RSTP Can Desginated Ports be Alternate Ports

nothimagainnothimagain Member Posts: 72 ■■□□□□□□□□
So im learning about RSTP but im a little confuse about Alternate ports. I know they are support to be the port with the second best root cost to the root bridge ,but does the port have to be in a discarding state?

On the Cisco Docs it claims that the alternate port is the port in a blocking state but receives the second best BPDUs over the port .

What if a switch does not have a blocking port? What if the designated port IS the port with the second best root cost and its Root ports fails? or will that switch just not have an alternate port in this case.


  • Danielh22185Danielh22185 Member Posts: 1,195 ■■■■□□□□□□
    This question heavily depends on your spanning-tree topology. An alternate port can serve as a root port in the event the root port goes down. However that does not mean it has to be discarding. Also ALL ports with the exception of the current root port are designated ports. You are confusing port ROLES vs port TYPES.

    Key operational note of spanning-tree: STP will block the least number of ports necessary to break the bridging loop.
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