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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.
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.
VLT Bandwidth Monitoring
When bandwidth usage of the VLTi (ICL) exceeds 80%, a syslog error message (shown in the following message) and an SNMP
trap are generated.
%STKUNIT0-M:CP %VLTMGR-6-VLT-LAG-ICL: Overall Bandwidth utilization of VLT-ICL-LAG (port-
channel 25)
crosses threshold. Bandwidth usage (80 )
When the bandwidth usage drops below the 80% threshold, the system generates another syslog message (shown in the
following message) and an SNMP trap.
%STKUNIT0-M:CP %VLTMGR-6-VLT-LAG-ICL: Overall Bandwidth utilization of VLT-ICL-LAG (port-
channel 25)
reaches below threshold. Bandwidth usage (74 )VLT show remote port channel status
VLT and IGMP Snooping
When configuring IGMP Snooping with VLT, ensure the configurations on both sides of the VLT trunk are identical to get the
same behavior on both sides of the trunk.
When you configure IGMP snooping on a VLT node, the dynamically learned groups and multicast router ports are automatically
learned on the VLT peer node.
Virtual Link Trunking (VLT)
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