MST region and instances

[deepak george]

Dear All,

I have a query regarding MST, I have attached the topology, i am suing 4500 switches as the CORE in the LAn network and many 2960,3850 as access switches, am also using Nexus switches. I would like to use Multiple spanning tree(MST) in the network. My plan was to make the 4500 VSS core as the root switch. However am confused as to how many regions and instances do i create. Please give ur valuable views on this as to how do i go about and apply MST on the network

[Reza Sharifi]

Hi,

Usually you don't need more than one region and one instance unless you have a lots of vlans and you want to load balance traffic. Multiple regions with multiple instance can make things complicated and hard to manage.

For switches to participate in multiple spanning-tree (MST) instances, you must consistently configure the switches with the same MST configuration information. A collection of interconnected switches that have the same MST configuration comprises an MST region as shown in Figure 19-1.

The MST configuration controls to which MST region each switch belongs. The configuration includes the name of the region, the revision number, and the MST VLAN-to-instance assignment map. You configure the switch for a region by using the spanning-tree mst configuration global configuration command, after which the switch enters the MST configuration mode. From this mode, you can map VLANs to an MST instance by using the instance MST configuration command, specify the region name by using the name MST configuration command, and set the revision number by using the revision MST configuration command.

A region can have one or multiple members with the same MST configuration. Each member must be capable of processing RSTP bridge protocol data units (BPDUs). There is no limit to the number of MST regions in a network, but each region can support up to 65 spanning-tree instances. Instances can be identified by any number in the range from 0 to 4094. You can assign a VLAN to only one spanning-tree instance at a time.

link:

http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3560/software/release/12-2_52_se/configuration/guide/3560scg/swmstp.html

HTH

[deepak george]

Hi Reza,

Thanks for the reply, i will be running around 23 VLAn's in the network. So will one or two instances be better ??

[Reza Sharifi]

Hi,

For that many vlans, just one instance will do the job. Keep it simple and easy to manage.

HTH