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Dell EMC Configuration Guide for the S4048T–ON System 9.14.2.4

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Overview

In a traditional switched topology as shown below, spanning tree protocols (STPs) are used to block one or more links to prevent loops in the network. Although loops are prevented, bandwidth of all links is not effectively utilized by the connected devices.

Figure 1. Traditional switched topology
Traditional switched topology

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 2. VLT providing multipath
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).

Figure 3. Example of VLT Deployment
Example of VLT Deployment

VLT 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.
  • Active-Active load sharing with VRRP.
  • Active-Active load sharing with peer-routing for Layer-3 VLAN
  • Graceful failover of LACP during reload
  • Agility in VM Migration under VLT domain.
CAUTION Dell EMC Networking does not recommend enabling Stacking and VLT simultaneously. If you enable both features at the same time, unexpected behavior may occur.

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 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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