MSTP Question

Answered Question
May 11th, 2010

Hi,

1. Can we configure two different MSTP regions in the same switch

2. We have two core switches interconnected between each other and we have two edge switches. each edge switch has uplinks to the two cores. In this scenario can we configure only one MSTP region and two local instances on all the switches

3. As each Edge siwtch has uplinks to the two cores, do I need to create two MSTP regions  (or can this be implemented with two MSTP regions as well)

Thanks,

Chandru

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Correct Answer by Giuseppe Larosa about 6 years 8 months ago

Hello Chandru,

you should use only one MST region spanning on all devices

core1 device can be the root bridge for selected MST instances

core2 device can be the root bridge for all the other MST instances.

you cannot have a single device in two MST regions and only the IST is runned between two different MST regions, single MST instances canno span over MST region boundaries.

if this is a campus network of an enterprise you need only one MST region and you can take advantage of different root bridge for different MST instances

The use of multiple MST regions is appropriate for a achieving scalability in a service provider network not in an enterprise.

Hope to help

Giuseppe

Correct Answer by Giuseppe Larosa about 6 years 8 months ago

Hello Chandru,

1) no, each switch can be only in one specific MST region

2) yes, all devices can be in the same MST region and you can deploy Vlans to MST instances mapping as you need

3) yes

very important:

MST region membership is checked when exchanging BPDUs so a change in mapping can cause the MST region to be partitioned

the recommendation is to provide mapping for all the 4094 possible vlans to all possible MST instances (16 or 64)

in this way when you need a new vlan you pick up one from the subset associated to a specific MST instance.

note: the vlans don't need to exist in vlan database to be mapped

This helps in avoiding unwanted partitioning during change

Hope to help

Giuseppe

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Correct Answer
Giuseppe Larosa Tue, 05/11/2010 - 03:10

Hello Chandru,

1) no, each switch can be only in one specific MST region

2) yes, all devices can be in the same MST region and you can deploy Vlans to MST instances mapping as you need

3) yes

very important:

MST region membership is checked when exchanging BPDUs so a change in mapping can cause the MST region to be partitioned

the recommendation is to provide mapping for all the 4094 possible vlans to all possible MST instances (16 or 64)

in this way when you need a new vlan you pick up one from the subset associated to a specific MST instance.

note: the vlans don't need to exist in vlan database to be mapped

This helps in avoiding unwanted partitioning during change

Hope to help

Giuseppe

csaravanan-sym Tue, 05/11/2010 - 03:27

Hi Giuseppe,

Thanks for your reply

I am still not understanding part 3 ( -- As each Edge siwtch has uplinks to the two cores, do I need to create two MSTP regions  (or can this be implemented with two MSTP regions as well)

so lets assume, I have one MST region mapped to each core (the cores are interconnected and the access switches are not interconnected with each other they reach each other through core switches) since each core is connected to both access switches. I will have to run two MSTP regions on the Access Switch right

It will not be possible to have two MST regions configured in this scenario right? Please explain

Thanks,

Chandru

Correct Answer
Giuseppe Larosa Tue, 05/11/2010 - 04:41

Hello Chandru,

you should use only one MST region spanning on all devices

core1 device can be the root bridge for selected MST instances

core2 device can be the root bridge for all the other MST instances.

you cannot have a single device in two MST regions and only the IST is runned between two different MST regions, single MST instances canno span over MST region boundaries.

if this is a campus network of an enterprise you need only one MST region and you can take advantage of different root bridge for different MST instances

The use of multiple MST regions is appropriate for a achieving scalability in a service provider network not in an enterprise.

Hope to help

Giuseppe

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