STP root port two path tie

TWXTWX Posts: 259Member
I made this for me. If it helps you too, good.

The curriculum did not do all that good of a job of reinforcing what happens when there are two equal-cost paths back to the root bridge. Clearly STP has to make a decision:

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When the costs tie, STP looks at the BID of the next hop for the two routes:

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It selects the route with the lower BID:

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That's all there is to it, at least at CCNA level.

Comments

  • james43026james43026 Posts: 303Member
    It's also important to consider if both paths go through the same switch. Then the BID will be the same on both ports. Then the PID becomes the tie breaker.
  • TWXTWX Posts: 259Member
    Yeah, after I made my chart I read-up on that and watched some videos.

    One thing that I don't care for in the deciding-switch is that when one looks at the spanning-tree summary information, it shows the PIDs that it advertises, not the PIDs that come from the switch that it has to decide against:

    idf3-sw2#sh spanning-tree interface po1

    Vlan Role Sts Cost Prio.Nbr Type

    ---- ---


    VLAN0001 Desg FWD 3 128.224 P2p


    idf3-sw2#sh spanning-tree interface po1 detail
    Port 224 (Port-channel1) of VLAN0001 is designated forwarding
    Port path cost 3, Port priority 128, Port Identifier 128.224.
    Designated root has priority 32769, address 24b6.5739.8f80
    Designated bridge has priority 32769, address 24b6.5739.8f80
    Designated port id is 112.224, designated path cost 0
    Timers: message age 0, forward delay 0, hold 0
    Number of transitions to forwarding state: 1
    Link type is point-to-point by default
    BPDU: sent 15136, received 422969

    In this particular case the 112.24 advertised by the other switch is the part that decides against as shown in the second command, not the 128.224 that it shows for itself in the first command.
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