Users Guide
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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 trac where alternative paths exist.
Virtual link trunking oers the following benets:
• 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 congurations 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.
Virtual Link Trunking (VLT)
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