subrouters

Subrouters are routers that act as subordinate nodes within a larger network, connecting one or more subnets or VLANs to the wider backbone. They sit between an edge or core router and the local network, enforcing policies, routing between internal segments, and sometimes providing NAT, DHCP relay, or VPN termination. In many networks the term describes a device that operates within a hierarchical design rather than as a single flat edge router.

Deployment and functions: In enterprise and service-provider networks, subrouters are placed at distribution or campus layers to limit broadcast domains, apply access control, and simplify route management. They exchange routing information with neighboring devices using protocols such as OSPF, EIGRP, or BGP, or rely on static routes. Virtualization allows subrouters to be realized as virtual routers or VRFs to isolate routing tables and policies within one device.

Architecture and features: A subrouter typically has interfaces or subinterfaces connected to multiple subnets or VLANs. It can perform route aggregation, ACL enforcement, NAT, firewalling, QoS, and VPN termination. In VLAN-based networks, subinterfaces on a single device can implement multiple subrouters for different VLANs (router-on-a-stick).

Advantages and considerations: Subrouters improve scalability and security by containing traffic and policies to specific segments. They also ease management in large networks. Drawbacks include increased hardware or configuration complexity and the need for consistent policy across devices to avoid routing problems.

Related concepts include hierarchical routing, VRFs, router-on-a-stick, and network segmentation.