Reference Guide

Virtual Link Trunking (VLT) | 1005

Virtual Link Trunking (VLT)

Virtual Link Trunking (VLT) is supported on the and platform.

Overview

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.) 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 Spanning Tree Protocol (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 VRRP

• Provides link-level resiliency

• Assures high availability

As shown in the following figure, 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 IGMP Snooping require state information to be coordinated

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.

Caution: Dell Force10 recommends not enabling Stacking and VLT simultaneously.

If both are enabled at the same time, unexpected behavior will occur.

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