Configuring Advanced Features

Spanning Tree Protocol (STP)

•Active path from node A to node B: 1—> 3

•Backup (redundant) path from node A to node B: 4 —> 2 —> 3

1

 

 

switch A

 

3

 

 

 

 

 

path cost:

 

 

2

 

path cost: 100

100

 

 

 

 

 

 

 

 

 

 

 

 

 

path cost: 100

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

switch B

 

switch C

 

switch D

 

 

4

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

path cost:200

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

node

 

 

 

 

 

 

node

 

A

 

 

 

 

 

 

B

 

 

 

 

 

 

 

 

 

 

 

 

Figure 9-71. Example of Redundant Paths Between Two NodesSTP 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 HP 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 connection 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 whenever 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 2512 and 2524 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.

Features

Advanced Configuring

 

 

9-109