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RFC1586 - Guidelines for Running OSPF Over Frame Relay Networks( 三 )

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configured manually.
STEP 3: Configure the IP network layer for the interface by
specifying the number of logical interfaces and the IP
address and subnet mask for each numbered logical
interface. Specify the VCs (by DLCI) associated with each
logical network interface if there is more than one. If an
address resolution protocol such as Inverse ARP [4] is
being used, it should suffice to specify a list of IP
addresses for the FR interface and have Inverse ARP create
the DLCI-IP address binding.
STEP 4: Configure OSPF to run over each logical interface as
appropriate, specifying the necessary interface parameters
such as area ID, link metric, protocol timers and
intervals, DR priority, and list of neighbors (for the DR).
OSPF interfaces consisting of one VC can be treated as
point-to-point links while multi-VC OSPF interfaces are
treated as NBMA subnets. In its internal OSPF MIB [5], the
router should create additional entries in the ospfIfTable
each with the appropriate ospfIfType (nbma or
pointTopoint).
3.2 Unnumbered Point-to-Point Logical Interfaces
OSPF uses the IP address to instance each numbered interface.
However, since an unnumbered point-to-point link does not have an IP
address, the ifIndex from the interface MIB is used instead [5].
This is straightforward for a physical point-to-point network, since
the ifIndex is assigned when the interface is configured. Logical
interfaces over FR however, do not have distinct and unique values
for ifIndex. To allow OSPF to instance unnumbered logical point-to-
point links, it is necessary to assign each such link a unique
ifIndex in STEP 3 above. This could lead to some confusion in the
interfaces table since a new ifTable entry would have to be created
for each logical point-to-point link. This type of departure from the
standard practice of creating interface table entries only for
physical interfaces could be viewed as an unnecessary complication.
Alternatively, it is possible to build a private MIB that contains
data structures to instance unnumbered logical links. However, making
recommendations for the structure and use of such a private MIB is
beyond the scope of this work. Even if unnumbered point-to-point
logical links were implemented in this manner, it would still be
necessary to allow a FR interface to be configured with multiple IP
addresses when a router is connected to multiple NBMA subnets through
a single physical interface. Hence, while it is possible to define
unnumbered logical point-to-point links in OSPF, we find this
alternative less attractive than using numbered logical point-to-
point links.
4. Using OSPF over FR
The ability to configure distinct logical interfaces over FR gives
users a great deal of flexibility in designing FR networks for use
with OSPF. Because routers can be partially interconnected over FR,
it is possible to design networks more cost-effectively than before.
The issues to consider are the price/cost structure for VCs (fixed,
distance-sensitive, banded) and ports, performance guarantees
provided, traffic distribution (local, long-haul), and protocol
efficiency. We have mentioned that the NBMA model provides some
economy in OSPF protocol processing and overhead and is recommended
for small homogeneous networks. In general, users should configure
their networks to contain several small "NBMA clusters," which are in
turn interconnected by long-haul VCs. The best choices for the number
of routers in each cluster and the size of the long-haul logical
point-to-point links depends on the factors mentioned above. If it is
necessary to architect a more "flat" network, the ability to assign

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