ospf design guide...

this document is awesome...  most texts have simply lifted this thing and changed the words a little bit... but the graphics are just killing me...

 http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094e9e.shtml

Point-to-Point Subinterfaces

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 the above diagram, 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. The following is a typical configuration for RTA and RTB:

the ellipses above are almost good, but what in the hell is that thing surrounding 128.213...

i'm actually digging the crudity of it all... the text is simply excellent... no frills, down and dirty...