Understanding IS-IS Mesh Groups
A mesh group is a set of routing devices that are fully connected. That is, they have a fully meshed topology.
Junos OS supports IS-IS mesh groups as documented in RFC 2973, IS-IS Mesh Groups.
When link-state PDUs are being flooded throughout an area, each router within a mesh group receives only a single copy of a link-state PDU instead of receiving one copy from each neighbor, thus minimizing the overhead associated with the flooding of link-state PDUs.
Mesh groups provide a scaling method for the flooding subsystem. We recommend that you deploy mesh groups when your network design has a dense flooding topology. For example, consider the classical overlay topologies of the 1990s where 200 routers were fully meshed using permanent virtual circuits (PVCs) over an ATM core, because ATM was the only high-speed technology at the time. A PVC is a software-defined logical connection in a network such as a Frame Relay network.
What has changed since the 1990s is that IP and MPLS technology have reduced the ATM layer and removed the need for overlay meshing. The flooding graphs have become sparse in almost all practical deployments. In service provider networks, overlay networks are no longer used.
In enterprise networks, dense flooding graphs that, for example, lease a Layer 2 VPN service (an overlay network) to fully mesh its WAN routers might continue to be a useful architecture. In such cases, mesh groups might be useful.