[c-nsp] Maximum spannig tree instances

[David Hughes]

On 15/07/2009, at 4:01 AM, Jon Lewis wrote:

> The cisco examples I saw say to leave MST0 empty and use MST1 and  
> MST2 for VLANs.

Good option.  Only non-MST speakers will end up in instance 0.  Spread  
your vlans over instance 1 and 2 (and root those instances  
appropriately) and all will be good.  We use blocks of 50 vlans for  
the "load sharing" which gives us what we need and keeps the config  
small.

> Will adding new VLANs to an MST instance disrupt traffic flow for  
> other VLANs in that MST instance?
>
> The topology I have is actually 2 core switches with a bunch of edge  
> switches redundantly uplinked to both cores.

Not sure.  We pre-configure the MST vlan mappings (see below) and just  
prune vlans on the trunks.  We run the same MST config on every switch  
in the network and will worry about changing the vlan mappings when we  
have more than 2000 vlans in a single layer 2 domain.  I can't see  
that being a problem for any of the L2's at any of our datacentres for  
a while.  For us, once we got MST in place it's been set-and-forget.   
It's worked flawlessly.

---
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
---


Thanks

David
...