Reference Guide

NOTE: When you launch the VLT link, the VLT peer-ship is not established if any of the following is
TRUE:
• The VLT System-MAC configured on both the VLT peers do not match.
• The VLT Unit-Id configured on both the VLT peers are identical.
• The VLT System-MAC or Unit-Id is configured only on one of the VLT peers.
• The VLT domain ID is not the same on both peers.
If the VLT peer-ship is already established, changing the System-MAC or Unit-Id does not cause VLT
peer-ship to go down.
Also, if the VLT peer-ship is already established and the VLT Unit-Id or System-MAC are configured
on both peers, then changing the CLI configurations on the VLT Unit-Id or System-MAC is rejected
if any of the following become TRUE:
• After making the CLI configuration change, the VLT Unit-Id becomes identical on both peers.
• After making the CLI configuration change, the VLT System-MAC do not match on both peers.
When the VLT peer-ship is already established, you can remove the VLT Unit-Id or System-MAC
configuration from either or both peers. However, removing configuration settings can cause the
VLT ports to go down if you configure the Unit-Id or System-MAC on only one of the VLT peers.
Overview
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).
VLT reduces the role of spanning tree protocols (STPs) by allowing link aggregation group (LAG)
terminations on two separate distribution or core switches, and by 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.
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
PMUX Mode of the IO Aggregator
221