STP parameters

[ikizoo4]

Hello

I configured 2 3550 switches with ISL trunk, and switch #1 is server and another is client under same VTP domain, switch #1 is root switch, There is a vlan named VLAN_A, here is thing, i'd like to change STP hello timer of VLAN_A

Q, I changed Hello timer of VLAN_A on root switch switch#1, but on switch#2 has still old Hello timer value, do I have to change Hello timer both switch manually? VTP is enabled and works fine.

thanks

[ankurbhasin]

Hi Iki,

The hello timer is the time interval (in seconds) at which the root device transmits a configuration message via BPDU packets.

So if you change the hello timer on root switch it will take care of the issue and you need not change it manually on the downstream switches.

BPDU always travel from the root to the other switches.

HTH

Ankur

[ikizoo4]

Hi Ankur

In first place, I thought exact same as you, but like I said before, on switch#2, hello timer of VLAN_A is unchanged,,, then just ignore those unchanged hello timer of downstream switches, is this what you mean?

thanks

[ankurbhasin]

Hi Iki,

Yes that is what I mean you need not change it on downstream switch because BPDU is always generated by root switch so if you change it on root it will send with the change timer and it will be fwded by the switches on way with same timer.

So for suppose you change it on root bridge to 1 it will keep on sending the BPDU every 1 second to the downstream switches and each downtream switches will just fwd it.

Remember if it is a small network of few switches like in your case 2 switches it is ok to change the STP default timers but if incase it is a big network there are chances of loop also if you play around with the timers so just take care of that part.

HTH

Ankur

[glen.grant]

It's normally not recommended to mess with the defaults for spanning tree . If you are worried about convergence time maybe you should think about implementing rapid spanning tree .

[ikizoo4]

Hi

Thanks for answering, one more thing, VTP makes share the vlan information between switches, once vlan information is changed on root switch(VTP server mode), then other switches under same VTP domain should share those vlan informations, how come switch#2 still have old Hello timer?

Regards.

[Francois Tallet]

VTP indeed propagates the vlan information but the spanning tree parameter is not a vlan parameter. Actually, Cisco CLI is not very clear in this regards because it mixes the vlan and the STP instance running on the vlan. When you see "spanning-tree vlan X", you must understand "spanning instance-running-on-vlan X blah". This is because until recently Cisco switches were only running PVST (per-vlan STP) where each vlan has its own STP instance.

Regards,

Francois

[ikizoo4]

Francois,

thanks for answering, so when i change STP parameter of a vlan on root switch then it will happen inconsistency of STP parameters and to fix this inconsistency, what do i have to do? changing STP parameters on every switches (include root switch) manually is only solution?

Thanks a lot.

[Francois Tallet]

Hi,

The timer values are propagated inside the BPDUs that are sent by the root. So if your other bridges see the correct root, they will also accept the timer values from this root. Considering the parallel you make with VTP, I have the feeling that you expect STP to change the configuration of your other switches to match the one of the root. This will not happen. Only the *operational* values of the timers (hello-time, max-age and forward-delay) are modified to match the root information. You should be able to see that with the "show spanning-tree [detail]" command. STP does not change the configuration of the switch: the "show run" will be unaffected.

Hope this is the source of your concern;-)

Regards,

Francois