Virtual Link Trunking (Vlt); Overview - Dell Force10 Z9000 Configuration Manual

Virtual Link Trunking (VLT)

Virtual link trunking (VLT) is supported on Dell Networking OS.

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 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.
The following example shows VLT deployed on switches. The switches appear as a single virtual switch from the point of view of the
switch or server supporting link aggregation control protocol (LACP).
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