Port fast query

Is there any difference in functionality when we implement port fast in global configuration mode V/S interface mode?

 

-->>port fast  at interface level:

switch(config-if)#spanning-tree
portfast

--> Port fast at Global Mode: 

Switch(config)#spanning-tree portfast
default 

Comments

  • Hari.sapkotaHari.sapkota ✭✭ ✭✭

    Is there any difference in functionality when we implement port fast in global configuration mode V/S interface mode?

    Both the commands work similarly. If you need to enable this feature on the selected interfaces, use interface specific command. Otherwise, you can use global mode command that will affect all the access ports.

    Good luck!

  • Is it possibile that we can enable port fast for a trunk port ??? [e.g. lets say a VMware machine generating BPDUs need to participate in STP]

  • you can enable portfast on trunk ports.

    int x/x

    spanning-tree portfast trunk

     

    You need the 'trunk' keyword otherwise no vlan on trunk port will be portfast.  when you do #sh spanning-tree int x/x portfast ,  you can see portfast status per vlan.

    best senario for this is when you have a trunk link to a router or asa , basically anything that does not generate bpdus  as portfast status is conditional upon not recieving any.

  • If the server is participating in STP/sending BPDUs you should NOT be enabling portfast. If fact, if a portfast interfaces receives a BPDU it loses portfast status anyway. So it would be pointless!

     

    You should only configure portfast at the edge of the STP domain, I.e links to hosts, routers etc.

     

    If a server is tagging frames, for example a VM host with guests on different VLANs then you should configure spanning-tree portfast trunk.

     

    Portfast trunk is only available at the interface level, not global.

     

    As for the original question, the global and interface level portfast commands work the same. You can think of the global command as a macro that enables portfast on all access mode interfaces.

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