2.2.8.1 Embedded Routers

A router may be a stand-alone computer system, dedicated to its IP router functions. Alternatively, it is possible to embed router functions within a host operating system that supports connections to two or more networks. The best-known example of an operating system with embedded router code is the Berkeley BSD system. The embedded router feature seems to make building a network easy, but it has a number of hidden pitfalls:

1. If a host has only a single constituent-network interface, it should not act as a router.

   For example, hosts with embedded router code that gratuitously forward broadcast packets or datagrams on the same net often cause packet avalanches.

2. If a (multihomed) host acts as a router, it is subject to the requirements for routers contained in this document.

   For example, the routing protocol issues and the router control and monitoring problems are as hard and important for embedded routers as for stand-alone routers.

   Internet router requirements and specifications may change independently of operating system changes. An administration that operates an embedded router in the Internet is strongly advised to maintain and update the router code. This might require router source code.

3. When a host executes embedded router code, it becomes part of the Internet infrastructure. Thus, errors in software or configuration can hinder communication between other hosts. As a consequence, the host administrator must lose some autonomy.

   In many circumstances, a host administrator will need to disable router code embedded in the operating system. For this reason, it should be straightforward to disable embedded router functionality.

4. When a host running embedded router code is concurrently used for other services, the Operation and Maintenance requirements for the two modes of use may conflict.

   For example, router O&M will in many cases be performed remotely by an operations center; this may require privileged system access that the host administrator would not normally want to distribute.