Configuring Spanning Tree Protocol (STP) and IronSpan Features
December 2005 © Foundry Networks, Inc. 7 - 55
Reconvergence Time
Spanning tree reconvergence using 802.1W Draft 3 can occur within one second.
After the spanning tree reconverges following the topology change, traffic also must reconverge on all the bridges
attached to the spanning tree. This is true regardless of whether 802.1W Draft 3 or standard STP is used to
reconverge the spanning tree.
Traffic reconvergence happens after the spanning tree reconvergence, and is achieved by flushing the Layer 2
information on the bridges.
• Following 802.1W Draft 3 reconvergence of the spanning tree, traffic reconvergence occurs in the time it takes
for the bridge to detect the link changes plus the STP maximum age set on the bridge.
• If standard STP reconvergence occurs instead, traffic reconvergence takes two times the forward delay plus
the maximum age.
NOTE: 802.1W Draft 3 does not apply when a failed root port comes back up. In this case, standard STP is
used.
Configuration Considerations
802.1W Draft 3 is disabled by default. To ensure optimal performance of the feature before you enable it:
• Configure the bridge priorities so that the root bridge is one that supports 802.1W Draft 3. (Use a Foundry
device or third-party device that supports 802.1W Draft 3.)
• Change the forwarding delay on the root bridge to a value lower than the default 15 seconds. Foundry
recommends a value from 3 – 10 seconds. The lower forwarding delay helps reduce reconvergence delays in
cases where 802.1W Draft 3 is not applicable, such as when a failed root port comes back up.
• Configure the bridge priorities and root port costs so that each device has an active path to the root bridge if
its root port becomes unavailable. For example, port 3/4 is connected to port 2/4 on Switch 2, which has the
second most favorable bridge priority in the spanning tree.
NOTE: If reconvergence involves changing the state of a root port on a bridge that supports 802.1D STP but not
802.1W Draft 3, then reconvergence still requires the amount of time it takes for the ports on the 802.1D bridge to
change state to forwarding (as needed), and receive BPDUs from the root bridge for the new topology.
Enabling 802.1W Draft 3
802.1W Draft 3 is disabled by default. The procedure for enabling the feature differs depending on whether single
STP is enabled on the device.
NOTE: STP must be enabled before you can enable 802.1W Draft 3.
Enabling 802.1W Draft 3 When Single STP Is Not Enabled
By default, each port-based VLAN on the device has its own spanning tree. To enable 802.1W Draft 3 in a port-
based VLAN, enter commands such as the following:
FESX424 Router(config)# vlan 10
FESX424 Router(config-vlan-10)# spanning-tree rstp
Syntax: [no] spanning-tree rstp
This command enables 802.1W Draft 3. You must enter the command separately in each port-based VLAN in
which you want to run 802.1W Draft 3.
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