Reference Guide

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
• You cannot enable S5000 stacking simultaneously with VLT. If you enable both at the same time,
unexpected behavior occurs. For more information, refer to VLT and Stacking.
• VLT is not supported on an S5000 configured for FCoE transit or NPIV proxy gateway.
• VLT port channel interfaces must be switch ports.
• If you include RSTP on the system, configure it before VLT. Refer to Configuring Rapid Spanning Tree.
• 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
device, refer to RSTP and 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.
Configuration Notes
When you configure VLT, the following conditions apply.
• 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.
978
Virtual Link Trunking (VLT)