Hi, I'm a bit confused about the spanning-tree portfast trunk command. Portfast should only be used on access ports connected to a single end device (PC/printer/server). What is the point then in having a portfast trunk command? When you configure it it gives the same error message and basically tells you not to do it:
%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.
A switchport before coming to the forwarding state, transitions through states of blocking, listening, learning. This takes about 50 sec for the port to come to forwarding state. But if the port is access port, where a pc or printer is connected, why should it wait so long. Access ports doesnt need any stp calculation & needs to come up as soon as posible for data transmission. portfast on an interface transitions it from directly blocking to forwarding.
However, as u mentioned when a switch is connected to it & the port recv a bpdu, stp would put the port in blocking state if bpduguard is configured.
Well, you can have a trunk to what looks like a host for a switch: a router for instance. A router is an L3 device that cannot introduce a bridging loop, that does not run STP but that can trunk with 802.1Q or even ISL. Portfast is clearly indicated in that case.
It used to be that trunks could only be made between switches, that's why trunks where not considered as valid for portfast. That's also the reason why the plain portfast CLI did not have any effect on trunks. That's also why a separate keyword had to be added to the CLI when Cisco evenutally realized that it could be useful to run portfast on trunks;-)
Thanks Francois, (and also the other 2 guys for your input).
I had a think over the weekend and came up with another reason which seems quite obvious to me now. If you've only got 1 uplink (which is a trunk) and therefore no chance of bridging loops then I think one should put portfast-trunk on that too.
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