DIGITAL clearVISN IP Switch Manager allows four types of interfaces: loopback, broadcast, point-to-point, and non-broadcast multi-access (nbma). Some implementations allow multiple protocol addresses per physical interface.
The loopback interface must have the address of 127.0.0.1. Packets sent to this interface are sent back to the originator. This interface also is used as a catch-all interface for implementing other features, such as reject and black hole routes. Although a netmask is reported on this interface, it is ignored.
It is useful to assign an additional address to this interface that is the same as the OSPF or BGP router id; this allows routing to a system based on the router id, which will work if some interfaces are DOWN.
The broadcast interface is a multi-access interface capable of a physical level broadcast, such as:
This interface has an associated subnet mask and broadcast address. The interface route to a broadcast network is a route to the complete subnet.
The point-to-point interface is a tunnel to another host, usually on a serial link. This interface has a local address and a remote address. (Although it may be possible to specify multiple addresses for a point-to-point interface, there does not seem to be a useful reason for doing so.) The remote address must be unique among all the interface addresses on a given router. The local address may be shared among many point-to-point interfaces and up to one non-point-to-point interface. This is technically a form of the router id method for addressless links. This technique conserves subnets, as none are required when using it.
If a subnet mask is specified on a point-to-point interface, it is only used by RIP version 1 and IGRP to determine which subnets may be propagated to the router on the other side of this interface.
The non-broadcast multi-access interface is multi-access, but not capable of broadcast. Examples are frame relay and X.25. This type of interface has a local address and a subnet mask.
The the routing daemon (Ipsrd) insures that there is a route available to each IP interface that is configured and up. For point-to-point interfaces, Ipsrd installs some special routes. If the local address on one or more point-to-point interfaces is not shared with a non-point-to-point interface, Ipsrd installs a route to the local address, pointing at the loopback interface with a preference of 110. This insures that packets originating on this host destined for this local address are handled locally. OSPF prefers to route packets for the local interface across the point-to-point link, where they will be returned by the router on the remote end. This verifies operation of the link. Since OSPF installs routes with a preference of 10, these routes override the route installed with a preference of 110.
If the local address of one or more point-to-point interfaces is shared with a non-point-to-point interface, Ipsrd installs a route to the local address with a preference of 0 (zero) that is not installed in the forwarding table. This prevents protocols like OSPF from routing packets to this address across a serial interface when this system could be functioning as a host.
When the status of an interface changes, Ipsrd notifies all protocols, which take appropriate action. Ipsrd assumes that interfaces that are not marked UP do not exist.
Ipsrd ignores any interfaces that have invalid data for the local, remote, or broadcast addresses, or the subnet mask. Invalid data includes zeros in any field. Ipsrd also ignores any point-to-point interface that has the same local and remote addresses; it assumes the interface is in a loopback test mode.