A subinterface is a logical way of defining an interface. The same
physical interface can be split into multiple logical interfaces, with
each subinterface being defined as point-to-point. This was originally
created in order to better handle issues caused by split horizon over NBMA
and vector based routing protocols.
A point-to-point subinterface has the properties of any physical
point-to-point interface. As far as OSPF is concerned, an adjacency is
always formed over a point-to-point subinterface with no DR or BDR
election. The following is an illustration of point-to-point subinterfaces:
In Figure
, on RTA, we can split Serial 0 into two
point-to-point subinterfaces, S0.1 and S0.2. This way, OSPF will consider
the cloud as a set of point-to-point links rather than one multi-access
network. The only drawback for the point-to-point is that each segment
will belong to a different subnet. This might not be acceptable since some
administrators have already assigned one IP subnet for the whole cloud.
Another workaround is to use IP unnumbered interfaces on the cloud.
This also might be a problem for some administrators who manage the WAN
based on IP addresses of the serial lines. Figure
shows a typical
configuration for RTA and RTB.