STP - Designated port confusion

Unanswered Question
Oct 7th, 2007

I've been reviewing my STP knowledge and have come across a problem that doesn't make sense to me.

It goes something like this:

SWa - bid = 1.xxxxx

SWb - bid = 2.xxxxx

SWc - bid = 3.xxxxx

SWA is connected to SWB via fa0/0.

SWA is connected to SWC via fa0/1.

SWB is connected to SWA via fa0/0.

SWB is connected to SWC via fa0/1.

SWC is connected to SWA via fa0/0

SWC is connected to SWB via fa0/1.

This is basically a triangle of 3 switches.

I know that SWa is the root bridge based on the fact that it has the lowest BID.

My studies tell me (or so I think) that

1. The ports on SWa are both designated ports because it is the root bridge.

2. The connection between B and C will be blocked since B can reach C via A (the root brige) and vice-versa.

Is this correct?


I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
royalblues Sun, 10/07/2007 - 09:47

You are correct.

Just remember the 4 step STP decision process.

1. Lowest root BID

2. Lowest path cost to root bridge

3. Lowest sender BID

4. Lowest port ID

In your case the SWa becomes root because of lowest BID and all its ports are designated ports.

FA 0/0 ports of the SWb & SWc become root ports based on 2nd step and are in forwarding mode.

Now to break the loop it becomes necessary to block one of the port connections between SWb and Swc. Here according to step 3, the switch with lower sender BID wins and hence you will find that Sw2 fa0/1 port would be in FWD state and SWc fa0/1 port would be blocking.

edit: sorry Edison didn't see your reply

HTH, rate if it does


swmorris Mon, 10/08/2007 - 19:34

One side of that link will be blocked. There is a series of elections that take place in spanning tree.

Root bridge comes first, and as you noted, all ports of the root bridge will be forwarding.

Next there is root ports. This is from each bridge's perspective of the shortest path to the root bridge. So bridge B and bridge each have a link going back to A. Those ports will be the root ports.

Next is designated ports, and that's from a link perspective (kinda put yourself in the middle of the link, and decide what the best path out is).

For root and designated ports, first you look at total spanning tree cost to get back to the root bridge. If the cost is the same, then you look at received bridge ID's.

Presuming that B has a lower BID than C does, the link between B and C on B's side will be the designated port.

Any non-root, non-designated ports (namely C's side of the B/C link) will be blocking.



[email protected]


This Discussion