Users Guide

840 Spanning Tree Protocol
DirectLink Rapid Convergence
The DirectLink Rapid Convergence (DRC) feature is designed for an access-
layer switch that has redundant blocked uplinks. It operates on ports blocked
by spanning tree. DRC can be configured for the entire switch; it cannot be
enabled for individual VLANs.
The DRC feature is based on the concept of an uplink group. An uplink group
consists of all the ports that provide a path to the root bridge (the root port
and any blocked ports). If the root port fails, the blocked port with next
lowest cost from the uplink group is selected and immediately put in the
forwarding state without going through the standard spanning tree listening
and learning states.
To accelerate convergence time once DRC has switched over to a new root
port, STP-PV transmits dummy packets out the new root port, with the
source MAC addresses taken from its forwarding table. The destination
address is an SSTP MAC address that ensures that the packet is flooded on
the whole network. The packets update the forwarding tables on the other
upstream switches. The rate at which the dummy multicasts are sent can be
configured by the administrator. RSTP-PV has a different mechanism
adopted from IEEE 802.1w that handles the update of the forwarding
database and the fast transition to a new uplink. DRC can be enabled on
RSTP-PV enabled switches but has no effect.
DRC is disabled when the administrator modifies the spanning-tree priority
of a VLAN and is re-enabled only when the default priority is restored.
DRC and Link Up Events
In the event of failure of the primary uplink, a replacement uplink is
immediately selected from the uplink group and put into the forwarding
state. If another port is enabled that, in accordance with STP rules, should
become the primary uplink (root port), the switch delays migrating to the
new port for twice the forwarding delay. The purpose of this delay is two-fold:
• Stability—If the primary uplink is flapping, reenabling the link
immediately can introduce additional instability into the network.
• Reduced Traffic Loss—DRC moves a port into the forwarding state as
soon as it is up, but the connected port obeys the usual STP rules; i.e. it
goes through the listening and learning stages, which take 15 seconds each