interfaces)

Comments

  • dsgmdsgm Posts: 228Member ■■■□□□□□□□
    Im going to take a stab at this being a newbie

    The fa0/1 to Gi1/1 is going to take the bandwidth of the lower link (cost of 19)

    Then i think from your diagram the gi1/2 is also linked to a gig port which has a cost of 4

    More experienced people correct me if i am wrong
  • bndeulogy2bndeulogy2 Posts: 8Member ■□□□□□□□□□
    dsgm,

    (Edit: I just noticed on my second picture they both have costs of 19. Maybe the election process was not complete on the first picture? Either way if anyone can clarify the process I would be grateful)

    Thanks for your reply! I assume you are talking about the link Fa0/2 to Gig1/1. I was under the impression that the cost of the link was local to the Port. This is where I am a bit confused with STP and basic cabling. If a Gig cable is attached to an Ethernet port at the other end of the link, I would assume the max speed of the link would be ethernet, giving the link a cost of 19. However, many examples have seemed to use the cost on a per-port basis, including the ICND2 exam.

    Is anyone able to clarify how this process works please?

    Thanks for your time,
    Kyle
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