Concept Guide

– 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 congure the primary role manually.
– In a VLT domain, the peer switches must run the same Dell EMC Networking OS software version.
– Separately congure 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 nd 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
dierent 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 conguring 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 trac only when there is a link failure that requires using VLTi in order for data packets to
reach their nal destination.
– Unknown, multicast, and broadcast trac can be ooded across the VLT interconnect.
– MAC addresses for VLANs congured 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 congured across the VLTi are the same on both VLT peer nodes.
– 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 congure 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 ap.
– In a VLT domain, the following software features are supported on VLTi: link layer discovery protocol (LLDP), ow control, port
monitoring, and jumbo frames.
– When you enable the VLTi link, the link between the VLT peer switches is established if the following congured information is true
on both peer switches:
◦ the VLT system MAC address matches.
◦ the VLT unit-id is not identical.
NOTE
: If you congure the VLT system MAC address or VLT unit-id on only one of the VLT peer switches, the link
between the VLT peer switches is not established. Each VLT peer switch must be correctly congured to establish the
link between the peers.
– If the link between the VLT peer switches is established, changing the VLT system MAC address or the VLT unit-id causes the link
between the VLT peer switches to become disabled. However, removing the VLT system MAC address or the VLT unit-id may
disable the VLT ports if you happen to congure the unit ID or system MAC address on only one VLT peer at any time.
– If the link between VLT peer switches is established, any change to the VLT system MAC address or unit-id fails if the changes
made create a mismatch by causing the VLT unit-ID to be the same on both peers and/or the VLT system MAC address does not
match on both peers.
– If you replace a VLT peer node, precongure the switch with the VLT system MAC address, unit-id, and other VLT parameters
before connecting it to the existing VLT peer switch using the VLTi connection.
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Virtual Link Trunking (VLT)