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Tag: 802.1s

CCIE: 802.1s (MST)

CCIE: 802.1s (MST)


MSTP, which uses RSTP for rapid convergence, enables VLANs to be grouped into a spanning-tree instance, with each instance having a spanning-tree topology independent of other spanning-tree instances. This architecture provides multiple forwarding paths for data traffic, enables load balancing, and reduces the number of spanning-tree instances required to support a large number of VLANs.

All the details can be found here.

My personal field experience:

It’s funny, I always hear how great MST is (and it does have advantages) but, I only saw it in a production enterprise environment a handful of times. The primary drivers are lots of VLANs (500+) and load balancing traffic (VLAN traffic engineering) across redundant links. Service providers could use this on small metro rings that utilize STP as the ring failover mechanism (RPR may be a better option).


For correct operation, all switches in the MST region must agree on the same CIST regional root. Therefore, any two switches in the region only synchronize their port roles for an MST instance if they converge to a common CIST regional root.

For two or more switches to be in the same MST region, they must have the same VLAN-to-instance map, the same configuration revision number, and the same name.

SW1 (Root for Instance 2 and 3, backup root for Instance 1)

spanning-tree mst configuration

instance 1 vlan 1-100

instance 2 vlan 101-200

instance 3 vlan 201-4094

spanning-tree mst 1 priority 4096
spanning-tree mst 2 priority 0
spanning-tree mst 3 priority 0
Additional information:


CCIE: STP (802.1d)

CCIE: STP (802.1d)

So, first a little history on Spanning tree protocol (STP). Based on an algorithm created by Radia Pearlman in 1985.

Became a standard IEEE protocol in 1990. Still widely deployed. Flavors of spanning tree. 802.1d (ieee), 802.1w (rapid), and 802.1s (mst). Evolution of STP, Cisco vPC (2-way non blocking, still requires STP) and Fabric Path (eliminates STP completely). TRILL is a standardized version of Fabric Path. Both TRILL and Fabric Path utilize a link state protocol (IS-IS) as their loop prevention method.

Specific Cisco enhancement to 802.1d (prior to 802.1w): UplinkFast and BackboneFast

UplinkFast: The UplinkFast feature is designed to run in a switched environment when the switch has at least one alternate/backup root port (port in blocking state), that is why Cisco recommends that UplinkFast be enabled only for switches with blocked ports, typically at the access-layer. Do not use on switches without the implied topology knowledge of a alternative/backup root link typically to distribution and core switches in Cisco multilayer design. ONLY enable on NON-ROOT switches. 

In order to be effective, the feature needs to have blocked ports that provides redundant connectivity to the root. As soon as Uplink Fast is configured on a switch, switch automatically adjusts some STP parameters are adjusted in order to help achieve this:

  • The bridge priority of the switch is increased to a significantly higher value than the default. This ensures that the switch is not likely to be elected root bridge, which does not have any root ports (all ports are designated).
  • All the ports of the switch have their cost increased by 3000. This ensures that switch ports are not likely be elected designated ports.