VSX LAGs span both aggregation switches. The two switches appear as one device to partner downstream or upstream devices or both when forming a LAG with the VSX pair. The two switches synchronize their databases and states over a user configured link referred to as an Inter-Switch Link (ISL). A VSX LAG must be LACP-based.

VSX LAGs are preferable to point-to-point transit VLANs for upstream connectivity when the routed only port is not an option, such with the case of multiple VRFs. This configuration reduces the number of transit VLANs and associated SVIs, simplifying operations and troubleshooting. You can also greatly optimize traffic path by enabling active forwarding and active gateway. When you enable active forwarding and active gateway, north-south and south-north traffic bypasses the ISL link.