Spanning tree question

hshah12

Hey guys

Another quick question for you all..

When a switch port(non root bridge) connecting "directly" to one of the port on the root bridge has less path cost than other ports on other switches, can this port be blocked under Spanning tree process?

thanks

EdTheLad

hshah12 wrote:

Hey guys

Another quick question for you all..

When a switch port(non root bridge) connecting "directly" to one of the port on the root bridge has less path cost than other ports on other switches, can this port be blocked under Spanning tree process?

thanks

The port that is connected to the root bridge is the root port, this port recieves BPDU's directly from the root bridge and relays them on to other switches.The root port has a cost associated with it that it adds to the bpdus before propogating them to the downstream switches.So its impossible that the downstream switches have a lower cost.

marlon23

1* All ports on Root Bridge are designated ports (all of them have cost 0).

Assume that SW1 is root, SW2 is non-root directly connected to root.

Port connecting SW2 to SW1(root) can be placed in blocking state when you have more direct connections to SW1 from SW2 (when they have same cost than port with least number wins and is forwarding(root port), other are in blocking state or in Rapid STP the 2nd is put as alternative).


Remember that you can configure etherchannel in that scenario. In that case all ports are forwarding as one logical port with multiple.port bandwidth.

Danman32

In class, to demonstrate spanning tree, we had several switches connected together with multiple links. One link was 1Gb, other link was 100BaseT. I wondered why the 100BaseT was the designated port on my switch (we were the root bridge). Turns out the lab group behind us connected both links to 100BaseT ports. Once they fixed their connections and had the link from our 1Gb port to their 1GB port, that port became the designated port as expected.