Reference Guide

Virtual Link Trunking (VLT)

Virtual link trunking (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 ToR. VLT reduces the role of Spanning Tree

protocols by allowing LAG terminations on two separate distribution or core switches, and by supporting

a loop free topology.

(A Spanning Tree protocol is still needed to prevent the initial loop that may occur prior to VLT being

established. After VLT is established, RSTP may be used to prevent loops from forming with new links that

are incorrectly connected and outside the VLT domain.)

Overview

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 illustration shows VLT deployed on S5000 switches. The switches appear as a single virtual

switch from the point of view of the switch or server supporting LACP.

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Virtual Link Trunking (VLT)