Reference Guide

upstream network. L2/L3 control plane protocols and system management features function normally in
VLT mode. Features such as VRRP and internet group management protocol (IGMP) snooping require
state information coordinating between the two VLT chassis. IGMP and VLT configurations must be
identical on both sides of the trunk to ensure the same behavior on both sides.
VLT Terminology
The following are key VLT terms.
• Virtual link trunk (VLT) — The combined port channel between an attached device and the VLT peer
switches.
• VLT backup link — The backup link monitors the vitality of VLT peer switches. The backup link sends
configurable, periodic keep alive messages between the VLT peer switches.
• VLT interconnect (VLTi) — The link used to synchronize states between the VLT peer switches. Both
ends must be on 10G or 40G interfaces.
• VLT domain — This domain includes both the VLT peer devices, VLT interconnect, and all of the port
channels in the VLT connected to the attached devices. It is also associated to the configuration
mode that you must use to assign VLT global parameters.
• VLT peer device — One of a pair of devices that are connected with the special port channel known
as the VLT interconnect (VLTi).
VLT peer switches have independent management planes. A VLT interconnect between the VLT chassis
maintains synchronization of L2/L3 control planes across the two VLT peer switches. The VLT
interconnect uses either 10G or 40G user ports on the chassis.
A separate backup link maintains heartbeat messages across an out-of-band (OOB) management
network. The backup link ensures that node failure conditions are correctly detected and are not
confused with failures of the VLT interconnect. VLT ensures that local traffic on a chassis does not
traverse the VLTi and takes the shortest path to the destination via directly attached links.
Configure Virtual Link Trunking
VLT requires that you enable the feature and then configure the same VLT domain, backup link, and VLT
interconnect on both peer switches.
Important Points to Remember
• VLT port channel interfaces must be switch ports.
• Dell Networking strongly recommends that the VLTi (VLT interconnect) be a static LAG and that you
disable LACP on the VLTi.
• If the lacp-ungroup feature is not supported on the ToR, reboot the VLT peers one at a time. After
rebooting, verify that VLTi (ICL) is active before attempting DHCP connectivity.
Configuration Notes
When you configure VLT, the following conditions apply.
• VLT domain
– A VLT domain supports two chassis members, which appear as a single logical device to network
access devices connected to VLT ports through a port channel.
– A VLT domain consists of the two core chassis, the interconnect trunk, backup link, and the LAG
members connected to attached devices.
– Each VLT domain has a unique MAC address that you create or VLT creates automatically.
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PMUX Mode of the IO Aggregator