Users 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 top-of-rack (ToR).
Topics:
• Overview
• VLT Terminology
• Configure Virtual Link Trunking
• RSTP Configuration
• PVST+ Configuration
• eVLT Configuration Example
• PIM-Sparse Mode Configuration Example
• Verifying a VLT Configuration
• Additional VLT Sample Configurations
• Troubleshooting VLT
• Reconfiguring Stacked Switches as VLT
• Specifying VLT Nodes in a PVLAN
• Configuring a VLT VLAN or LAG in a PVLAN
• Proxy ARP Capability on VLT Peer Nodes
• VLT Nodes as Rendezvous Points for Multicast Resiliency
• Configuring VLAN-Stack over VLT
• IPv6 Peer Routing in VLT Domains Overview
Overview
VLT reduces the role of spanning tree protocols (STPs) by allowing link aggregation group (LAG) terminations on two separate
distribution or core switches and 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.
VLT 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.
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