Technical data

Gateway Routing Daemon (GATED) Configuration Reference
A.10 Interface Statements
A.10.2 IP Interface Addresses and Routes
The BSD 4.3 and later networking implementations allow the following four
types of interfaces. Some implementations allow multiple protocol addresses per
physical interface, but these are mostly based on BSD 4.3 RENO or later.
Loopback This interface must have the address of 127.0.0.1. Packets sent
to this interface are sent back to the originator. This interface
is also used as an interface for implementing other features,
such as reject and blackhole 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
routerid ; this allows routing to a system based
on the router ID that will work if some interfaces are down.
Broadcast This is a multiaccess interface capable of a physical level
broadcast, such as Ethernet, Token Ring, and FDDI. This
interface has an associated subnet mask and broadcast
address. The interface route to a broadcast network is a
route to the complete subnet.
Point-to-point This is a tunnel to another host, usually on some sort of serial
link. This interface has a local address and a remote address.
The remote address must be unique among all the interface
addresses on a given router.
If a subnet mask is specified on a point-to-point interface, it is
only used by RIP version 1 to determine which subnets may be
propagated to the router on the other side of this interface.
Nonbroadcast multi-
access (NBMA)
This type of interface is multiaccess, but not capable of
broadcast, for example, frame relay and X.25. This type
of interface has a local address and a subnet mask. (Not
supported.)
The GATED daemon ensures that there is a route available to each IP interface
that is configured and up. Normally this is done by the SET INTERFACE
command that configures the interface; GATED also does it to ensure consistency.
For point-to-point interfaces, GATED installs some special routes. GATED
installs a route to the local address pointing at the loopback interface with a
preference of 110. This ensures 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 is used
to verify operation of the link. Because OSPF installs routes with a preference of
10, these routes override the route installed with a preference of 110.
When the status of an interface changes, GATED notifies all the protocols, which
take the appropriate action. The GATED daemon assumes that interfaces that
are not marked UP do not exist.
The GATED daemon 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. The GATED daemon also ignores any point-to-point interface that
has the same local and remote addresses; it assumes it is in some sort of loopback
test mode.
A–14 Gateway Routing Daemon (GATED) Configuration Reference