Setup Guide

BPDUs use the MAC address of the primary VLT peer as the RSTP bridge ID in the designated bridge ID field. The primary VLT
peer sends these BPDUs on VLT interfaces connected to access devices. The MAC address for a VLT domain is automatically
selected on the peer switches when you create the domain (refer to Enabling VLT and Creating a VLT Domain).
Configure both ends of the VLT interconnect trunk with identical RSTP configurations. When you enable VLT, the show
spanning-tree rstp brief command output displays VLT information (refer to Verifying a VLT Configuration).
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, take the following 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. BPDUs generated by an RSTP-enabled access device are only processed
by the primary VLT switch. 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 (primary)
Dell_VLTpeer1(conf)#protocol spanning-tree rstp
Dell_VLTpeer1(conf-rstp)#no disable
Dell_VLTpeer1(conf-rstp)#bridge-priority 4096
Configure RSTP on VLT peers to prevent forwarding loops—VLT peer 2
(secondary)
Dell_VLTpeer2(conf)#protocol spanning-tree rstp
Dell_VLTpeer2(conf-rstp)#no disable
Dell_VLTpeer2(conf-rstp)#bridge-priority 8192
NOTE:
When you remove the VLT configuration, RSTP is recommended as a backup solution to avoid spanning—tree
loops. In this case, you can use the following RSTP default values:
● hello–time: 2 seconds
● forward–delay: 15 seconds
Virtual Link Trunking (VLT) 883