HOW DOES MST WORK
- MST works by decoupling VLAN and STP Instance
- STP instance to VLAN mapping is user defined
- Topology calculation is done by RSTP
- Result is higher scalability
- (Rapid)PVST+ uses one instance per VLAN
- As VLANS scale, control plane dies
MST Regions
- Devices that agree in these three parameters are in the same region
- Instance name
- Revision number
- VLAN to STP instance mappings
Intra vs Inter Region
- Intra Region
- Details of the region are known within the region
- VLAN to STPIs are manually defined
- Undefined VLANS fall into CIST (MST 0)
- Inter Region
- details between regions are not known
- Different regions see each other as virtual bridges
- Result is simplified Inter-region calculation
- Intra-region MSTIs are collapsed into CIST
MST Configuration
- Define the following in MST configuration mode
- region name
- revision number
- vlan to instance mappings
- Enable MST globally
- real deployment must start at Root and work out
MST Path Selection
- Same election process as CST/PVST
- Root bridge
- Lowest BID
- Root port
- Lowest cost
- Lowest upstream BID
- Lowest port ID
Changing MST Root Bridge Election
- Manually change BID priority
- spanning-tree mst (instance) priority
- Lower is better
- use root bridge macro
- spanning-tree mst (instance) root (primary|secondary)
- sets local priority based on current root bridge
-Verification
- show spanning-tree mst (instance)
- show spanning-tree root
Changing an MST Port's Role
- Modify the port's cost
- spanning-tree mst (instance) cost
- bandwidth (bps)
- Modify the brdge ID
- spanning-tree mst (instance) priority
- Modify the Port ID
- spanning-tree mst (instance) port-priority
- Verification
- show spanning-tree interface (int) detail
- show spanning-tree mst (instance) detail
MST Interoperability
- MST is backwards compatible with legacy CST and PVST+
- behaves like inter-region MST
- CST Root must be within MST domain
No comments:
Post a Comment