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VLT Terminology
The following are key VLT terms.
● Virtual link trunk (VLT) — The combined port channel between an attached device and the VLT peer switches.
● VLT backup link — The backup link monitors the vitality of VLT peer switches. The backup link sends configurable, periodic
keep alive messages between the VLT peer switches.
● VLT interconnect (VLTi) — The link used to synchronize states between the VLT peer switches. Both ends must be on 10
GbE or 40 GbE interfaces.
● VLT domain — This domain includes both the VLT peer devices, VLT interconnect, and all of the port channels in the VLT
connected to the attached devices. It is also associated to the configuration mode that you must use to assign VLT global
parameters.
● VLT peer device — One of a pair of devices that are connected with the special port channel known as the VLT
interconnect (VLTi).
VLT peer switches have independent management planes. A VLT interconnect between the VLT chassis maintains
synchronization of L2/L3 control planes across the two VLT peer switches. The VLT interconnect uses either 10G or 40G
user ports on the chassis.
A separate backup link maintains heartbeat messages across an out-of-band (OOB) management network. The backup link
ensures that node failure conditions are correctly detected and are not confused with failures of the VLT interconnect. VLT
ensures that local traffic on a chassis does not traverse the VLTi and takes the shortest path to the destination via directly
attached links.
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.
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Virtual Link Trunking (VLT)