Administrator Guide

VLT not only overcomes this caveat, but also provides a multipath to the connected devices. In the example shown below, the
two physical VLT peers appear as a single logical device to the connected devices. As the connected devices consider the VLT
peers as a single switch, VLT eliminates STP-blocked ports. However, the two VLT devices are independent Layer2/Layer3
(L2/L3) switches for devices in the upstream network.
Figure 139. VLT providing multipath
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 alternate paths exist.
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 coordination between the two VLT chassis. The
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 how VLT is deployed. 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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Virtual Link Trunking (VLT)