I covered RSTP again last week and had a few notes on it, these notes are far from conclusive, but here goes.

RSTP is defined by IEEE 802.1w and improves upon STP which is defined in IEEE 802.1d. In a nutshell RSTP converges networks more “Rapidly” than STP.

Traditional STP sends hellos every 2 seconds and has a max age timer of 20 seconds, therefore 10 hellos would go missing before STP would start to converge. RSTP only waits for 3 times the hello time, the hello timer is still 2 seconds by default but as with traditional STP this can be changed. With traditional STP BDPUs are only sent from the root bridge and relayed by the other switches, with RSTP every switch sends it own BPDUs even if the root bridge is down, they are sent every hello timer.

Cisco created some cool features for traditional STP to help it converge quicker these are PortFast, UplinkFast, & BackboneFast, these are standardised in RSTP.

RSTP also defines some additional port roles;

• Root Port – Same as in STP
• Designated Port – Same as in STP
• Alternate Port – An alternative Root Port for a segment, same as Cisco UplinkFast feature
• Backup Port – A backup Designated Port for a shared segment.

RSTP also classifies the links in one of 3 types

• Point to Point – Fully Duplex links between 2 switches where hellos are exchanged are treated as Point to Point
• Shared – A Shared link is a port which connects to a hub
• Edge – A switch port which connects to an end system

If RSTP fails to receive a hello on a Point to Point link it immediately asks the other switch about its status, if its path to the root is down it immediately starts to converge. This is a standardised version of BackboneFast.

Traditional STP has 5 port states, Disabled, Blocking, Listening, Learning & Forwarding, RSTP on the other hand only has 3, Discarding, Learning & Forwarding. It converges faster as it does not need to go through the Listening stage as it actively queries its neighbors ensuring there are no loops.

RSTP helps networks achieve far speedier convergence times, traditional STP networks take around 30 to 50 seconds to converge where as RSTP can converge (dependant on topology) less than a second.

A fantastic link on RSTP – http://www.cisco.com/warp/public/473/146.html