Dell S4820T, Configuration Manual

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Virtual Link Trunking (VLT)
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 S4820T switches. The S4820T switches appear as a
single virtual switch from the point of view of the switch or server supporting link aggregation control
protocol (LACP).
Virtual Link Trunking (VLT)
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