STP Portfast forwarding delay

[shanemoss]

Hi Folks,

I recall at some stage reading about a delay before a port starts forwarding frames when the link comes up.  Normally with STP a port has to go through the listening and learning state etc but with portfast enabled the switch will forward frames much more quickly.  My understanding is that the forwarding is not immediate and that there is still some delay but I can't seem to find what this delay is.  My typical ports are configured for portfast (actually portfast trunk as they are trunk ports for IP phones) as well as BPDUGuard and I can observe that there is a few seconds delay beforfe the switch actually forwards frames after the liink comes up.  Does anyone know what this delay is and is there a way to shorten it?

Many thansk in advance.

[alexander.milyar · Accepted solution]

Checkout

http://www.cisco.com/en/US/products/hw/switches/ps708/products_tech_note09186a00800b1500.shtml

Because PortFast is enabled, only 1 second passed before STP started 

forwarding (instead of 30 seconds).

So there is only 1 second of portfast operation.

Further tuning is setting DTP, PAgP, speed and duplex to manual preconfigured mode, if you need it. It is discussed later in document.

[glen.grant]

   If you want to speed it up even more instead of using portfast use the "switchport host" command on every user switchport  which turns on portfast and turns off trunking, channeling negotiation and a couple other things. 

[alexander.milyar]

glen.grant написал(а):
   If you want to speed it up even more instead of using portfast use the "switchport host" command on every user switchport  which turns on portfast and turns off trunking, channeling negotiation and a couple other things.  

Actually it does only

spanning-tree portfast
switchport mode access
no channel-group

without "and a couple other things".

example:

Switch(config-if)#switchport host

switchport mode will be set to access

spanning-tree portfast will be enabled

channel group will be disabled

Switch(config-if)#

[shanemoss]

On my trunk ports I now configure spanning-tree portfast trunk and switchport nonegotiate and frames are forwarded pretty much instantly upon the link coming up.

Thanks.

[Joseph W. Doherty]

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Posting

My typical ports are configured for portfast (actually portfast trunk as they are trunk ports for IP phones) . . .

Hmm, you might be able to configure them as access ports with a voice vlan.  Don't know whether this would make any significant link up difference.