White Papers

Stacking and VLT
Dell EMC Technical White Paper
3.2 VLT operation
Both the VLT nodes of domain always continue to forward data plane traffic in active/active mode. With the
instantaneous synchronization of MAC and ARP entries, both the nodes remain active/active and continue to
forward the data traffic seamlessly.
3.3 VLT functionality
The same VLT domain-id should be configured on both VLT nodes. The unit-id 1 and 2 for the nodes is
configured automatically. For rapid convergence and optimal service, the same VLT MAC address should be
configured on both the nodes using vlt-mac command. The priority of the primary node election is based on
the lower system MAC-address of the switch, however with the primary-priority command, the VLT node with
the least configured priority takes over as primary. This election will not be preempted. For example, when the
primary node is reloaded, it is assigned the secondary role. The role change avoids disruptions in traffic flow
due to the election process.
Election happens only during the initial configuration or when VLT is first launched. The VLT role election has
no significance for the data traffic flowing through the VLT domain. It is only used for the control protocol
exchange. VLAN ID 4094 is assigned automatically and internally reserved as a control VLAN for the exchange
of VLT-related information between the nodes. The IPv6 address that is automatically assigned within the
reserved range is mapped for VLAN 4094 for reachability between the VLT nodes.
For the VLT interconnect (VLTi) link, the discovery interfaces are configured on both the nodes, port-channel
1000 is automatically configured, mapping the physical discovery interfaces. The ports should be configured as
no switchport from the default layer-2 mode while configuring the discovery interfaces.
For VLT port channels, the user should explicitly assign the vlt-port-channel ID to the configured port channel
on both the nodes. This port channel identifier should be same across both the nodes.
3.4 VLT backup
VLT backup link is an additional link used to check the availability of the peer nodes in the VLT domain. When
the VLTi interface goes down, the backup link helps to differentiate the VLTi link failure from peer node failure.
If the VLTi link fails, all the VLT nodes exchange node liveliness information through the backup link.