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STP Tiebreakers

Hi guys... Not sure if this is the right place to post this, but I've been reading about STP tiebreakers and I have a little question:

As far as I know,  STP tiebreakers for electing the root ports are: 

1. BID (Electing the root bridge)
2. Lower Path Cost (Selecting the best route to the root brige
3. Lower BID (If two or more paths have the same cost to the root brige)
4. Lower Recieved Port-Priority (This only happens if I a switch have multiple links connected to the same neighboor switch)
5. Lower Local Port-Priority (This only happens if a switch have multiple ports connected to the same port of a neighboor switch. I think using a hub, for example)

I've made some test and tiebreakers work fine. But I've also noticed that this tiebreakers apply for electing root ports, and not when electing designated ports.

Let's say that I have a root switch (Switch A) connected to (Switch B). Switch A have two ports connected to the same segment ( Port 1 and 2 through a hub). Switch B have one port to that segment (Port 1). Obviosly, switch B is going to elect that port as the root port, but Switch A have to choose one designated port and it also have to block the the other one. 

In that case, if you see the tiebreakers rules, switch A should go to rule number 4 (Lower Received port priority), because It received the same path coost to root (0) on both ports, and the same BID (his BID). Port 1 should receive a port priority of 128.2, which is the default priority sent by port 2.  And port 2 should receive a priority of 128.1 which is the default priority sent by port 1. The rule says that lowe received por priority should break the tie, so port 2 recieve the lower priority and should be elected as the designated port.

When I did my test, port 1 was elected as the designated port, ignoring rule number 4 and going directly to rule number 5.

So, when electing designated ports, does the tiebreak rules apply?

I'm sorry if this post is too long or if it is in the wrong place, but this seems a good place to ask because I found really good information on this site.

Thank you all.

  • LAN Switching and Routing
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