Spanning-Tree Operation

802.1D Spanning-Tree Protocol (STP)

You can also include STP general parameters in this command. See “Recon­ figuring General STP Operation on the Switch” on page 5-28.

For example, the following configures ports C5 and C6 to a path cost of 15, a priority of 100, and fast mode:

ProCurve(config)# spanning-tree c5-c6 path-cost 15 priority 100 mode fas

 

STP Fast Mode

 

For standard STP operation, when a network connection is established on a

 

device that is running STP, the port used for the connection goes through a

 

sequence of states (Listening and Learning) before getting to its final state

 

(Forwarding or Blocking, as determined by the STP negotiation). This

 

sequence takes two times the forward delay value configured for the switch.

 

The default is 15 seconds on ProCurve switches, per the IEEE 802.1D standard

 

recommendation, resulting in a total STP negotiation time of 30 seconds. Each

 

switch port goes through this start-up sequence whenever the network con­

 

nection is established on the port. This includes, for example, when the switch

 

or connected device is powered up, or the network cable is connected.

 

A problem can arise from this long STP start-up sequence because some end

 

nodes are configured to automatically try to access a network server when­

 

ever the end node detects a network connection. Typical server access

 

includes to Novell servers, DHCP servers, and X terminal servers. If the server

 

access is attempted during the time that the switch port is negotiating its STP

 

state, the server access will fail. To provide support for this end node behavior,

 

the switches covered by this manual offer a configuration mode, called “Fast

 

Mode”, that causes the switch port to skip the standard STP start-up sequence

 

and put the port directly into the “Forwarding” state, thus allowing the server

 

access request to be forwarded when the end node needs it.

 

If you encounter end nodes that repeatedly indicate server access failure when

 

attempting to bring up their network connection, and you have enabled STP

 

on the switch, try changing the configuration of the switch ports associated

 

with those end nodes to STP Fast Mode.

 

 

C a u t i o n

The Fast Mode configuration should be used only on switch ports connected

 

to end nodes. Changing the Mode to Fast on ports connected to hubs, switches,

 

or routers may cause loops in your network that STP may not be able to

 

immediately detect, in all cases. This will cause temporary loops in your

 

network. After the fast start-up sequence, though, the switch ports operate

 

according to the STP standard, and will adjust their state to eliminate continu­

 

ing network loops.

 

 

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