Introduction to Multiple spanning tree protocol-MST

Today I am going to talk about MST and stands for Multiple spanning tree protocol. The purpose of MST is basically providing the capabilities of rapid convergence and load balancing in a VLAN environment. As many of you studied about STP, RSTP, PVSTP and PVSTP+, the convergence time of MST is much faster than PVSTP+ and has compatibility with both PVSTP+ and RSTP LAN network design.

MST permits to construct multiple spanning trees over trunks. We can group and associate VLANs to spanning tree instances. Every instance can have a topology self-determining of other spanning tree instances. This design offers multiple redirecting paths for data traffic and permits load balancing. Network error tolerance is enhanced because a catastrophe in one occurrence does not disturb other cases.

Fig 1.1- Sample MST Topology-Physical and Virtual
Well talk about Spanning tree, A spanning tree instance can occur only on bridges that have well-matched VLAN occurrence allocations. We need to design a set of bridges with the same MST configuration data, which permits them to contribute in a precise set of spanning tree instances.

MST Configuration Restrictions and Guidelines
  • Do not deactivate spanning tree on any VLAN in any of the PVST bridges.
  • Do no use PVST bridges as the root of CST.
  • Do not connect switches with access links since access links may partition a VLAN.
  • Ensure that all PVST root bridges have lower, priority than the CST root bridge.
  • Ensure that trunks carry all of the VLANs mapped to an instance or do not carry any VLANs at all for this instance.
  • Complete any MST configuration that incorporates a large number of either existing or new logical VLAN ports during a maintenance window because the complete MST database gets reinitialised for any incremental change

RouteXP_SW # configure terminal
RouteXP_SW (config)# spanning-tree mode mst
RouteXP_SW (config)# spanning-tree mst configuration

Now let’s have a look on how to configure MST instance parameters
RouteXP_SW (config)# spanning-tree mst 1 priority?
<0-61440> bridge priority in increments of 4096

RouteXP_SW (config)# spanning-tree mst 1 priority 1
% Bridge Priority must be in increments of 4096.
% Allowed values are:
0 4096 8192 12288 16384 20480 24576 28672
32768 36864 40960 45056 49152 53248 57344 61440

RouteXP_SW (config)# spanning-tree mst 1 priority 49152
RouteXP_SW (config)# spanning-tree mst 0 root primary
RouteXP_SW # show spanning-tree mst

###### MST00 vlans mapped: 11-4094
Bridge address 00d0.00b8.1400 priority 24576 (24576 sysid 0)
Root this switch for CST and IST
Configured hello time 2, forward delay 15, max age 20, max hops 20

Interface Role Sts Cost Prio.Nbr Status
---------------- ---- --- --------- -------- --------------------------------
Fa4/4 Back BLK 1000 240.196 P2p
Fa4/5 Desg FWD 200000 128.197 P2p
Fa4/48 Desg FWD 200000 128.240 P2p Bound (STP)

###### MST01 vlans mapped: 1-10
Bridge address 00d0.00b8.1400 priority 49153 (49152 sysid 1)
Root this switch for MST01

Interface Role Sts Cost Prio.Nbr Status
---------------- ---- --- --------- -------- --------------------------------
Fa4/4 Back BLK 1000 160.196 P2p
Fa4/5 Desg FWD 200000 128.197 P2p
Fa4/48 Boun FWD 200000 128.240 P2p Bound(STP)


Configuring MST Instance Port Parameters

RouteXP_SW (config)# interface fast Ethernet 4/2
RouteXP_SW (config-if) # spanning-tree mst 1?
RouteXP_SW (config-if) # spanning-tree mst 1 cost 1234567
RouteXP_SW (config-if) # spanning-tree mst 1 port-priority 240
RouteXP_SW # show spanning-tree mst 1 interface fast Ethernet 4/2

FastEthernet4/2 of MST01 is backup blocking
Edge port: no (default) port guard: none (default)
Link type:point-to-point (auto) bpdu filter:disable (default)
Boundary :internal bpdu guard :disable (default)
Bpdus (MRecords) sent 125, received 1782

Instance Role Sts Cost Prio.Nbr Vlans mapped
-------- ---- --- --------- -------- -------------------------------
1 Back BLK 1234567 240.196 1-10

RouteXP_SW #

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