Reference Guide

○ 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.
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:
● Configure any ports at the edge of the spanning tree’s operating domain as edge ports, which are directly connected to end
stations or server racks. Disable RSTP on ports connected directly to Layer 3-only routers not running STP or configure
them as edge ports.
● Ensure that the primary VLT node is the root bridge and the secondary VLT peer node has the second-best bridge ID in the
network. If the primary VLT peer node fails, the secondary VLT peer node becomes the root bridge, avoiding problems with
spanning tree port state changes that occur when a VLT node fails or recovers.
● Even with this configuration, if the node has non-VLT ports using RSTP that you did not configure as edge ports and are
connected to other Layer 2 switches, spanning tree topology changes are still detected after VLT node recovery. To avoid
this scenario, ensure that you configure any non-VLT ports as edge ports or disable RSTP.
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Virtual Link Trunking (VLT)