Administrator Guide

Configuration Notes
When you configure VLT, the following conditions apply.
● With VLT, when an L3 interface is created, the local DA of that interface is added as an L2 entry pointing to the ICL
interface on the peer chassis. This ensures that the L3 packets reaching the peer, by LAG hashing on ToR, get forwarded to
the actual chassis via ICL and then get routed. When this interface is removed, the entry pointing to ICL on the peer chassis
is deleted.
● 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.
○ ARP tables are synchronized between the VLT peer nodes.
○ VLT peer switches operate as separate chassis with independent control and data planes for devices attached on non-
VLT ports.
○ One device in the VLT domain is assigned a primary role; the other device takes the secondary role. The primary and
secondary roles are required for scenarios when connectivity between the chassis is lost. VLT assigns the primary chassis
role according to the lowest MAC address. You can configure the primary role manually.
○ In a VLT domain, the peer switches must run the same Dell EMC Networking OS software version.
○ Separately configure each VLT peer switch with the same VLT domain ID and the VLT version. If the system detects
mismatches between VLT peer switches in the VLT domain ID or VLT version, the VLT Interconnect (VLTi) does not
activate. To find the reason for the VLTi being down, use the show vlt statistics command to verify that there
are mismatch errors, then use the show vlt brief command on each VLT peer to view the VLT version on the peer
switch. If the VLT version is more than one release different from the current version in use, the VLTi does not activate.
○ The chassis members in a VLT domain support connection to orphan hosts and switches that are not connected to both
switches in the VLT core.
● VLT interconnect (VLTi)
○ The VLT interconnect can consist of 1G or 10G ports. A maximum of eight ports are supported.
○ The port channel must be in Default mode (not Switchport mode) to have VLTi recognize it.
○ The system automatically includes the required VLANs in VLTi. You do not need to manually select VLANs.
○ VLT peer switches operate as separate chassis with independent control and data planes for devices attached to non-
VLT ports.
○ Port-channel link aggregation (LAG) across the ports in the VLT interconnect is required; individual ports are not
supported. Dell EMC Networking strongly recommends configuring a static LAG for VLTi.
○ The VLT interconnect synchronizes L2 and L3 control-plane information across the two chassis.
○ The VLT interconnect is used for data traffic only when there is a link failure that requires using VLTi in order for data
packets to reach their final destination.
○ Unknown, multicast, and broadcast traffic can be flooded across the VLT interconnect.
○ MAC addresses for VLANs configured across VLT peer chassis are synchronized over the VLT interconnect on an egress
port such as a VLT LAG. MAC addresses are the same on both VLT peer nodes.
○ ARP entries configured across the VLTi are the same on both VLT peer nodes.
○ In certain scenarios, both the compute node NICs configured in active-passive mode erroneously respond to an
ARPREQUEST initiated on VLTi (ICL), and both VLT nodes sync the ARPREPLY on VLTi multiple times resulting in
frequent MAC moves. In this scenario, disable the additional ARP refresh on VLTi using the command no arp
additional-refresh in vlt-domain configuration mode.
○ If you shut down the port channel used in the VLT interconnect on a peer switch in a VLT domain in which you did not
configure a backup link, the switch’s role displays in the show vlt brief command output as Primary instead of
Standalone.
○ When you change the default VLAN ID on a VLT peer switch, the VLT interconnect may flap.
○ In a VLT domain, the following software features are supported on VLTi: link layer discovery protocol (LLDP), flow
control, port monitoring, and jumbo frames.
○ When you enable the VLTi link, the link between the VLT peer switches is established if the following configured
information is true on both peer switches:
￭ the VLT system MAC address matches.
￭ the VLT unit-id is not identical.
Virtual Link Trunking (VLT)
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