STP Vs RSTP

Unanswered Question
Jan 6th, 2008

guys i kno wthe differenc ebetween them but lil mor ethings i wanna ask which i m not sure about first of all port fst, uplink fast, backbone fast.......does stp has these features as wlel or its the only feature of RSTP now my second question is whats the difference between uplink fast and backbone fast?????? cheers

I have this problem too.
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ohassairi Sun, 01/06/2008 - 21:23

uplink fast:

if a forwarding link that is directly connected to the switch gets down, the switch unblock the blocked port and lets it in forwarding state without: listening and learning

BB fast:

if a forwarding link that is NOT directly connected to the switch gets down, the switch unblock the blocked port and lets it in forwarding state without: listening and learning

bvsnarayana03 Sun, 01/06/2008 - 23:21

I'm not sure of your 1st question. Probably these features came in with enhancement in IOS versions.

I had to read & re-read a number of times to understand this. I would be happy if I could make one understand what I understood.

Uplinkfast:

Lets say a switch A connected to Switch R & switch B, where R=root switch & B=Backup root(consider a triangular topology).

Ideally, A has port connected to R as fwding & the one connected to backup root as blocking.

When A's uplink to Root fails, it has an alternate path to root via B, which is in blocking state. Now this port will take 40-45 sec to transition from blocking to fwding. To overcome this delay of transition through states, uplinkfast is configured so that when a switch detects a directly connected port going down then the other port immediately transitions to fwding. This happens within 5 sec & saves a lot of time which is usually taken for transition.

Backbonefast:

Consider the same topology. This time lets say the link between Root & Backup root fails. This makes backup root to assume, that the root has gone down & sends the BPDU with itself as root to C. C when see's BPDU from Backup while root already active in topology undsrtands that there was a failure in its non directly connected link. It thus transitions the blocked port connected to backup to froward state.

hope that clarifies.

pls rate all helpful posts.

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