Administrator Guide

The following example shows how VLT is deployed. The switches appear as a single virtual switch from the point of view of the switch or
server supporting link aggregation control protocol (LACP).
VLT on Core Switches
You can also deploy VLT on core switches.
Uplinks from servers to the access layer and from access layer to the aggregation layer are bundled in LAG groups with end-to-end Layer
2 multipathing. This set up requires “horizontal” stacking at the access layer and VLT at the aggregation layer such that all the uplinks
from servers to access and access to aggregation are in Active-Active Load Sharing mode. This example provides the highest form of
resiliency, scaling, and load balancing in data center switching networks.
The following example shows stacking at the access, VLT in aggregation, and Layer 3 at the core.
The aggregation layer is mostly in the L2/L3 switching/routing layer. For better resiliency in the aggregation, Dell Networking recommends
running the internal gateway protocol (IGP) on the VLTi VLAN to synchronize the L3 routing table across the two nodes on a VLT system.
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 10G or 40G
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.
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 Configure 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.
• 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.
Virtual Link Trunking (VLT)
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