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New Member

Portfast on 3500 Switch

Hi, I understand that portfast should be disabled for any switch ports that have multi access devices hanging off them but what if a switch had a hub off it and portfast was enabled and there was definitely no loops to cause problems with STP? Would there be any negative effects?

Thanks, Lee.

5 REPLIES
hvn
New Member

Re: Portfast on 3500 Switch

Dear Lee,

Note: The portfast feature should never be used on switch ports that connect to other switches, hubs, or routers. These connections may cause physical loops and it is very important that spanning tree go through the full initialization procedure in these situations. A spanning tree loop can bring your network down. If portfast is turned on for a port that is part of a physical loop, it can cause a window of time where packets could possibly be continuously forwarded (and even multiply) in such a way that the network cannot recover.

The above paragraph is from technical tips of cisco.Hope u have read that and understood how dangerous portfast command can be on switches connected hubs.

New Member

Re: Portfast on 3500 Switch

Lee,

There would be no negative effects. HOWEVER, if you ever change your network such that you create a loop, then the loop will destroy useability of part or all of your network. You will not be able to diagnose this loop remotely, but will have to resort to unplugging connections to find the problem. It's just not worth it.

If you have BPDUGUARD enabled AND your switch/hub generates BPDUs (the cheap ones don't), then the port will disable itself. This could be considered a "negative effect" I suppose.

New Member

Re: Portfast on 3500 Switch

Thanks Guys. I understand that only single workstations should have portfast enabled and that hubs, switches etc should not, for spanning tree reasons. I've had various speculations as to what would happen if a hub was plugged into a switchport that had portfast enabled and just wanted to clarify if there were any negative effects when I definitely knew that there were no loops.

New Member

Re: Portfast on 3500 Switch

As long as you have no physical loops you should be fine. Hubs don't participate in spanningtree

New Member

Re: Portfast on 3500 Switch

Thanks for that!

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