turning off portfast

Answered Question
Sep 12th, 2008

If you have portfast set on a port but now want it to participate in spanning tree, will the port go from it's current forwarding state and transition through listening/learning and back to forwarding.

Or to put it another way will it drop the connected device for 30 seconds?

I have this problem too.
0 votes
Correct Answer by paul.matthews about 8 years 3 months ago

a portfast port still participates in spanning tree - the difference is that it jumps straight in forwarding.

If you disable portfast, normal behaviour will apply.

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Correct Answer
paul.matthews Fri, 09/12/2008 - 01:58

a portfast port still participates in spanning tree - the difference is that it jumps straight in forwarding.

If you disable portfast, normal behaviour will apply.

francisco_1 Fri, 09/12/2008 - 02:44

Stuart,

just in addition to paul's comment, enabling portfast you are forcing the switchport into forwarding mode immediately. The port still participates/running the spanning tree protocol in the event that if the port is to be a part of the loop, it will eventually transition into STP blocking mode and when you enable the port, it transition straight to forwarding again. Without the portfast, it has to go through listening, learning process (about 30secs!!) instead of transition straight to forwarding.

francisco

phaniraj01 Fri, 09/12/2008 - 03:54

Without portfast, switchport undergoes follwoing states:

blocking (20sec)-> listening (15sec)-> learning state (15 sec)-> Forwarding state

i.e. 50 sec from blocking to forwarding state

by enabling portfast on access ports, switchport will come directly form blocking to forwading state

thanks,

phani

Joseph W. Doherty Fri, 09/12/2008 - 04:06

If I remember correctly, features like portfast were developed by Cisco to handle standard STP issues. Most (all?) of the issues are also handled in the newer STP variant standards, which can be used across vendors. I.e. if not already doing so, if still using STP, consider converting to one of the later variants where, features such as portfast, may be implicit.

paul.matthews Fri, 09/12/2008 - 04:46

The features are not implicit - with Rapid spanning tree, the quick transition to forwarding on an inter switch link is due to acknowledgement, rather than timers.

With rapid spanning tree, there is the concept of an edge port, which transitions immediately to forwarding without waiting for acknowledgement.

How is a port designated an edge port? by configuring portfast!

Paul.

Joseph W. Doherty Fri, 09/12/2008 - 06:36

Semantics, perhaps? Fast convergence features are implicit as they are part of the newer STP protocols, such as in RSTP 802.1w vs. STP 802.1D, i.e. they're no longer proprietary Cisco extensions.

However, you're quite right about using the portfast command within the Cisco implementation to identify edge ports.

Others might find this whitepaper, http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml, on the differences between STP and RSTP interesting.

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