62Virtual Link Trunking (VLT)Virtual link trunking (VLT) allows physical links between two chassis to appear as a single virtual link to the network core or otherswitches such as Edge, Access, or top-of-rack (ToR).OverviewVLT reduces the role of spanning tree protocols (STPs) by allowing link aggregation group (LAG) terminations on two separatedistribution 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, youmay use rapid spanning tree protocol (RSTP) to prevent loops from forming with new links that are incorrectly connected andoutside the VLT domain.VLT provides Layer 2 multipathing, creating redundancy through increased bandwidth, enabling multiple parallel paths betweennodes 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 featuresat 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 havea 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 functionnormally in VLT mode. Features such as VRRP and internet group management protocol (IGMP) snooping require state informationcoordinating between the two VLT chassis. IGMP and VLT configurations must be identical on both sides of the trunk to ensure thesame 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 theswitch or server supporting link aggregation control protocol (LACP).900 Virtual Link Trunking (VLT)
