stp blocking ports

[varlotbolt]

Hi all, can anyone explain why the "blue" ports are blocked and not any of the "red" ports?

the rule "the switch with the higher bridge id will end up blocking its side of the connection" doesn't seem to apply here.

Regards.

[ysaadi2009]

the role of selection is in this Order (the rule to elect the designated Port)

1 - Cost to Root Bridge the Lowest make his port in Designated (Forwarding).

2 - If they have the same Cost than : the Bridge ID the loest make his port as Designated(Forwarding)

Best Regards

[Arunagiri Nallusamy]

Hello Aristeidis,

++ In this case, Switch1 has 2 equal paths in order to reach the STP ROOT bridge.
++ During the initial STP convergence, Root Bridge BPDU’s will be advertised by both Switch0 and Switch2 connecting to Switch1.
++ These BPDU’s when they arrive at the corresponding ports of Switch1, they will have same cost to reach the STP ROOT.
++ Now, when STP cost is a tie, Switch1 chooses the lower Sender Bridge ID.
++ In this case, Switch 2 (0002.1791.6ee5) has a sender bridge ID lower than that of Switch0 (00e0.f713.8076).
++ Thus, Switch1 prefers to put the link towards Switch2 in forwarding state and the links towards Switch1 in blocking state.
++ RED ports in Switch0 will not be blocked as in that segment they advertise the BPDU’s that has best cost to reach the ROOT.
++ For example, if Switch0 advertises the cost as X, Switch1 advertises the cost as 2X.

Hope this helps.