Rapid PVST+ and MST Spanning Tree

What it is

Rapid Spanning Tree Protocol (RSTP) and Multiple Spanning Tree (MST) keep a switched network free of loops while converging quickly.

RSTP is defined in IEEE 802.1w. On Cisco switches it runs per VLAN as Rapid PVST+. MST is defined in IEEE 802.1s and groups many VLANs into a few spanning tree instances, which saves switch resources.

How it works

Both protocols elect a root, pick the best paths and block the rest.

The root bridge

All switches elect one root bridge, and the commands here are IOS XE. The winner has the lowest bridge ID. The bridge ID is made of a priority and the MAC address, and the priority moves in steps of 4096 because part of it carries the VLAN number.

Switch(config)# spanning-tree mode rapid-pvst
Switch(config)# spanning-tree vlan 10 root primary
Switch(config)# spanning-tree vlan 20 priority 4096

The root primary keyword sets a priority lower than the current root, and the macro does this once at the time it is entered. Setting priority gives you exact control. The default priority is 32768, and the extended system ID adds the VLAN number to it.

RSTP port roles and states

RSTP has four port roles and three states.

RSTP merges the old blocking, listening and disabled states into discarding. Alternate and backup ports are discarding. If the root port fails, an alternate port can take over at once, which is why convergence is fast.

Why RSTP is faster

RSTP does not depend on timers on point to point links. Neighbouring switches use a proposal and agreement exchange to move a link to forwarding. An edge port goes straight to forwarding, because it connects to an end device and cannot form a loop.

Switch(config)# interface GigabitEthernet1/0/5
Switch(config-if)# spanning-tree portfast

RSTP treats a full duplex link as point to point and a half duplex link as shared. Fast transitions need a point to point link.

Path cost

The path cost decides which port becomes the root port. Default costs depend on the speed and the cost method.

The command spanning-tree pathcost method long selects the long method. All switches in the network should use the same method. Rapid PVST+ uses the short method by default on Catalyst switches, though defaults can differ by platform.

MST basics

MST maps many VLANs to a small number of instances. Each instance has its own tree and its own root. Instance 0 is the IST, and it carries the information for the region as a whole.

Switches belong to the same region only if the name, the revision number and the VLAN to instance mapping all match.

Switch(config)# spanning-tree mode mst
Switch(config)# spanning-tree mst configuration
Switch(config-mst)# name REGION1
Switch(config-mst)# revision 1
Switch(config-mst)# instance 1 vlan 10,20
Switch(config-mst)# instance 2 vlan 30,40
Switch(config-mst)# exit
Switch(config)# spanning-tree mst 1 root primary
Switch(config)# spanning-tree mst 2 priority 4096

VLANs that you do not map belong to instance 0. A mismatch in any of the three items puts a switch in a different region, and it then connects to the others as if it were on another spanning tree.

Comparing the protocols

Verifying

Switch# show spanning-tree vlan 10
Switch# show spanning-tree summary
Switch# show spanning-tree mst
Switch# show spanning-tree mst configuration

show spanning-tree vlan 10 shows the root, the local bridge ID, and each port with its role, state and cost. In MST, show spanning-tree mst configuration shows the name, revision and mapping that must match between switches.