White Papers

Stacking and VLT
Dell EMC Technical White Paper
The back-up heartbeat messages are exchanged between the VLT peers through the back-up links of the
management network. Therefore, if the VLTi link fails and the peers continue to exchange the heartbeat
messages, the primary VLT peer knows that the secondary VLT peer is up. Since the MAC/ARP entries cannot
be synchronized between the two VLT peers, the secondary VLT node closes the VLT port-channel as shown
in the Figure 5. Similarly, the north-south traffic only flows through the primary VLT peer. When the VLTi link
is restored, the secondary peer waits for the pre-configured time for MAC/ARP entries to synchronize before
passing the traffic.
Figure 5: The VLTi failure (split brain scenario)
If the VLT peers don’t even exchange the heartbeat messages, both nodes take the role of primary node and
continue to pass the traffic.
3.6 VLT configuration
In order to configure VLT you must verify that both VLT peers are running the same operating system version.
To prevent loops in a VLT domain, enable STP globally using the spanning-tree mode command.
Figure 6: Lab setup for VLT configuration
VLT configurations on Peer 1
VLTPeer1(config)# spanning-tree mode rstp
VLTPeer1(config)# interface ethernet 1/1/40
VLTPeer1(conf-if-eth1/1/40)# no switchport
VLTPeer1(conf-if-eth1/1/40)# exit