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the master or backup for all VRRP groups configured on its interfaces. For more information, refer
to 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 vrrp command on each peer.
– Also configure the same L3 routing (static and dynamic) on each peer so that the L3 reachability
and routing tables are identical on both VLT 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 show vrrp 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 avoid flooding.
– When a VLT switch determines that a VLT port channel has failed (and that no other local port
channels are available), the peer with the failed port channel notifies the remote peer that it no
longer has an active port channel for a link. The remote peer then enables data forwarding across
the interconnect trunk for packets that would otherwise have been forwarded over the failed port
channel. This mechanism ensures reachability and provides loop management. If the VLT
interconnect fails, the VLT software on the primary switch checks the status of the remote peer
using the backup link. If the remote peer is up, the secondary switch disables all 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, the VLT management system uses the backup link interface to
determine whether the failure is a link-level failure or whether the remote peer has failed entirely.
If the remote peer is still alive (heartbeat messages are still being received), the VLT secondary
switch disables 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 of message forwarding across the
interconnect trunk, the two VLT peers resynchronize any MAC addresses learned while
communication 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 Peers
Primary and secondary VLT peers are supported to prevent issues when connectivity between peers is
lost on the switch.
You can elect or configure the Primary Peer. By default, the peer with the lowest MAC address is selected
as the Primary Peer. You can configure another peer 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 the failure. If the failed peer can still transmit heartbeat messages, the
Secondary Peer disables all VLT member ports and any Layer 3 interfaces attached to the VLAN
associated with the VLT domain. If heartbeat messages are not received, the Secondary Peer forwards
traffic assumes the role of the Primary Peer. If the original Primary Peer is restored, the VLT peer
reassigned as the Primary Peer retains this role and the other peer must be reassigned as a Secondary
Peer. Peer role changes are reported as SNMP traps.
Virtual Link Trunking (VLT)
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