Advanced STP: MST with multiple regions.

Lessons learned:

MST region is defined by:

-Vlan to instance mapping

-Region name

-Region number

Inter-Region path selection uses a CST (common spanning tree)

-intra-region MSTI’s are collapsed into a CIST

Note: multiple MST regions are collapsed into a CIST (common internal spanning-tree) – this is represented by instance #0

MST is backward compatible with legacy CST (802.1d) and PVST+

Behaves like Inter-regions MST

CST root MUST be within the MST Domain

The MST basically spoofs a single CST if doing backwards compatibility Or spoof an individual instance on a per-vlan basis

From a PVST perspective if you have a 100 vlans, you’d have 100 instances of STP.  When the MST domain talks to the PVST + domain, The MST instance 0 will replicate the same information over and over into the PVST instances.

Basically the root have to be elected within the MST domain, if elected in the CST or PVST – the interchangeability will not work.

Configuration -

Only best to configure regions in MST in a large environments.

On Switch 1 – I have the following config:

My MST region 1 has instances One and Two.  

Switch01#sh run | b spanning-tree

spanning-tree mode mst

spanning-tree extend system-id

!

spanning-tree mst configuration

 name MST1

 revision 1

 instance 1 vlan 10, 20, 40, 50

 instance 2 vlan 60, 70, 80

For Switch 2:

I’m going to configure a new MST

spanning-tree mst configuration

 name MST2

 revision 2

 instance 3 vlan 10, 20, 40, 50

 instance 4 vlan 60, 70, 80

I will still need the same vlans overall, because the job of L2 is still to span the broadcast domains.

Switch01#sh spanning-tree mst

Notice now we have what’s know as , “Regional Root this switch”

##### MST0    vlans mapped:   1-9,11-19,21-39,41-49,51-59,61-69,71-79

                               81-4094

Bridge        address 000c.303b.ba80  priority      32768 (32768 sysid 0)

Root          address 0003.e347.b900  priority      32768 (32768 sysid 0)

              port    Fa0/1           path cost     200000  

Regional Root this switch

Operational   hello time 2 , forward delay 15, max age 20, txholdcount 6

Configured    hello time 2 , forward delay 15, max age 20, max hops    20

Interface        Role Sts Cost      Prio.Nbr Type

---------------- ---- --- --------- -------- --------------------------------

Fa0/1            Root FWD 200000    128.1    P2p Bound(STP)

Fa0/24           Desg FWD 200000    128.24   P2p Pre-STD-Rx

Notice on SW2: it says “IST master  this switch” This is the internal STP

Switch-02#sh spanning-tree mst

###### MST00        vlans mapped:   1-9,11-19,21-39,41-49,51-59,61-69

                                     71-79,81-4094

Bridge      address 0013.60bb.4980  priority  32768 (32768 sysid 0)

Root        address 0003.e347.b900  priority  32768 (32768 sysid 0)

            port    Fa0/24          path cost 400000  

IST master  this switch

Operational hello time 2, forward delay 15, max age 20

Configured  hello time 2, forward delay 15, max age 20, max hops 20

Interface        Role Sts Cost      Prio.Nbr Type

---------------- ---- --- --------- -------- --------------------------------

Fa0/24           Root FWD 200000    128.24   P2p Bound(RSTP)

=========================================================

Basically with regions you’re collapsing a bunch of switches into one virtual instance. So you could have a thousand switches in each region represented by a single virtual bridge ID for Inter-Region communication.

This is the region root. His job is to talk to all the other regions.

Note:

Interface        Role Sts Cost      Prio.Nbr Type

---------------- ---- --- --------- -------- --------------------------------

Fa0/24           Boun FWD 200000    128.24   P2p Bound(RSTP) – this shows this link is a boundary link that is connecting to another RSTP Domain. Since we’re talking from MST to MST this is a RSTP interaction.

Inside the the individual regions – any type of path selection that we do is not going to go outside the region. You cannot change the path selection between the Inter-region on a per-instance basis, for example traffic between vlan 10 and vlan 20 form the Inter-region perspective.

If you want to modify the path section inside the each region, you need to change the MST cost.

Config between the MST and Inter-region MST is relatively similar, the key-point is that the common spanning-tree root which is for all the regions.  Densest matter where it is, it only matters that it’s NOT in the legacy STP domain or the PVST domain.