Impact of moving Spanning-tree Root Bridge for multiple VLANs?

[pmchandler]

We had a new network installed by a third party which, due to the wonder of VSS, is resilient but does not contain any loops. Unfortunately they did not fix the root bridges to the core switch (VSS pair). As we now have spanning tree root bridges spread all across our network (including some in some very undesirable locations), I would like to manually configure the core switch as our root bridge for all VLANs.

We have tested this with some user vlans and some vlans in the data centre whilst maintaining SSH sessions and pings etc and did not notice any impact. I now need to do this for the remaining VLANs including the data centre farm VLANs and a legacy VLAN 1 whose root bridge currently lies in a different data centre across a high speed LAN extension.

Based on the Root Bridges we have moved so far, I am not anticipating any impacts. However, I would like to tap the greater minds out there to see if anyone knows of any theoretical or real-world reasons why there may be some impact. This chage will be done via change control in a maintenance window but I would like to assure the 'pwers that be' that there will be no impact to live services.

Thanks in advance.

[Florin Barhala]

Hi Paul,

What STP flavor you run? Do you have the network diagram? If all other equipments are to default, I see no problem, probably some ports will maintain their status while other will be shut. As long as we don't know each VLAN network span we cannot predict all the changes, but as I told you if no special configuration are made on ports, things will go right.

[pmchandler]

Thanks for getting back to me. We are only running legacy PVST+ (had we realised, we would have requested RPVST given that all switches support it - all 6509E).

I have uploaded a schematic that is a fair representation of our network. As you can see, it is a star based topology which is resilient (due to VSS) but does not have any inherent loops.We have several dozen VLANs and almost every switch you see in the diagram is a root bridge for at least one of them. We would like to make the core switch the root bridge for all vlans.

I should add that the Primary Data Centre has its own VLANs and the Secondary Data has separate VLANs so I am only talking about the primary DC VLANs. However, for legacy reasons, we have VLAN 1 which spans both DCs and the root bridge is currently located at the secondary Data Centre. We would also like to make its root bridge the core switch in the main DC. Although I am concerned about all VLANs it is VLAN 1 that I am most concerned about as it is this (and only this) VLAN that traversses the link from the primary to the secondary data centre. My concern is whether it may disrupt the DC interconnect.

Regards

Paul.

[pmchandler]

I should add, just to make it clear, that as there are no loops, all links are forwarding. Moving the root bridge should keep all links in the forwarding state - at least that is my understanding and experience based on the root bridges for the user edge vlans we have already moved to the Core.

The reason I want to make the core root bridge for all VLANs is so that everything is much more predictable in the event an unintended loop is introduced into the network (as has happened due to end user error).

Regards

Paul