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MSTP configuration between Core and Distribution block of a network

Posted on 2012-09-16
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Regarding this topic, we are experiencing problems in my company because of different spanning-tree modes in Core and Distribution blocks so we want to migrate to a topology with MSTP protocol in all the network and I attach a map with a reduced version or our network including extracts of the MSTP configuration for every block.

We are going to use 3 regions:
WAN. It contains one instance for the vlan that interconnect Core with WAN routers (Internet in network map)
DIST. It contains one instance for the vlan that interconnect Core with Distribution Layer3 switches.
ACC. It contains 2 instances with the final user vlans.

The Core equipment uses only WAN and DIST regions, the Distribution equipment uses only DIST and ACC regions and the Access equipment uses only ACC region.

The main doubt we have is if it makes sense that we use a dedicated region for the network that interconnect Core and Distribution equipment. Is it going to work properly without loops?. Is there any other possibility more efficient? (the same situation applies to the network between Core and WAN routers).

Thank you very much for your help.

P.D. We understand that it's not necessary to include ACC region in Core equipment since the final user vlans are configured in Distribution equipment but not in Core. Do you agree?
MSTP-Core-Dist.jpg
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Question by:eltenista10
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by:Don Johnston
ID: 38403107
Why do you want separate regions?

Typically, if you want to limit the spanning tree domain, you would use a layer 3 device (like your distribution switches).
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Author Comment

by:eltenista10
ID: 38403153
But between Core and Distribution equipment there are as well redundant links with possibility of loops so I think we would have to use any spanning-tree protocol to achieve that "Core1" is the root for vlan25 (see map).

Assuming this and that we have used SMTP in Distribution equipment, I think that it's mandatory to use as well SMTP in Core equipment but I'm not sure. If that's correct, I see logical to configure a region that contains only this vlan 25 which interconnects Core-Distribution.

However I'm not sure if this solution would work properly and stable or if there are other possibilities more efficient that you could provide. Thanks a lot.
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Expert Comment

by:Don Johnston
ID: 38403193
Why do you want to have separate regions? Why not use a single region?
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Author Comment

by:eltenista10
ID: 38403238
Yes, we have thought about the possibility of configuring in all the switches of the network the same region, for example:

spanning-tree mst configuration
 name MyRegion
 revision 1
 instance 5 vlan 1,5
 instance 15 vlan 10,15
 instance 25 vlan 25
 instance 30 vlan 30
!

However, it means that we have to configure in Core equipment instances of SMTP that include final user vlans (1,5,10,15) which are not configured in Core equipment but in Distribution and Access equipment.

Likewise, it also means configuring in access switches instances with vlan 25 or 30 which are defined in Distribution or Core blocks. Is that correct?. I think this can cause problems in the network...
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Expert Comment

by:Don Johnston
ID: 38403250
I would either configure MSTP the same in all switches or make the layer 2 boundary at the distribution switches and route traffic to the core.
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Author Comment

by:eltenista10
ID: 38403294
In case of making layer 2 boundary at the distribution switches, which type of spanning-tree should we configure in Core equipment to achieve that CORE1 is root for vlan25 (interconnect Distribution-Core)?

We maybe are going to test with a single SMTP region for the entire network but, do you think the other solution with 3 regions should work fine as well?. Thanks.
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Expert Comment

by:Don Johnston
ID: 38403317
I think that using three regions is unnecessarily complicated. Which will make troubleshooting significantly more difficult.  But it will work.

If you put the L2 boundary at the distribution layer, I would do layer 3 above that (no STP). Let the routing protocol handle path selection.
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Author Comment

by:eltenista10
ID: 38403396
OK, thank you very much for your help Don.

The last doubt I have is to know if I can configure in a switch a SMTP instance associated with a vlan which is not configured at layer 2 level in that switch. For example the vlan 4 in the following configuration:

spanning-tree mst configuration
 name MyRegion
 revision 1
 instance 1 vlan 1,2
 instance 2 vlan 3,4
!

myswitchl3# sh vlan brief

VLAN Name                             Status    
---- -------------------------------- --------- -------------
1    default                               active    
2   MyVLAN2                            active    
3   MyVLAN3                            active
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Don Johnston earned 1500 total points
ID: 38403423
The last doubt I have is to know if I can configure in a switch a SMTP instance associated with a vlan which is not configured at layer 2 level in that switch

No. IIRC, you'll get an error message when you try to associate a non-existing VLAN with an MST instance.
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