Spanning Tree Protocol (STP)

Summary

The Spanning Tree Protocol (STP for short) is a protocol used by switches to avoid switching loops, while still providing link redundancy in the network.

They do this by electing a root switch and defining which link, for each switch, is the shortest path to the root switch. Ports are then selectively disabled, and the network graph is turned into a tree, which is by definition loop-free.

Note

The terms “switch” and “bridges” both refer to the same devices.

What STP solves

  • broadcast storms
  • corrupted MAC address tables
  • duplicate packet deliveries

Root selection criteria

In order:

  1. switch priority (default = 32768)
  2. lowest MAC address

Port roles

Role Explanation
root the port on a non-root bridge that is closest to the root bridge, in terms of cost
designated the port on a network segment that is closest to the root bridge, in terms of cost
non-designated ports that block traffic, in order to preserve a loop-free Layer 2 topology
disabled a port that is administratively shut down

Note

A “network segment” is any non-switched link between two switches. The most simple network segment you can find is a cable connection between two switches.

Default port cost

Speed Old cost New cost
10 Mbps 100  
100 Mbps 19  
1 Gbps 4  
10 Gbps 2  

Note

When selecting the root port on a switch, if two ports have the same cost to root, then the tie breaker is the remote switch’s port priority.

STP convergence times

If a used link goes down, it takes 50 seconds for a blocking link to start forwarding again (with the old 802.1d STP variety).

Port state Minimum time to next state
Blocking 20s
Listening 15s
Learning 15s
Forwarding  

Rapid Spanning Tree Protocol greatly reduces the time it takes to recover from a broken link.

STP Flavors

Abbreviation Description
STP The “Common” Spanning Tree as explained above.
PVST+ Per VLAN Spanning Tree
MSTP

Multiple Spanning Trees Protocol, sometimes referred to as MST.

Similar to PVST+ except that multiple VLANs are assigned the same root.
Rapid PVST+ Modified version of PVST+ that takes roughly 3s to converge.

Rapid PVST+

Synchronization

Rapid STP synchronization is a 5 step process between two switches. It is triggered whenever a switch, A, gets a new Root port.

  1. Switch A blocks ports that are on the opposite side of designated port from switch B
  2. Switch A sends a Proposal for the new route to root to switch B
  3. If this is a new best route to root for switch B, the port will change from designated port to root port.
  4. Switch B will send an Agreement back to switch A
  5. Switch A changes its port state from Blocking to Forwarding

This process cascades down the switch chain. At step 3, switch B will also start the process as switch A did during step 1.

Port roles

Instead of non-designated ports, we have alternate and backup ports.

alternate port
An alternate port can reach the root but is not the lowest cost port to it.
backup port

A backup port exists when we have more than one port going from a bridge to a shared media (eg. a hub). In that case, only one of those ports will be a designated port, and the other ones will be backup ports.

A backup port is blocking.

Port states

State Description
Discarding data is not being forwarded on the port. (seen on Alternate, Backup and Disabled ports)
Learning the switch is learning MAC addresses available off of the port. (seen when a port is transitioning to Forwarding)
Forwarding data is being forwarded on the port. (seen on Root and Designated ports)