Rapid Spanning-Tree (RSTP)

Hello Juan

Classic IEEE 802.1D STP used the blocking → listening → learning → forwarding set of port states. Each of these states exists for a predetermined amount of time. The forward delay timer is used to determine how long a port must stay in both the listening and learning states. The default value is 15 seconds for each state, for a total of 30 seconds.

In addition, there is the Max Age timer which is by default 20 seconds, and it defines the amount of time a switch port will consider a BPDU it has received valid. This timer essentially helps to determine how long a switch should wait for a BPDU on a port before it decides the link is down. So if a link goes down, STP can potentially wait up to 20+15+15 = 50 seconds before it reconverges, something which in today’s networks is no longer acceptable.

So what’s the difference with RSTP? RSTP also has states, including discarding → learning → forwarding. They’re different, there are fewer, but they are there. The difference is the fact that RSTP no longer depends upon timers to move from one state to the next. The sync mechanism is a negotiation mechanism that completes within milliseconds. Moving from state to state is no longer a matter of dozens of seconds, but sub-second time intervals. Does that make sense?

I hope this has been helpful!

Laz