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. 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.