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When to use spanning-tree portfast trunk

Posted on 2009-05-18
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Last Modified: 2012-08-14
I have 2 switches with 2 redundancy conections,  in all the ports, it is configures the command spanning-tree portfast trunk.

I am getting a lot of topology changes from this port, I know portfast should be use to access port, I want to figure out why, they configure it with the portfast trunk.

Does anyone have idea of what this command do?
Thanks
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Question by:dsanc
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by:that1guy15
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I myself have never ran into a situation that would need STP port-fast on a trunk link. But here are a few post with your same question.

http://www.techexams.net/forums/ccnp/38674-spanning-tree-porfast-trunk.html
http://www.groupstudy.com/archives/ccielab/200408/msg00579.html
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by:equarando
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Yes, you are right about spanning-tree portfast should be only used on access ports. But I guess for some odd reason if you want to turn portfast on your trunk links cisco lets you with that command.

Taken from cisco's command database...

"Router(config-if)# spanning-tree portfast trunk
%Warning:portfast should only be enabled on ports connected to a single
 host. Connecting hubs, concentrators, switches, bridges, etc... to this
 interface  when portfast is enabled, can cause temporary bridging loops.
 Use with CAUTION"
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Portfast trunk is for when you are quite positive that there will be no loops or redundant routes on this link. A trunk to a dot1q aware host, a dot1q aware point to point, etc. Never to a network that needs a redundant l2 path. Look at fast failove r or rapid spantree if the ordinary time periods are excessive for your needs.
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One reason I am seeing for it is when you connect an ip phone up to a port and need to set it to trunk due to multiple clans (voice and data). But from what I'm reading setting a voice vlan on the interface is the recommended solution.

A use for it was suggested in one of the links I provided, which was when connecting a virtualization server (hosting
multiple vlans to a switch.
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by:lanboyo
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Yes, or a router switch tagged connection. If you have any issues with.using the command portfast bpduguard disable, don't use portfast.
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