[c-nsp] Question about Multiple Spanning Tee (MST)

[David Hughes]

Hi Andy,

How is your PVST+ configured?  Usually your STP configuration is based  
on your L2 topology. If you have a "normal" dual-uplinked access layer  
then you are probably "load balancing" your vlans over your uplinks on  
a per-vlan basis using root bridge priorities.

If that's the sort of setup you are looking for then In MST mode  
there's really no reason to have more than 2 instances.  Having more  
instances than you have paths to your root bridges will just result in  
multiple instances blocking on the same ports.  If you have dual paths  
and run 2 instancess then you can "load balance" your vlans by mapping  
them into the 2 different instances.  The granularity of your "load  
balancing" will depend on your traffic patterns.  Something like the  
below can work well.  Just swap the priorities on the second root  
switch.

----
spanning-tree mode mst
!
spanning-tree mst configuration
  instance 1 vlan 1-49, 100-149, 200-249, 300-349, 400-449, 500-549,  
600-649
  instance 1 vlan 700-749, 800-849, 900-949, 1000-1049, 1100-1149,  
1200-1249
  instance 1 vlan 1300-1349, 1400-1449, 1500-1549, 1600-1649, 1700-1749
  instance 1 vlan 1800-1849, 1900-1949
  instance 2 vlan 50-99, 150-199, 250-299, 350-399, 450-499, 550-599,  
650-699
  instance 2 vlan 750-799, 850-899, 950-999, 1050-1099, 1150-1199,  
1250-1299
  instance 2 vlan 1350-1399, 1450-1499, 1550-1599, 1650-1699, 1750-1799
  instance 2 vlan 1850-1899, 1950-1999
!
spanning-tree mst 0-1 priority 8192
spanning-tree mst 2 priority 16384
----


David
...


On 24/04/2009, at 12:58 PM, Andy Saykao wrote:

> Hi All,
>
> Our switch network needs to be migrated from PVST+ to MST in order for
> our Cisco switches to be able to speak RSTP to some non-cisco  
> switches.
> Given that we have a few hundred vlans configured, is there some best
> practice to determine how many instances we need or can we basically  
> do
> whatever we want?
>
> For example:
>
> Option 1/ If we have 500 vlans, do we split them into two instances
> (instance 1 vlan 1-250 and instance 2 vlan 251-500).
>
> Options2/ Or would you split the vlans up based on their business
> requirements.
> Eg: vlan 1,10,100 Marketing-related vlans, vlan 2,20,200 Sales-related
> vlans - therefore create instance 1 vlan 1,10,100 and instance 2 vlan
> 2,20,200???
>
> Secondly, what if you went with option 2, and added a new vlan into
> Marketing (eg: vlan 1000). Would you have to manually update the
> spanning-tree mst config on all MST switches to include the new vlan  
> in
> the instance???
>
> Thanks.
>
> Andy
>
>
>
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