Rapid PVST+ Spanning Tree Operations

What it is

Spanning Tree blocks the redundant ports that would otherwise create a switching loop.

Redundant links between switches are good for resilience, but they create a danger. A broadcast frame can circle round a loop forever, and because Ethernet frames have no hop count, the loop does not end. The result is a broadcast storm that can bring a network down within seconds.

Spanning Tree Protocol (STP) prevents this. It chooses a loop-free path through the network and blocks the ports that would create a loop. If an active link fails, a blocked port can take over.

Rapid PVST+ is Cisco's fast version. It uses the Rapid Spanning Tree standard (802.1w) for quick convergence, and it runs a separate instance for each VLAN. That lets different VLANs use different paths.

How it works

The switches elect a root, each port takes a role, and protection features guard the edges.

Electing the root bridge

The switches elect one root bridge for each VLAN. Every other switch then finds its best path to the root.

The election uses the bridge ID, which is made of a priority value, the VLAN number (the extended system ID) and the switch MAC address. The lowest bridge ID wins.

The default priority is 32768. Because the VLAN number is added to it, the priority for VLAN 10 shows as 32778.

If priorities are equal, the lowest MAC address wins, which often picks an old switch. You should choose the root yourself:

spanning-tree mode rapid-pvst
spanning-tree vlan 10 root primary
spanning-tree vlan 10 root secondary

root primary lowers the priority, to 24576 or lower if needed, so the switch becomes the root. root secondary sets 28672 so the switch is the backup. You can also set the priority directly, in multiples of 4096:

spanning-tree vlan 10 priority 4096

Port roles

Each switch port gets a role:

• Root port: the best path from a non-root switch to the root bridge. Each non-root switch has one.
• Designated port: the best path on a segment, away from the root. Every segment has one. All ports on the root bridge are designated.
• Alternate port: a backup path to the root, on a different switch. It is the port that takes over if the root port fails.
• Backup port: a backup path to a segment that the same switch already reaches. This appears with hubs or shared links, and is rare.

The best path is the one with the lowest path cost. Cost depends on link speed.

These are the short method values, which Cisco switches use by default. The long method gives larger values. If costs tie, the lowest sender bridge ID decides, then the lowest sender port priority, then the lowest port number.

Port states

In Rapid PVST+ there are three states:

• Discarding: does not forward user frames and does not learn MAC addresses. This covers the older blocking and listening states.
• Learning: builds the MAC table but does not yet forward.
• Forwarding: sends and receives normally.

Classic STP has five states, including blocking and listening, and takes 30 to 50 seconds to react to a change. Rapid PVST+ uses a handshake of proposal and agreement between neighbours, so it usually recovers within a few seconds.

Protection features

PortFast is for ports connected to end devices. It skips the waiting period, so a PC gets network access quickly. It must never be used on a link to another switch, because it could allow a loop.

BPDU guard protects PortFast ports. If a switch is plugged into such a port, a BPDU arrives, and the port is put into the err-disabled state.

BPDU filter hides BPDUs from a port. It is risky, because it can allow loops.

Root guard keeps the intended root in place. If a port receives a better BPDU, it goes into a root-inconsistent state, and recovers when the better BPDUs stop.

Loop guard protects against a one-way link failure, where a blocking port stops getting BPDUs and wrongly starts forwarding.

Verifying

show spanning-tree
show spanning-tree vlan 10
show spanning-tree summary

The output shows the root bridge ID, this switch's bridge ID, and a table of ports with their role (Root, Desg, Altn), state (FWD, LRN, BLK) and cost.