Administrator Guide

○ In a VLT domain, VRRP interoperates with virtual link trunks that carry traffic to and from access devices (refer to
Overview). The VLT peers belong to the same VRRP group and are assigned master and backup roles. Each peer actively
forwards L3 traffic, reducing the traffic flow over the VLT interconnect.
○ 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 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
To prevent issues when connectivity between peers is lost, you can designate Primary and Secondary roles for VLT peers . 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.
RSTP and VLT
VLT 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 changes are distributed to the entire layer 2 network, which can cause a network-wide flush of learned MAC and
ARP addresses, requiring these addresses to be re-learned. However, enabling RSTP can detect potential loops caused by
non-system issues such as cabling errors or incorrect 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 on both VLT peers:
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Virtual Link Trunking (VLT)