Portfast Trunk

N3tWrkNutN3tWrkNut Member Posts: 30 ■■□□□□□□□□
Does anyone know why you would use this? I am new to a company and in my studies I have read not to enable portfast on a trunk link. I get here and I see that portfast trunk is enabled and I can't seem to find any good reason to have it enabled and the only reason I get for it being enabled is because the vendor enabled it.

Help or guidance please! icon_razz.gif


  • down77down77 Member Posts: 1,009
    CLI tells you why:

    6509(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

    CCIE Sec: Starting Nov 11
  • N3tWrkNutN3tWrkNut Member Posts: 30 ■■□□□□□□□□
    I have seen that statement when I put it on an access port. That is why I am asking why would you use it on a trunk port especially to another switch. That is how it is setup here and it makes me nervous.
  • N3tWrkNutN3tWrkNut Member Posts: 30 ■■□□□□□□□□
    Never mind....I see what you are saying now. Reading is very important....I still see a port that is trunked to a switch with portfast trunk....I will have to remove that one since the others do not have that. The others go to the esx hosts hense the single host ;)

    Thanks for your reply!!!!
  • down77down77 Member Posts: 1,009
    Typically your trunk port is connected to another switch which means you may run the risk of creating a spanning-tree loop... which in turn could temporarily take down your segment or network.

    Maybe they have a need to bypass STP directly into the forwarding state for convergence? Or it could be for IP Phones?

    Found an older thread with the same question:

    CCIE Sec: Starting Nov 11
  • elphrank0elphrank0 Member Posts: 67 ■■□□□□□□□□
    Portfast reduces the amount of time for convergence if I remember correctly. It would stop the 50 seconds it takes for convergence and do it in like 5 or 10..
  • elphrank0elphrank0 Member Posts: 67 ■■□□□□□□□□
    "Using it means the port won't spend the usual 50- seconds to come up into forwarding mode while STP is converging."

    Lammle, Todd, 2007, CCNA Study Guide, 6th, PG. 512
  • GT-RobGT-Rob Member Posts: 1,090
    We use portfast on trunks all the time.

    On our blade switches, the interfaces make a trunk with the server. Since its impossible to have a switch there, and the server needs portfast for DHCP/PXE to work correctly, we need it on.

    If you were to connect a switch to a trunk and were 100% sure there were no loops, there is no problem with port fast (or bpdufilter/guard if you wanted).
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