Configuring Layer 2 Multipathing for Virtual Chassis Link Aggregation (VLAG)


You can aggregate links between two switches by configuring Layer 2 multipathing and virtual chassis Link Aggregation.


A virtual chassis Link Aggregation Group (VLAG) allows links that are physically connected to two different switches to appear as a single Ethernet trunk to a third device. The third device can be a server, switch, or any other networking device. A VLAG can create Layer 2 multipathing which allows you to create redundancy, enabling multiple parallel paths between nodes.


A VLAG requires that a least one cross connection between the two switches, also called peers, where the VLAG links terminate. The specific ports that connect the different switches, do not require explicit configuration before creating a VLAG.


VLAGs can provide the following benefits:


  • Allows a single device to use an Ethernet trunk across two access layer (Top of Rack) switches.
  • Eliminates Spanning Tree Protocol (STP) blocked ports.
  • Provides a loop-free topology
  • Provides fast convergence if a link or device fails.
  • Provides link-level resiliency.
  • Helps ensure high availability.


Netvisor One performs VLAG synchronization to coordinate active-standby and active-active configurations using the following rules:


  • Active-Standby VLAGs — For VLAG, only one side, ports on the cluster node, may be up at anytime. The other side is in standby mode. If there is conflict, the side with the longest time up remains up and the other side is disabled.
  • Active-Active VLAGs — If both sides of the VLAG are up, Netvisor One adds port egress rules to drop any packets that egress the VLAG port if the ingress port is the cluster link. This prevents loops.


Netvisor One reports the state as up or down and synchronizes the state. For active-standby VLAGs, port up timestamps are exchanged to resolve any contest if both ports are up.


Netvisor One performs synchronization from the primary node to the secondary node. If the secondary node requires synchronization, the secondary node sends a request to the primary node to perform the synchronization.


Synchronization messages are sent on a per-VLAG basis, and compare the local VLAG port state with the peer VLAG port state. The port state then determines any port enable or disable actions for active-standby VLAGs or port egress rule changes for active-active VLAGs.


VLAG synchronization occurs when a trigger happens on the configuration:


  • A VLAG is created or modified.
  • A VLAG member port is up or down.
  • A cluster-link is up or down.


For any port in an active-standby VLAG, Netvisor One records the time up of the port, and sends it as part of the VLAG synchronization message. the time up values are compared on both nodes to determine the active port.