http://www.ciscopress.com/articles/article.asp?p=170741&seqNum=5
Configuring Frame Relay Subinterfaces On partially meshed Frame Relay networks, the problem of split horizon can be overcome by using Frame Relay subinterfaces. Frame Relay provides a mechanism to allow a physical interface to be partitioned into multiple virtual interfaces. In a similar way, using subinterfaces allows a partially meshed network to be divided into a number of smaller, fully meshed point-topoint networks. Generally, each point-to-point subnetwork is assigned a unique network address. This allows packets received on one physical interface to be sent out from the same physical interface, albeit forwarded on VCs in different subinterfaces. There are two types of subinterfaces supported by Cisco routers: point-to-point and multipoint subinterfaces.

Open Shortest Path First Reference:
http://docwiki.cisco.com/wiki/Open_Shortest_Path_First
Explanation:
Additional OSPF features include equal-cost, multipath routing, and routing based on upper-layer type-ofservice
(TOS) requests. TOS-based routing supports those upper-layer protocols that can specify particular
types of service. An application, for example, might specify that certain data is urgent. If OSPF has high-priority
links at its disposal, these can be used to transport the urgent datagram.
OSPF supports one or more metrics. If only one metric is used, it is considered to be arbitrary, and TOS is not supported. If more than one metric is used, TOS is optionally supported through the use of a separate metric (and, therefore, a separate routing table) for each of the eight combinations created by the three IP TOS bits (the delay, throughput, and reliability bits). For example, if the IP TOS bits specify low delay, low throughput, and high reliability, OSPF calculates routes to all destinations based on this TOS designation. IP subnet masks are included with each advertised destination, enabling variable-length subnet masks. With variable-length subnet masks, an IP network can be broken into many subnets of various sizes. This provides network administrators with extra network-configuration flexibility.