Reference Guide

Virtual Link Trunking (VLT) | 299
You must configure both ends of the VLT interconnect trunk with identical RSTP configurations. When
VLT is enabled, the
show spanning-tree rstp brief command output displays VLT information.
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, you must take the following steps:
Sample RSTP Configuration
Using Figure 22-1 as a sample VLT topology, the primary VLT switch will send 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 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 figures show an example of the RSTP configuration that you must perform on each peer
switch to prevent forwarding loops.
Figure 22-2. Configuring RSTP on VLT Peers to Prevent Forwarding Loops (VLT Peer 1)
Figure 22-3. Configuring RSTP on VLT Peers to Prevent Forwarding Loops (VLT Peer 2)
Step Task Command Syntax Command Mode
1 Configure RSTP in the core network and on each peer switch as described in Chapter 27, 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.
no disable
PROTOCOL
SPANNING TREE
RSTP
3 Configure each peer switch with a unique bridge
priority.
bridge-priority
PROTOCOL
SPANNING TREE
RSTP
FTOS_VLTpeer1(conf)#protocol spanning-tree rstp
FTOS_VLTpeer1(conf-rstp)#no disable
FTOS_VLTpeer1(conf-rstp)#bridge-priority 4096
FTOS_VLTpeer2(conf)#protocol spanning-tree rstp
FTOS_VLTpeer2(conf-rstp)#no disable
FTOS_VLTpeer2(conf-rstp)#bridge-priority 0