Spanning tree question.

[Nikos Nicolaides]

Hello community,

I had a spanning-tree issue a while ago with 3 switches and I am cracking my head of why this happened. I need an explenation for future reference.

I had a 3750 acting as a L3 switch and a 2960 trunked behind the 3750. All switches had IP phones plugged in.

I have put an old 2900 or 3550 switch behind the 2960, in trunk and configured to allow only the Voice VLAN. No redundant uplinks.

When i put the old switch, phones in the 2960 (middle) were disconnected and were in 'Registering' phase.

The old switch was the STP root for the voice VLAN. I have changed STP priority and everything worked fine.

My question is: why did this happen? STP is a loop prevending mechanism. I had no loops. But I caused partial downtime.

And even if the old switch was the root bridge, it would still try and send traffic to the default gateway, right?

Can anyone explain please?

TIA,

Nicos Nicolaides       

[Jon Marshall]

Nicos

How long was the downtime.

When you added the switch and if the trunk allowed all vlans then it may be that -

1) the switch added had a lower STP priority for the vlans. This is unlikely unless you manually set it up

or

2) more likely alll switches had the default STP priority but the new switch had the lowest mac address. If this is the case the new switch will become the root bridge and so the other switches need to reconverge because now they have different ports to get to the root bridge.

Whilst this is happening data will not be forwarded on these links.

Here it doesn't really matter whether there are redundant links or not. STP still has to work out a loop free topology in relation to the root bridge even if there are no loops in your network.

The way to avoid this happening again is to pick a switch (probably your 3750) and manually set a lower STP priortiy for all vlans on this switch.

Jon

[Nikos Nicolaides]

Well the downtime was much more than the normal 50-second one, it was more like 5-10 minutes.

The switch had the lowest MAC address. This is not what I am asking.

What I am asking is whether this is the normal operation? Should switch 2960 not be able to forward any packets to the 3750 because it forwards everything to the 'root' bridge (being the 2900)?

[Jon Marshall · Accepted solution]

Nicos

What I am asking is whether this is the normal operation? Should switch 2960 not be able to forward any packets to the 3750 because it forwards everything to the 'root' bridge (being the 2900)?

STP is only used to work out a loop free topology ie. it creates loop free paths within the network. Once it has done this there is no requirement for traffic to go via the root bridge, it simply goes the most direct route on the the active links.

So the 2960 would send packets direct to the 3750 ie. it does not have to go via the root bridge as long as it's link to the 3750 is not blocking which in your setup it won't be.

Jon