DELL 9.7(0.0) User Manual

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.
• If you include RSTP on the system, configure it before VLT. Refer to 

• Dell Networking strongly recommends that the VLTi (VLT interconnect) be a static LAG and that you 

disable LACP on the VLTi.

• Ensure that the spanning tree root bridge is at the Aggregation layer. If you enable RSTP on the VLT 

 for guidelines to avoid traffic loss.

• If you reboot both VLT peers in BMP mode and the VLT LAGs are static, the DHCP server reply to the 

DHCP discover offer may not be forwarded by the ToR to the correct node. To avoid this scenario, 

configure the VLT LAGs to the ToR and the ToR port channel to the VLT peers with LACP. If supported 

by the ToR, enable the lacp-ungroup feature on the ToR using the lacp ungroup member-
independent port-channel command.

• 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.

• When you enable IGMP snooping on the VLT peers, ensure the value of the delay-restore 

command is not less than the query interval.

• When you enable Layer 3 routing protocols on VLT peers, make sure the delay-restore timer is set to a 

value that allows sufficient time for all routes to establish adjacency and exchange all the L3 routes 

between the VLT peers before you enable the VLT ports.

• Only use the lacp ungroup member-independent command if the system connects to nodes 

using bare metal provisioning (BMP) to upgrade or boot from the network.

• Ensure that you configure all port channels where LACP ungroup is applicable as hybrid ports and as 

untagged members of a VLAN. BMP uses untagged dynamic host configuration protocol (DHCP) 

packets to communicate with the DHCP server.

• If the DHCP server is located on the ToR and the VLTi (ICL) is down due to a failed link when a VLT 

node is rebooted in BMP mode, it is not able to reach the DHCP server, resulting in BMP failure.

• If the source is connected to an orphan (non-spanned, non-VLT) port in a VLT peer, the receiver is 

connected to a VLT (spanned) port-channel, and the VLT port-channel link between the VLT peer 

connected to the source and TOR is down, traffic is duplicated due to route inconsistency between 

peers. To avoid this scenario, Dell Networking recommends configuring both the source and the 

receiver on a spanned VLT VLAN.

• After you enter the clear arp command on a Z9500 configured as the primary VLT peer switch, an 

ARP request destined for the secondary VLT peer that arrives from a host and is tunneled through the 

primary peer updates the ARP entry in the Route Processor (RP) on the primary peer. However, the 

same ARP request packet is dropped on the Control Processor (CP) because it is not destined to the 

primary peer and the CP has no corresponding ARP entry that can be refreshed with this packet. As a 

result, there is an ARP entry mismatch in the RP and CP tables. There is no impact on switch behavior.

• Bulk synchronization happens only for global IPv6 Neighbors; link-local neighbor entries are not 

synced.

• If all of the following conditions are true, MAC addresses may not be synced correctly:

– VLT peers use VLT interconnect (VLTi)
– Sticky MAC is enabled on an orphan port in the primary or secondary peer
– MACs are currently inactive

Virtual Link Trunking (VLT)

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