Users Guide

Preventing Forwarding Loops in a VLT Domain
During the bootup of VLT peer switches, a forwarding loop may occur until the VLT configurations are
applied on each switch and the primary/secondary roles are determined.
To prevent the interfaces in the VLT interconnect trunk and RSTP-enabled VLT ports from entering a
Forwarding state and creating a traffic loop in a VLT domain, follow these steps.
1 Configure RSTP in the core network and on each peer switch as described in
Rapid Spanning Tree Protocol (RSTP).
Disabling RSTP on one VLT peer may result in a VLT domain failure.
2 Enable RSTP on each peer switch.
PROTOCOL SPANNING TREE RSTP mode
no disable
3 Configure each peer switch with a unique bridge priority.
PROTOCOL SPANNING TREE RSTP mode
bridge-priority
Sample RSTP Configuration
The following is a sample of an RSTP configuration.
Using the example shown in the Overview section as a sample VLT topology, the primary VLT switch sends
BPDUs to an access device (switch or server) with its own RSTP bridge ID. The primary VLT switch process
BPDUs generated by an RSTP-enabled access device. The secondary VLT switch tunnels the BPDUs that it
receives to the primary VLT switch over the VLT interconnect. Only the primary VLT switch determines the
RSTP roles and states on VLT ports and ensures that the VLT interconnect link is never blocked.
In the case of a primary VLT switch failure, the secondary switch starts sending BPDUs with its own bridge ID
and inherits all the port states from the last synchronization with the primary switch. An access device never
detects the change in primary/secondary roles and does not see it as a topology change.
The following examples show the RSTP configuration that you must perform on each peer switch to prevent
forwarding loops.
Configure RSTP on VLT Peers to Prevent Forwarding Loops
(VLT Peer 1)
Dell_VLTpeer1(conf)#protocol spanning-tree rstp
Dell_VLTpeer1(conf-rstp)#no disable
Dell_VLTpeer1(conf-rstp)#bridge-priority 4096
Virtual Link Trunking (VLT) 1137