Filter
Top Products
Chapter 40: STP, RSTP and MSTP Protocols
566
Forwarding Delay and Topology Changes
If there is a change in the network topology due to a failure, removal, or
addition of any active components, the active topology also changes. This
may trigger a change in the state of some blocked ports. However, a
change in a port state is not activated immediately.
It might take time for the root bridge to notify all bridges that a topology
change has occurred, especially if it is a large network. If a topology
change is made before all the bridges have been notified, a temporary
data loop could occur, and that could adversely impact network
performance.
To forestall the formation of temporary data loops during topology
changes, a port designated to change from blocking to forwarding passes
through two additional states—listening and learning—before beginning to
forward frames. The amount of time a port spends in these states is set by
the forwarding delay value. This value states the amount of time that a
port spends in the listening and learning states prior to changing to the
forwarding state.
The forwarding delay value is adjustable on the switch. The appropriate
value for this parameter depends on a number of variables, with the size
of your network being a primary factor. For large networks, you should
specify a value large enough to allow the root bridge sufficient time to
propagate a topology change throughout the entire network. For small
networks, you should not specify a value so large that a topology change
is needlessly delayed, which could result in the delay or loss of some data
packets.
Note
The forwarding delay parameter applies only to ports on the switch
that are operating STP-compatible mode.