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Silver

STP root port

hello

in spanning tree protocol, when choosing root port, we search for the port with the min cost to the root.

if we have 2 paths to the root with the same cost, then we look to the lowest port identifier that is in the received BPDU.

now if both port identifier values are equal what criteria will we use to choose the root port ? the local port id?

thanks for your help

1 ACCEPTED SOLUTION

Accepted Solutions
Purple

STP root port

Hi,

the first tiebreaker when root path cost is the same is lowest sender bridge id and that's what wil be used here.

port priority and port id are only used when you have parallel links between two switches.

Regards.

Alain

Don't forget to rate helpful posts.
4 REPLIES
Bronze

STP root port

You can not have the same port ID for every port on the switch. The value is 128.1, 128.2, 128.3 ..and so on. Every single port on the switch is unique.

If you have port 1 and port 2 connected to another switch, port 1 will become root if all other values are equal (128.1 is lower than 128.2).

Hope this helps

Eugen

Silver

STP root port

if we have 2 paths to the root with the same cost, then we look to the lowest port identifier that is in the received BPDU. this is not a local port. this port identifier is related to the remote port (in the adjacent bridge) from where we received the BPDU.

so it is possible to receive 2 BPDU from 2 switches having same cost and coming from same port ID !!!

in the example, bridg whith PCB receives 2 BPDU with cost = 4 and port id in switch A and C are the same 128.3

e

Purple

STP root port

Hi,

the first tiebreaker when root path cost is the same is lowest sender bridge id and that's what wil be used here.

port priority and port id are only used when you have parallel links between two switches.

Regards.

Alain

Don't forget to rate helpful posts.
Silver

STP root port

ok !

thanks alain

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