E.2.2 Ospf Basics; E.2.3 Ospf Challenges; Figure E-1. Satellite Shared Outbound Network - Comtech EF Data SLM-5650A Installation And Operation Manual

SLM-5650A Satellite Modem
Appendix E
E.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 the 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.
E.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

Figure E-1. Satellite Shared Outbound Network

E–2
Revision 8
MN-SLM5650A