spanning-tree portfast 1

[bubarooni]

I am getting ready to add a switch and had fired off a note on the subject to a Cisco tech. He responded with a couple of different pointers including the following:

DO NOT USE PORT FAST ON PORTS
THAT GO TO SWITCHES, HUBS OR ROUTERS!

His emphasis, not mine. Anyway, this has me a wee bit concerned as all my Cisco devices are on interfaces with the spanning-tree portfast present.

What does the spanning-tree portfast do? Should I disable it on the ports with Cisco devices present? They are working right now. Should I just leave it alone?

Thanks

 

[Draz]

Portfast should be enabled on all ports on which a station, server or router are attached.
This option stops the spanning tree protocol operating over these ports, and sends the port directly into forwarding mode when the links come up.
This is desirable on this type of port because it is attached to devices that do not run spanning tree.

NB – if a router is bridging to a port on a switch it will be running spanning tree and therefore portfast must be disabled on that port.

 

[CiscoKidd]

If you set portfast on ports that have switches hubs or routers on those ports then it is possible that you can cause a loop in your network. This can cause your whole network to stop.

Ciscokid

 

[bierhunter]

Basically what happens is this. When you connect a device (or power up a device) on a port, it goes through various algorithms to determine whether or not the switch should pass traffic through that port or not. This is part of the Spantree portocol (STP). This is designed to help prevent loops at layer-2. Similar to routing protocols preventing loops at layer-3.

Portfast basically disables STP on that port. Therefore, the port is always in what's called forwarding mode (traffic is forwarded through that port). The standard is that you only enable portfast on ports connected to hosts (workstations, etc), and never on uplinks, trunks, whatever. The reasoning behind this is that a network will usually have redundant paths to a destination (switch). The STP prevents loops from occuring through those paths, therefore, you don't want to enable portfast on links to other switches; or STP won't work on those ports...thus possibly causing traffic loops.

Now...on the other hand. If your network has NO redundant paths, you can safely enable portfast on a link to another switch. But, if you ever add a redundant path to that switch, you must remember to remove portfast from that port.

Why would you want to enable portfast? Good question... When a device is connected or turned on without portfast, you can experience up to about a 50-second delay before traffic flows through that port. This is because STP is checking out everything. This can cause "domain not found" errors in an NT environment due to timeouts. Therefore, you should enable portfast on host ports to prevent this from happening.

Normally, you won't need to worry about portfast on ports going to other switches. Those ports are normally always on. It's a good idea to just leave it off on those ports.

Hope this helps.