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Portfast - to enable or not to enable?

Our internet router (3640) is connected to a Catalyst 3524XL (standalone).

eth0 on the 3640 is 10/half (comes fixed that way on the interface it came with) and the port on the 3524 that connects to eth0 is manually set to 10/half.

STP is disabled, which means all of the ports are in portfast mode.

I have read that portfast on a port connecting to a router is a bad thing, and that it might cause routing loops.

a) I am confused as to how a loop can occur on a single cable between 2 devices.

b) WHY is it a bad thing, besides the loop-issue?

c) What symptoms might appear in such a scenario?

Thanks.

4 REPLIES

Re: Portfast - to enable or not to enable?

Hi,

a) Loop can't occure on a single cable, of course.

b) It is NOT a bad thing, the only dangerous portfast feature is a possibility of a very-short-time loop in the case you connect two portfast-enabled ports into one hub, e.g. But the switch is still sending BPDUs on portfast ports, so even in that case loop will be disconnected after first BPDU received (2 sec default).

So I think there is no problem with portfast on port connecting a router.

But you are saying "STP is disabled, which means all of the ports are in portfast mode". Have you really disabled STP on the switch or have you just enabled portfast on all ports (i.e. STP is still running)? I would be afraid of disabling STP on the switch, this is really dangerous.

Regards,

Milan

Anonymous
N/A

Re: Portfast - to enable or not to enable?

An admin at a previous job decided to turn off STP to save the bandwidth consumed by STP generated packets flying around every 2 seconds. Since the network was designed without any redundant links, STP was not needed.

Until . . .

One day a tech plugged a cable from one switch to another. Why? I have no idea. But, it brought the entire network to its knees until we found this long cable running down the hall.

The admin's respose was - don't do that.

I agree - turning off STP can have some severe unintended consequences - at the most unfortunate time.

New Member

Re: Portfast - to enable or not to enable?

The reason why it was suggested to disable portfast or let the switch do its STP checking to make sure it is okay to forward on a router port is because

---- Routers can be enabled to do bridging !!!!! . in that case, router is a bridge/switch and participate in spanning tree.

So as a rule of thumb, it is safe to not to run portfast on ports connecting to routers.

It is a good idea to enable Spanning tree and configure port-fast on relevant ports rather then disable spanning tree which may cause loops as below.

in your switch, you can connect cross-over cable between two ports and you will cause a loop because STP is disable and there is no protection.

hope this helps

New Member

Re: Portfast - to enable or not to enable?

If we enable portfast on our 2924 router port, we lose the link immediately. Not good.

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