portfast "trunk"

Unanswered Question
Feb 11th, 2007

What is the true use of portfast with the trunk keyword?

I have this problem too.
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albert.remo Sun, 02/11/2007 - 20:08

Hi!

Good day! Portfast causes a L2 interface access ports to enter forwarding state immediately, it bypassing the listening and learning state. Newer versions allows portfast on trunks. You should proceed with utmost caution when using portfast on trunk ports.

Use PortFast only when connecting a single end station to a Layer 2 access port. Otherwise, you might create a network loop.

Check on this link for a better understading of PortFast trunk

http://www.cisco.com/en/US/products/hw/routers/ps368/products_configuration_guide_chapter09186a008007e6bd.html#wp1042489

Hope this helps,

Regards,

Albert

Francois Tallet Mon, 02/12/2007 - 08:47

This keyword is just the result of history. Initially, there was a collusion between edge port and portfast: we assumed that a port that was not trunking was necessarily connected to a host, and that a trunking port was necessarily connected to a switch (at that time, ISL was the only trunking encapsulation, and was only supported by cisco switches). Because of that, some bright mind decided that portfast could only be allowed on access ports, for safety reason (to avoid the user from shooting themselves in the foot and enabling portfast between two switches).

Then later, we discovered that it was nice to have portfast configured on trunks going to a router;-) As we could not just change the meaning of the existing portfast command (the stability of some network might depend on the fact that it did not apply on trunk ports), we had to introduce a new keyword for this "new" feature.

Regards,

Francois

yvanog Mon, 02/12/2007 - 09:58

Thank you very much! I've been wracking my brain to figure out how that command got in there and finding info on the command is difficult.

Again, thanks!

Rob

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