bridge question

[Traveler]

I'm wondering...
if bridges in the network have the same priorities, they can't decide spanning tree topology.
So are they going to keep exchanging BPDU, or start data flow but each claim itself root bridge?
Will data flow or not?

 

[cmetz]

Traveler, the root bridge is elected according to a set of rules set out in the spec, while priority is a major part of the election algorithm it is not the only piece of data available with which to elect - the bridge ID (MAC address) is a tie-breaker. If you have a set of spanning-tree bridges all of which have the same bridge ID and the same priority, then I don't remember off the top of my head, but I think then you're dead.

Same priority, different bridge IDs (MAC addresses), it'll work fine.

 

[Traveler]

So you mean, if the bridges (no distinction) can't decide which one is the root, then the connection is dead?!

 

[cmetz]

Yes. But if you have two STP switches with the same MAC address, you have bigger problems. If you hand configure the bridge IDs to be the same (normally they're derived from MAC addr), then you deserve to not have it working.

These are both pathological cases you should never see in the real world.

 

[spidey07]

cmetz is on the money.

priority is normally 32768 out of the box (its a 16 bit number).

bridge with lowest bridge ID (priority.mac address) is the root bridge and the tree is built from there.
If all priorities are the same, and this is VERY common as most people don't even think to mess with priorities then lowest mac address is the root.

just remember that bridge ID is a combination of priority and mac address.

In practice its best to choose your root bridge wisely and set the priority to something like 5. In a properly designed spanning tree blocking and forwarding ports are well documented and tuned so there is no guess work and the tree will converge quickly.

Or just design the net to not rely on spanning-tree at all.
Spidey and Cmetz really love this approach. Really, really, love it.
:beer:

 

[AFB]

Originally posted by: spidey07
cmetz is on the money.


Or just design the net to not rely on spanning-tree at all.
Spidey and Cmetz really love this approach. Really, really, love it.
:beer:

Please explain.