Before selecting an OSPF configuration
strategy for a Frame Relay network (or legacy X.25 network), you must
first understand the different NBMA topologies. Fundamentally, two
possible physical topologies exist for Frame Relay networks :
- Full-mesh topology
- Partial-mesh topology (including the hub-and-spoke topology)
The following sections describe how to configure OSPF in both full-mesh
and partial-mesh Frame Relay networks.
Full-Mesh Frame Relay
Organizations deploy Frame Relay primarily because it supports more than
one logical connection over a single interface, making it an affordable
and flexible choice for WAN links. A full-mesh topology takes advantage of
Frame Relay's capability to support multiple permanent virtual circuits (PVCs)
on a single serial interface. In a full-mesh topology, every router has a
PVC to every other router. 
For OSPF to work properly over a multiaccess full-mesh topology that
does not support broadcasts, you must manually enter each OSPF neighbor
addresses on each router, one at a time. The OSPF
neighbor
command tells a
router about its neighbors' IP addresses so that it can exchange routing
information without multicasts. The following example illustrates how the
neighbor
command is used:
RTA(config)#router ospf 1
RTA(config-router)#network 3.1.1.0 0.0.0.255 area 0
RTA(config-router)#neighbor 3.1.1.2
RTA(config-router)#neighbor 3.1.1.3
Specifying each router's neighbors is not the only option to make OSPF
work in this type of environment. The following section explains how
configuring subinterfaces can eliminate the need for the
neighbor
command.
Configuring Subinterfaces to Create Point-to-Point Networks
The IOS subinterface feature can be used to break up a multiaccess network
into a collection of point-to-point networks.
In Figure ,
a different IP subnet is assigned to each PVC. OSPF automatically
recognizes this configuration as point-to-point, not NBMA, even with Frame
Relay configured on the interfaces. Recall that OSPF point-to-point
networks do not elect a DR. Instead, the Frame Relay router uses Inverse
ARP or a Frame Relay map to obtain the link partner's address so that
routing information can be exchanged.
A full-mesh topology offers numerous advantages, including maximum fault
tolerance. Unfortunately, full-mesh topologies can get expensive because
each PVC must be leased from a provider. An organization would have to
lease 45 PVCs to support just 10 fully meshed routers! If subinterfaces
are used to create point-to-point networks, then the 45 IP subnets must
also be allocated and managed, which is an additional expense.
|