Advanced STP: Rapid Spanning tree protocol.

Lessons Learned:

RSTP:

Rapid convergence based on sync process.

 Enabled through

-spanning tree mode mst

- spanning-tree mode rapid-pvst

Sync process only occurs on point-to-point non-edge ports.

-implies link-type must be accurate.

-spanning-tree link-type (point-to-point | shared)

- spanning tree portfast.

Notes:

When you’re running mst or rpvst, the switches are using a sync process to see if they agree who is the root bridge is, and what should be the rot, vs designate back up or alternate ports.

How the process works:

A device is elected as the root: the root bridge then tell the downstream neighbor the it is the Root.

Is the downstream neighbor agrees – it automatically transitions to root port status and starts forwarding.

Once the downstream switch does that, every other downstream interface goes into the blocking mode.

Once the interfaces are in the blocking mode. The downstream switch then sends its own proposals downstream to the other neighbors.

It tells the downstream neighbors the I know of a root port through me, so the downstream neighbors will transition their ports to root ports and send an acknowledgement – the proposal response. Then each downstream switch will continue this process.

The key – is we start at the root bridge and work our way down the tree with the proposal an proposal response. It makes the switches agree quickly.

Note: the proposal process will only happen if each switches downstream interfaces are point-to-point and non-edge links.

A p2p link essentially means “full-duplex”. If the link negotiates to half-duplex or is plugged into a hub, then it cannot be part of the proposal process

For Non- P2P links, the normal STP algorithm is run.

An Edge port in RPVST is equivalent of an portfast port on PVST.

If at the edge of the network you do not configure the links as portfast or edge ports, you will NOT have rapid convergence.

Note: basically the links between the switches are configure as p2p non-edge ports and end host ports are configured as edge ports.

Configure globally

# spanning-tree mode rapid-pvst

Note: When you have rapid-pvst on – you don’t need to configure UplinkFast or BackboneFast.