stp root bridge change

[gopann]

I have a lan consists of 27 access switches (2950s/3548s). There is a core switch 6006. Access switches are stacked in diffrent floors. Total there are 7 stacks. From each stack there are two fiber uplinks collapsing to core. There are 22 VLANs.

Issue here is the STP root bridge one access switch for some VLAN and the router for one VLAN, not the core switch. This happened due to STP algorithm which takes the switch with lowest MAC address as root switch when all switch has same bridge priority. And in one VLAN where router is there, because of bridging enabled in router the router becomes STP root bridge, same MAC address way. Rest all VLANs have root bridge that simple access switch.

This is giving me issues when I change some backbone cables. I am planning to manipulate STP and make my core switch as root bridge. I will do this per vlan basis. In what all way I can achieve this? Do this can make any serious impact on the network uptime. My setup is very critical as of now.

Please help me. Thanx

Regds

Gopa

[konigl · Accepted solution]

Gopa,

On the 6006, you can either use the macro command "set spantree root x" or the specific command "set spantree priority yyy x" where "x" is the VLAN number, and "yyy" is a bridge priority 0-65535.

If you use the macro command, it will run an algorithm that determines a bridge priority number to make the 6006 the Root Bridge (RB) for that VLAN. (On a 6506 I manage, it defaulted the priority on my test VLAN to 8192; clearing it set the priority back to 32768.)

I prefer to run the specific command and give it my own priority number, like 10 for the primary RB, and 20 for my secondary RB. That way I can be sure that I control the RB.

If STP is running with default timers on that VLAN, it will take 30 to 50 seconds for that VLAN to reconverge. No traffic other than BPDUs flows during that time.

Once your 6006 is the Root Bridge for that VLAN, you can tune the parameters (hello, maxage, fwddelay) that affect how fast STP reconverges on that VLAN. You need to know your topology exactly before you start tinkering with these; and what your Network Diameter on that VLAN is.

Since all the access switches you're using are Cisco 2950s and 3548s, you're probably better off just configuring Spanning Tree UplinkFast on them to cut their reconvergence time down to 3 to 5 seconds.

Suggestion: Get the 6006 to be the RB for all 22 of your VLANs first; then put UplinkFast on all your access switches. There should be no STP downtime on any of the VLANs when you enable UplinkFast this way; and after it's done, you can change fiber backbone cables around as you wish with minimal downtime.

Hope this helps.

[milan.kulik]

Hi,

just one addition:

It's possible to run

set spantree root x dia y

(or even set spantree root x dia y hello z)

macro which will set the timers correctly for you.

A good link to understand STP timers details:

http://www.cisco.com/warp/customer/473/122.html

Regards,

Milan

[gopann]

Hi,

Thank you very much, friends. I completed this activity using "set spantree priority" command for individual vlans. Reduced the bridge priority of the core switch to 4096 manually. Surprisingly there were no complaints from users of network disconnection. Only when I did this for VLAN1 there was around one minute downtime for the previous root bridge, ie one access switch which became root bridge on election. Rest all were fine. For other VLANs no downtime at all. Great stuff..

Thank you very much for your support.

Best regards

Gopa