Spanning tree

Unanswered Question
Apr 26th, 2010

As shown in the attached diagram, I have a connectivity between two data centres.


Switch A an B are in same data centre and C & D are in same data centre.

currently two data centres are connected via a L2 trunk link and I need to have a another connectivity for resiliency. The new connectivity will be between Switch A in data centre 1 and Switch D in data centre 2.


Currently Spanning Tree priority is as below for the 900 vlan.

SW B - 8192

SW A – 16384


SW C - 8192
SW D - 16384


Can you please advice on the Spanning Tree priority to be followed when we connect XR0306SW D and SW A with the new Link.

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Giuseppe Larosa Mon, 04/26/2010 - 09:46

Hello Mathew,

you have already joined the two datacenters in vlan 900.

so one of the two switches with the lowest priority ( SW B or SW C)  has been elected the root bridge

you can check this with

sh spanning-tree vlan 900

STP priorities are global parameters and cannot be changed per link.

Switch B or Switch C is the root bridge for vlan 900.

When you add the A-D link the new link triggers an STP calculation, that should end up on a blocked link.

Hope to help

Giuseppe

mathew-varghese Mon, 04/26/2010 - 10:03

Hi Giuseppe,

Thanks alot for your reply ..

I know STP configuration is a global configurtaion and cannot be changed per link. My question is that do I need to have different STP priority for each switch when I will have another connectivity between SW A and SW D.

Regards

Mathew

Giuseppe Larosa Mon, 04/26/2010 - 12:43

Hello Mathew,

whatever choice of priorities you do one link will be in STP blocking state so I would not make any change.

You will have redundancy but not load balancing.

Hope to help

Giuseppe

mathew-varghese Mon, 04/26/2010 - 21:50

Hi Giuseppe,

Thanks for your reply.

As all the four switches will be in STP domain , Do I need to keep four differenet priority or I can do as below

SW B - 8192

SW A –  16384


SW C -  8192
SW D - 16384

Regards

Mathew

shailesh.h Mon, 04/26/2010 - 15:09

Few of lines in L-2 extended Data Centre are

Cisco does not recommend extending the Spanning Tree Protocol domain beyond the campus. Spanning Tree

Protocol is a very conservative protocol that favors loss of connectivity over temporary looping during its operation.

As a result, a spanning tree reconvergence generally generates momentary interruption in frame forwarding.

Because the different data centers are independent bridged domains, it is beneficial to isolate their respective

spanning trees. This way, a change in a particular data center will not cause transient connectivity problems or

superfluous flooding in another data center.

Use STP filtering across two DCs

mathew-varghese Mon, 04/26/2010 - 21:51

Hi Shailesh,

Thanks for your feedback.

Our data centres are designed as the same way I explained, still eager to know how to do the filtering . Can you please provide some documents on this ?

Regards

Mathew

Giuseppe Larosa Mon, 04/26/2010 - 23:30

Hello Mathew,

you can use STP BPDU filtering only if you have ONLY one link between the two datacenters

if you want to have two links, the existing one between SW B and SW C , and the planned one between SW A and SW D you need to use STP and as I have described one link of the four links interconnecting the switches will be in STP blocking state.

WARNING: if you use STP BPDU filtering with two links between the two datacenters you build a loop and the network will be turned down.

Shailesh's suggestion can be used only on current scenario.

Eventually the single link can be changed in an etherchannel to have link redundancy but you cannot achieve node redundancy in interconnecting the two data centers without using  STP or without forming a VSS pair or a stack between colocated switches (this depends on platform in use)

Hope to help

Giuseppe

mathew-varghese Mon, 04/26/2010 - 23:36

Hi,

I agree with your comments to have STP if I am connecting SW A and SW D but do I need to keep four different priority ?

Regards

Mathew

Giuseppe Larosa Tue, 04/27/2010 - 00:18

Hello Mathew,

>> I agree with your comments to have STP if I am connecting SW A and SW D  but do I need to keep four different priority ?

even if you use equal priorities on the 4 switches they will be ordered based on the lowest MAC address

Bridge ID =  STP priority + MAC address

lowest wins

of the 4 links one will be blocked and changing the priorities will only influence which link of the 4 will be blocked.

With current settings it is likely that new link between the SW A and SW D will be the one with one side in STP blocking state

I would keep the current STP priorities for this reason.

Hope to help

Giuseppe

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