Reference Guide

• 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 on the S4820T platform.
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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