Reference Guide

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 Stacking
You cannot enable stacking with VLT.
If you enable stacking on a unit on which you want to enable VLT, you must first remove the unit from
the existing stack. After you remove the unit, you can configure VLT on the unit.
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.
VLT Port Delayed Restoration
When a VLT node boots up, if the VLT ports have been previously saved in the start-up configuration,
they are not immediately enabled.
To ensure MAC and ARP entries from the VLT per node are downloaded to the newly enabled VLT node,
the system allows time for the VLT ports on the new node to be enabled and begin receiving traffic.
The delay-restore feature waits for all saved configurations to be applied, then starts a configurable
timer. After the timer expires, the VLT ports are enabled one-by-one in a controlled manner. The delay
between bringing up each VLT port-channel is proportional to the number of physical members in the
port-channel. The default is 90 seconds.
If you enable IGMP snooping, IGMP queries are also sent out on the VLT ports at this time allowing any
receivers to respond to the queries and update the multicast table on the new node.
This delay in bringing up the VLT ports also applies when the VLTi link recovers from a failure that caused
the VLT ports on the secondary VLT peer node to be disabled.
VLT Routing
VLT routing is supported on the Aggregator.
Layer 2 protocols from the ToR to the server are intra-rack and inter-rack. No spanning tree is required,
but interoperability with spanning trees at the aggregation layer is supported. Communication is active-
active, with no blocked links. MAC tables are synchronized between VLT nodes for bridging and you can
enable IGMP snooping.
Spanned VLANs
Any VLAN configured on both VLT peer nodes is referred to as a Spanned VLAN. The VLT Interconnect
(VLTi) port is automatically added as a member of the Spanned VLAN. As a result, any adjacent router
connected to at least one VLT node on a Spanned VLAN subnet is directly reachable from both VLT peer
nodes at the routing level.
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PMUX Mode of the IO Aggregator