16-10
Spanning Tree Protocol (STP)
How STP Operates
Spanning Tree Protocol
(STP)
Figure 16-3. Example of Redundant Paths Between Two Nodes
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 HP switches, per the IEEE 802.1D standard
recommendation, resulting in a total STP negotiati on 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 sequ ence 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 Switch 4108GL offers a configuration mode, ca lled “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.
node
A
switch A
node
B
switch Dswitch B switch C
1
path cost:
100
3
path cost: 100
4
path cost:200
•Active path from node A to node B: 1—> 3
•Backup (redundant) path from node A to node B: 4 —> 2 —> 3
2
path cost: 100