DELL N3000 User Manual

Configuring the Spanning Tree Protocol

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.

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 

by default. Delaying the switchover allows the connected port to go to 

through the listening and learning states while the switch is still 

transmitting packets on the original uplink.

The optimal behavior is to keep the current uplink active and hold the new 

port in the blocked state for twice the forwarding delay.