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Configuring STP Bridge Parameters Configuring Spanning Tree Parameters

page 6-18 OmniSwitch 6800/6850/9000 Network Configuration Guide June 2006

Configuring the Bridge Forward Delay Time

The bridge forward delay time specifies how long, in seconds, a port remains in the learning state while it

is transitioning to a forwarding state. In addition, when a topology change occurs, the forward delay time

value is used to age out all dynamically learned addresses in the MAC address forwarding table. For more

information about the MAC address table, see Chapter 2, “Managing Source Learning.”

The forward delay time propagated in a root bridge Configuration BPDU is the value used by all other

bridges in the tree for their own forward delay time. Therefore, if this value is changed for the root bridge,

all other bridges associated with the same instance will adopt this value as well.

If the switch is running in the 1x1 Spanning Tree mode, then a forward delay time value is defined for

each VLAN instance. If the switch is running in the flat Spanning Tree mode, then the forward delay time

value is defined for the flat mode instance. In both cases, the default forward delay time used is 15

seconds.

Note that specifying a low forward delay time may cause temporary network loops, because packets may

get forwarded before Spanning Tree configuration or change notices have reached all nodes in the

network.

To change the bridge forward delay time value for a VLAN instance, specify a VLAN ID with the bridge

forward delay command when the switch is running in the 1x1 mode. For example, the following

command changes the forward delay time for VLAN 455 to 10 seconds:

> bridge 455 forward delay 20

The explicit bridge 1x1 forward delay command configures the forward delay time for a VLAN instance

when the switch is running in either mode (1x1 or flat). For example, the following command performs the

same function as the command in the previous example:

-> bridge 1x1 455 forward delay 20

To change the forward delay time value for the flat mode instance, use either the bridge forward delay

command or the bridge cist forward delay command. Note that both commands are available when the

switch is running in either mode (1x1 or flat) and an instance number is not required. For example, the

following commands change the forward delay time for the flat mode instance to 10:

-> bridge forward delay 10

-> bridge cist forward delay 10

As in previous releases, it is possible to configure the flat mode instance with the bridge forward delay

command by specifying 1 as the instance number (e.g., bridge 1 forward delay 30). However, this is only

available when the switch is already running in the flat mode and STP or RSTP is the active protocol.

Note that the forward delay time is not configurable for 802.1s Multiple Spanning Tree Instances (MSTI).

These instances inherit the forward delay time from the flat mode instance (CIST).

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