White Papers

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.)

Topics:

• Overview

• VLT Terminology

• Congure Virtual Link Trunking

• RSTP Conguration

• PVST+ Conguration

• VLT Sample Conguration

• eVLT Conguration Example

• PIM-Sparse Mode Conguration Example

• Verifying a VLT Conguration

• Additional VLT Sample Congurations

• Troubleshooting VLT

• Reconguring Stacked Switches as VLT

• Specifying VLT Nodes in a PVLAN

• Conguring a VLT VLAN or LAG in a PVLAN

• Proxy ARP Capability on VLT Peer Nodes

• VLT Nodes as Rendezvous Points for Multicast Resiliency

• Conguring VLAN-Stack over VLT

• IPv6 Peer Routing in VLT Domains Overview

Overview

VLT provides Layer 2 multipathing, creating redundancy through increased bandwidth, enabling multiple parallel paths between nodes and

load-balancing trac where alternative paths exist.

Virtual link trunking oers the following benets:

• 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.

Virtual Link Trunking (VLT) 995