Reference Guide

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Virtual Link Trunking (VLT)
Virtual link trunking (VLT) is supported on the S4820T platform.
Overview
VLT allows physical links between two chassis to appear as a single virtual link to the network core or other switches
such as Edge, Access, or top-of-rack (ToR).
VLT reduces the role of spanning tree protocols (STPs) by allowing link aggregation group (LAG) terminations on two
separate distribution or core switches, and by supporting a loop-free topology. (To prevent the initial loop that may
occur prior to VLT being established, use a spanning tree protocol. After VLT is established, you may use rapid spanning
tree protocol (RSTP) to prevent loops from forming with new links that are incorrectly connected and outside the VLT
domain.)
VLT provides Layer 2 multipathing, creating redundancy through increased bandwidth, enabling multiple parallel paths
between nodes and load-balancing traffic where alternative paths exist.
Virtual link trunking offers the following benefits:
• Allows a single device to use a LAG across two upstream devices.
• Eliminates STP-blocked ports.
• Provides a loop-free topology.
• Uses all available uplink bandwidth.
• Provides fast convergence if either the link or a device fails.
• Optimized forwarding with virtual router redundancy protocol (VRRP).
• Provides link-level resiliency.
• Assures high availability.
CAUTION: Dell Networking does not recommend enabling Stacking and VLT simultaneously. If you enable both
features at the same time, unexpected behavior occurs.
As shown in the following example, VLT presents a single logical Layer 2 domain from the perspective of attached
devices that have a virtual link trunk terminating on separate chassis in the VLT domain. However, the two VLT chassis
are independent Layer2/Layer3 (L2/L3) switches for devices in the upstream network. L2/L3 control plane protocols and
system management features function normally in VLT mode. Features such as VRRP and internet group management
protocol (IGMP) snooping require state information coordinating between the two VLT chassis. IGMP and VLT
configurations must be identical on both sides of the trunk to ensure the same behavior on both sides.
The following example shows VLT deployed on S4820T switches. The S4820T switches appear as a single virtual switch
from the point of view of the switch or server supporting link aggregation control protocol (LACP).
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