Spanning Tree

Spanning Tree blocks ports

When a loop is found, a link is chosen to be BLOCKED.

Spanning Tree wants to find the root bridge

The protocol will attempt to keep a path open towards the ROOT BRDIGE.

Spanning Tree is simple

It works with simple or complex topologies.

The Spanning Tree Protocol (802.1d)

• Spanning Tree prevents broadcast storms
• .. by breaking Layer 2 loops
• Spanning Tree flavours are plentiful
  • STP (802.1d): the original Spanning Tree Protocol
  • RSTP (802.1w): Rapid STP; significantly speeds up network convergence
  • PVST: Per-VLAN Spanning Tree; Cisco-proprietary; forms one spanning tree for each VLAN
  • Rapid PVST+: Cisco-proprietary; PVST that is faster.
  • MSTP (802.1q-2014): Multiple Spanning Tree Protocol; like PVST and RSTP, but creates groups of VLANs that share spanning trees (one per group of VLANs, rather than one per VLAN.)
  • Cisco defaults to PVST+ or Rapid PVST+ (depending on the type of switch)
• Arista defaults to MSTP

Switches and Bridges

• Spanning Tree often refers to switches as bridges
• A bridge is just a switch with very few ports
• When you see “bridge” in a modern networking context, just think “switch”

STP’s general mechanism

• Switches regularly send BPDU (Bridge Protocol Data Unit) packets out of every port
• BPDU contains unique identifier of the Switch
• BPDU also contains a “path cost” field. Cost depends on the speed of the interface.
• BPDU contains a “bridge priority” field too. Lowest bridge priority wins.
• BPDUs received from other switches are forwarded out of every port except the port it was received on
• the Path Cost in the received BPDU is increased before it’s forwarded
• if a switch receives its own BPDU:
  • it knows there must be a loop
  • it does not forward the BPDU

Root Bridge

A simple layer 2 topology showing the root bridge.

STP Root Bridge Election

• the Spanning Tree root bridge is elected based on the bridge priority field in the received BPDUs
  • the Root Priority is configured manually!
• lowest bridge priority becomes the root bridge
• now, the job of every switch is to find the cheapest path to the root

Root Path Selection

• a given switch may have received multiple BPDUs from the root bridge
  • (one per path!)
• if there are multiple paths to the root bridge, one must be chosen
  • there can be only one
• the path with the cheapest cost is chosen
  • this interface is called the ROOT port
• the other paths are blocked
  • these interfaces are labelled as ALTERNATE

Port types

STP might decide that these are the port states:

Next…

A lab with experiments!