Dell PowerEdge FN I/O Module Configuration Guide 9.10(0.0)

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 peer devices have independent management planes. A chassis interconnect trunk between the VLT chassis maintains synchronization of L2/L3 control planes across the two VLT peers. The chassis interconnect trunk uses 10GE or 40GE user ports on the chassis.

VLT provides Layer 2 multipathing, creating redundancy through increased bandwidth, enabling multiple parallel paths between nodes and load-balancing traffic where alternative paths exist.

A separate backup link maintains heartbeat messages across an out-of-band management network. The backup link ensures that node failure conditions are correctly detected and are not confused with failures of the chassis interconnect trunk. VLT ensures that local traffic on a chassis does not traverse the chassis interconnect trunk and takes the shortest path to the destination via directly attached links.

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.