notes: RSTP 802.1w

RSTP

- takes advantage of STP features;
a. portfast - now called edge port, not participating in STP.
b. uplinkfast - built-in
c. backbonefast - built-in

RSTP port roles

1. Root port - Is the port that has the best root path cost to the root.

2. Designated port - Is the downstream port that has the best root path cost to the root.
- Is a downstream interface pointing away from the root bridge.
- By default, all ports on the root bridge are DP

3. Alternate port - Is a port that has an alternate path to the root. An alternate port, is less desirable than the root port.
- In blocking state will receive STP info, but not send any out that interface.

4. Backup port - Is a backup designated port.
- quickly converging backup to the root port for a system.

RSTP Port States

RSTP port states are simplified from 802.1D and consist of the following:

1. Discarding
- Incoming frames are simply dropped; no MAC addresses are learned.
- Combines the 802.1D (STP) disabled, blocking, and listening states.

2. Learning

- Incoming frames are dropped, but MAC addresses are learned.

3. Forwarding
- Incoming frames are forwarded according to MAC addresses that have been (and are being) learned.

Implementing RSTP

On most Cisco switches, configuring 802.1s (Multiple Spanning Tree, MST) automatically enables RSTP. Cisco did invent a mode of operation, PVST+ mode, that enables you to use RSTP without the implementation of MST.

command to enable rstp/pvst+:

spanning-tree mode rapid-pvst