1 Reply Latest reply: Sep 24, 2017 12:06 PM by Martin RSS

    Rapid spanning-tree and duplex




      according to CCNP SWITCH official certification guide, I read that there a 3 port type in RSPT:


      Edge port

      Root port

      Point-to-point port


      "Point-to-point ports are automatically determined by duplex mode in use".

      "Full duplex ports are considered point-to-point", while "Half duplex ports are considered on a shared medium with  possibly more than two switches".


      I don't understand that.


      This is "sh spanning tree vlan 10" on my switch:


      Interface           Role Sts Cost      Prio.Nbr Type

      ------------------- ---- --- --------- -------- --------------------------------

      Gi0/5               Desg FWD 4         128.5    P2p Edge

      Gi0/6               Root FWD 4         128.6    P2p

      Gi0/9               Desg FWD 4         128.9    P2p

      Gi0/11              Desg FWD 4         128.11   P2p Edge

      Gi0/12              Desg FWD 4         128.12   P2p Edge

      Gi0/13              Desg FWD 4         128.13   P2p

      Gi0/16              Desg FWD 19        128.16   P2p Edge

      Gi0/17              Desg FWD 4         128.17   P2p Edge

      Gi0/18              Desg FWD 4         128.18   P2p Edge

      Gi0/24              Desg FWD 4         128.24   P2p


      My PC is connected to Gi0/5, interface is in Full Duplex, but this is "Edge port", not P2p (honestly, the type "P2p Edge" is a bit confusing).

      What could be type for "shared" medium? It means that, with "sh interfaces status", should I see port in "half" duplex mode?

      Thank you

        • 1. Re: Rapid spanning-tree and duplex

          To speed up things, switch will use auto negotiation to put ports in perspective roles. auto negotiation determines speed and duplex mode of every port in switch. Note that a link must be up and comes up first before STP can run on it.


          Half duplex --think about old hubs, not too many of those use today's networks. Anyway, RSTP calls Half duplex ports "Shared" ports cause it cannot be sure how many devices can be connected to such port.


          Point-to-point means there is only one other neighbor. having that means no chances for loops. it means Rapid switch can safety negotiate root bridge and port roles.

          so u can have a switch, server, printer, or a pc on p2p port.


          P2P port becomes the Edge port when u have portfast command.  once BPDU is received on the Edge port, it will lose its edge status and becomes regular port .

          Note; u must have portfast command (global or interface) for port to become Edge (P2p Edge). Otherwise, PC connected to p2p shows as P2p.