RSTP -- timers?

mguy

I'm trying to understand STP and RSTP speeds. Below is my thread for STP.

http://www.techexams.net/forums/ccna-ccent/78838-stp-timers-convergence.html

Now, please check if I have RSTP timers correct.


RSTP converges faster right by doing two things

1.) Reduce max age to 6 sec (from 20 sec)
2.) Consolidates port states to 3 (from 4)

== Scenario ===

SW1 fails.

6 seconds (hello.. hello.. hello)

SW2 blocks and listens (discarding), and starts to learn

15 seconds (learning... learning.. learning)

SW2 is now learned! and starts forwarding

Total time: 6 + 15 = 21 seconds


Is this right?

sizeon

the speed comes when a link fails stp has to re-calculate a loop free path. But with RSTP, it uses ALT state ports which will become live once a link fails. So in other words, RSTP doesn't have to calculate which links to block since it already has an ALT port on standby.

mguy

but how long (or how much faster than STP) does it take for RSTP enabled switches to converge?

sizeon

mguy wrote: »

but how long (or how much faster than STP) does it take for RSTP enabled switches to converge?

There is no set timer on how fast a STP and RSTP network converges.

oli356

A quote from wiki "While STP can take 30 to 50 seconds to respond to a topology change, RSTP is typically able to respond to changes within 3 × Hello times (default: 3 times 2 seconds) or within a few milliseconds of a physical link failure."

When CBT nuggests did his demonstration RSTP was a lot quicker than STP..

If you can get packet tracer and have a look for yourself the differences in network converges. Have 2 computers, 1 pinging the other continuously. Shutdown a port on the root bridge switch (making the 3rd switch come alive)... The pings will stop and start and you will be able to see how long it takes.. Just do this for rstp and STP.
I think there was something called pvst+ which needed to be enabled on the host pc port though so the PC had connectivity again quicker.
Hopefuly this is right and makes sense, very tired! Heck I just had to edit out the word router with switch!

YFZblu

oli356 wrote: »

I think there was something called pvst+ which needed to be enabled on the host pc port though so the PC had connectivity again quicker.
Hopefuly this is right and makes sense, very tired! Heck I just had to edit out the word router with switch!

It's called Portfast, it's used to place an access port from blocking status to forwarding status immediately. So it doesn't really have an affect in this situation. IIRC the major advantage to Portfast is so you don't have to wait around for network connectivity if you've just plugged in, or if your computer starts up faster than STP can place your switchport in a forwarding position - Causing users to call you and say "when I start my computer why does it take so long before I can connect to the internet?"

I think you're referring to the command that enables RSTP on a switch: spanning-tree mode rapid-pvst

YFZblu

OP: Yes, you are correct - The major advantaged to RSTP over STP to my knowledge is that RSTP can detect a failed neighbor significantly faster than STP can, because ALL bridges in an RSTP LAN are manufacturing and sending BPDU's, while in STP only the Root Bridge is manufacturing BPDU's, with the non-root Briges simply forwarding those to neighbors.

STP: 10 Hello's (20 sec) before the LAN recognizes a neighbor has gone down >Blocking > Listening (15 sec) > Learning (15 sec) > Forwarding = 50 seconds

RSTP: 3 Hello's (6 sec) before the LAN recognizes a neighbor has gone down > Discarding > Learning (15 sec) > Forwarding = 21 seconds