Comtech EF Data SLM-5650A Installation And Operation Manual page 495

H.2.2 OSPF Basics
Certain critical features of the OSPF protocol that impact a shared outbound satellite network are
as follows:
•
OSPF requires that the communication channel between the interfaces that connect any
two given OSPF nodes must have ability to pass traffic bidirectionally.
•
A router running the OSPF algorithm will send out "Hello" packets on each interface,
when the response from a node on the same subnet as the interface is received, OSPF
will create a neighbor adjacency with that node. In this way, the subnets and segments of
the OSPF network are discovered, mapped and an OSPF database is created. This
database is used by all OSPF routers to determine open shortest paths to all remote
networks. OSPF nodes that do not follow these rules will not be added to a router's
neighbor adjacency database.
H.2.3 OSPF Challenges
Use of a split path violates the requirement whereby OSPF messaging is sent and received on
the same interface. Therefore, the hub modems will not achieve "neighbor adjacency" with any of
the remote routers. This means that running OSPF on the hub routers will not add any value.
In the example diagram, the SLM-5650A NP Router is essentially a two-port router, thereby
requiring that another OSPF node must always be in place "downstream" at the remote.
However, if this downstream router is running OSPF, could the high costs associated with a
satellite link be managed and reduced by specifically configuring the router interface? Such a
configuration would allow the overall OSPF network to control whether traffic should pass to the
satellite link, or to a lower-cost link, such a terrestrial fiber, based upon availability. The possibility
of such a solution is examined in the following use case.
H.2.4 OSPF Maritime Use Cases
The maritime use cases presented here depicts application of OSPFv2 as part of a deployed
network of routers that includes the SLM-5650A NP Interface.
The primary objective of these use cases is to describe the process of automatically re-routing
traffic when alternative transmission links are available in a maritime environment. Specifically,
the satellite link should be used when at sea, and the high-speed terrestrial link should be used
when at shore. This switch should occur automatically.
If other communication links such as ship-to-ship or back-up lower-speed satellite networks are
available, then the OSPF routing protocol will select the best path as defined by link cost.
Appendix I H–3
SLM-5650A Satellite Modem
Revision 10
MN-SLM-5650A