• VRRP elects the router with the highest priority as the master in the VRRP group. To ensure VRRP operation in a VLT domain,configure VRRP group priority on each VLT peer so that a peer is either the master or backup for all VRRP groups configured on itsinterfaces. For more information, see Setting VRRP Group (Virtual Router) Priority.• To verify that a VLT peer is consistently configured for either the master or backup role in all VRRP groups, use the show vrrpcommand on each peer.• Configure the same L3 routing (static and dynamic) on each peer so that the L3 reachability and routing tables are identical on bothVLT peers. Both the VRRP master and backup peers must be able to locally forward L3 traffic in the same way.• In a VLT domain, although both VLT peers actively participate in L3 forwarding as the VRRP master or backup router, the showvrrp command output displays one peer as master and the other peer as backup.• Failure scenarios• On a link failover, when a VLT port channel fails, the traffic destined for that VLT port channel is redirected to the VLTi to avoidflooding.• When a VLT switch determines that a VLT port channel has failed (and that no other local port channels are available), the peerwith the failed port channel notifies the remote peer that it no longer has an active port channel for a link. The remote peer thenenables data forwarding across the interconnect trunk for packets that would otherwise have been forwarded over the failed portchannel. This mechanism ensures reachability and provides loop management. If the VLT interconnect fails, the VLT software on theprimary switch checks the status of the remote peer using the backup link. If the remote peer is up, the secondary switch disablesall VLT ports on its device to prevent loops.• If all ports in the VLT interconnect fail, or if the messaging infrastructure fails to communicate across the interconnect trunk, theVLT management system uses the backup link interface to determine whether the failure is a link-level failure or whether the remotepeer has failed entirely. If the remote peer is still alive (heartbeat messages are still being received), the VLT secondary switchdisables its VLT port channels. If keepalive messages from the peer are not being received, the peer continues to forward traffic,assuming that it is the last device available in the network. In either case, after recovery of the peer link or reestablishment ofmessage forwarding across the interconnect trunk, the two VLT peers resynchronize any MAC addresses learned whilecommunication was interrupted and the VLT system continues normal data forwarding.• If the primary chassis fails, the secondary chassis takes on the operational role of the primary.• The SNMP MIB reports VLT statistics.Primary and Secondary VLT PeersTo prevent issues when connectivity between peers is lost, you can designate Primary and Secondary roles for VLT peers . You can elect orconfigure the Primary Peer. By default, the peer with the lowest MAC address is selected as the Primary Peer. You can configure anotherpeer as the Primary Peer using the VLT domain domain-id role priority priority-value command.If the VLTi link fails, the status of the remote VLT Primary Peer is checked using the backup link. If the remote VLT Primary Peer is available,the Secondary Peer disables all VLT ports to prevent loops.If all ports in the VLTi link fail or if the communication between VLTi links fails, VLT checks the backup link to determine the cause of thefailure. If the failed peer can still transmit heartbeat messages, the Secondary Peer disables all VLT member ports and any Layer 3interfaces attached to the VLAN associated with the VLT domain. If heartbeat messages are not received, the Secondary Peer forwardstraffic assumes the role of the Primary Peer. If the original Primary Peer is restored, the VLT peer reassigned as the Primary Peer retainsthis role and the other peer must be reassigned as a Secondary Peer. Peer role changes are reported as SNMP traps.RSTP and VLTVLT provides loop-free redundant topologies and does not require RSTP.RSTP can cause temporary port state blocking and may cause topology changes after link or node failures. Spanning tree topology changesare distributed to the entire layer 2 network, which can cause a network-wide flush of learned MAC and ARP addresses, requiring theseaddresses to be re-learned. However, enabling RSTP can detect potential loops caused by non-system issues such as cabling errors orincorrect configurations. To minimize possible topology changes after link or node failure, RSTP is useful for potential loop detection.Configure RSTP using the following specifications.The following recommendations help you avoid these issues and the associated traffic loss caused by using RSTP when you enable VLT onboth VLT peers:Virtual Link Trunking (VLT) 987
