Spanning Tree Portfast

Portfast - didn't think I would have enough for a whole post, but there are a few items worth mentioning.


What is it - Typically, if a host (server, PC, etc.) plugs into a port, spanning tree will run to ensure there is not a network loop before allowing the host to talk.  If you are running the default, Cisco per-vlan spanning tree 802.1d, this takes 30 seconds for both listening and learning mode to run.

The problem - Thirty seconds is a long time to wait, especially when it means you are not getting DHCP responses and your computer is deciding that it must not be on a real network so it doesn't need to run all the network startup scripts.

The solution - configuring portfast on an interface tells that switch that only a host (nothing that could cause a loop) is connected.  The switch then allows the port to immediately transition to forwarding (but will loose it's portfast status if a BPDU is recieved)

The dangers - if you mis-configure this, and turn portfast on for an uplink to another switch, hub, etc. a loop could form and crash your network before spanning tree ever has a chance to prevent it.

The terms:  in rapid spanning tree protocol (802.1w) they are called "edge ports" but are still configured with the same portfast command. 

The commands:

interface fa0/1
spanning-tree portfast
exit

or

interface fa0/1
switchport host
 *This turns on portfast and also disables channeling and trunk negotiation*

or globally from privileged exec mode

#spanning-tree portfast default
*In the global form of portfast, immediate forwarding is enabled for all access ports (NOT ANY TRUNK PORTS) but you should still manually input the command no spanning-tree portfast for any ports that may be connected to other switches*


spanning-tree portfast trunk
*This command is to turn portfast on for a trunk.  You would typically have a trunk enabled for a host such as a server that needs multiple vlans, or a VoIP phone*


If a BPDU is ever recieved on an edge/portfast port, it loses it's portfast status AND sends a Topology Change Notification to all other switches in the STP domain.