stp root and secondary switches

[richardmcmahon]

Hi,

We have a pair of 7609 routers for core switches. Each access switch (2950T-48SI) has a connection to each 7609 using its gig ports. If I always want one 7609 to be the stp root and the other to be the stp root secondary would the following commands work as I would expect

Primary

spanning-tree vlan #id# priority 0

Secondary

spanning-tree vlan #id# priority 1

And also issue the global spanning-tree uplinfast command on all access switches.

I realise that there is the following command "spanning-tree vlan #id# root primary/secondary" but the docs I've read indicate that this runs a macro on the switch and is not an actual config command as such in the running-config. Can anyone clear up what the best set of commands would be in my instance.

Thanks,

Richard

[sundar.palaniappan]

Richard,

If you want to designate one switch to be the primary root and the other one as a secondary root then configuring them with a priority of 0 and 1 should suffice.

The second method that you mentioned, root primary or secondary keyword, would be a good option if you aren't familiar with your LAN topology and are unsure about the priorities set on other switches. When configuring 'spanning-tree vlan (id) primary root' the switch would lower the priority to 100 less than the current root bridge to force it to become the root. This is a dynamic approach to make one switch to be the root bridge for a specific VLAN.

You can choose option 1 provided all other distribution/access switches are set to use the default priority of 32768.

HTH

Sundar

[johnnylingo]

We have a similar situation but with 6513s.

What is the impact of changing the spanning tree roots? Will there be an outage while the topology table is rebuilt?

[mahmoodmkl]

Hi

There will be a temporary outage as STP takes time to converge may be around 50 seconds.But take care before u change the u r spanning tree roots u need to have complete understanding for u network topology.

Thanks

Mahmood

[Anand Narayana]

hi,

i have got 6509(catOS)with L3 module as a main & other as 3750 standby, running hsrp, so all the edge switches are one connected to 6509 & other to 3750, i never done one as root primary & the other as root secondary. so in my case how do i do that? since u say that special care has 2 taken care b4 making as root, i wanted to the how to go abt this.

[mahmoodmkl]

Hi

As u have said that u have one 6509 and 3750.which one is acting as the root right now.i think 6509.

And i think u r runngin pvst+ in u r environment.so as u know that each vlan will be having a seprate stp instance.so if u modify the spanning-tree priorities u may end up having differnt priorities for differnt vlans on differnt switches accordning to it the switches will become root for some vlans and others may be root for other vlans.

So to have a stable STP topology u can have one primary and one secondry.for example if u make u r 6509 as the primary assign it a priority for 8192 for all the vlans.and 3750 u can go ahead with 16384 for all the vlans in this case 6509 will be ur primary and 3750 will be u r secondry.

Thanks

Mahmood

[Anand Narayana]

since i am not that much familiar with the STP, can u help me outhow to make the configuration related to that?

[mahmoodmkl]

HI

hope this link will help you.

http://www.cisco.com/en/US/products/hw/switches/ps700/products_configuration_guide_chapter09186a008007fa34.html

Thanks

Mahmood

[rparunr015068]

Just to clarify, if we change a root bridge in the existing network, will that cause the 50 sec outage always (of course between root switch and other switches)?

[Anand Narayana]

this is my present condition in my 6509 & 3750, juz to know whether this behaviour is normal or not. find the attachement