Cisco ONS 15327, User Documentation

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4-9
Cisco ONS 15327 User Documentation, R3.3
June 2002
Chapter 4 IP Networking
ONS 15327 IP Addressing Scenarios
4.2.6 Scenario 6: Using OSPF
Open Shortest Path First (OSPF) is a link state Internet routing protocol. Link state protocols use a “hello
protocol” to monitor their links with adjacent routers and to test the status of their links to their
neighbors. Link state protocols advertise their directly-connected networks and their active links. Each
link state router captures the link state “advertisements” and puts them together to create a topology of
the entire network or area. From this database, the router calculates a routing table by constructing a
shortest path tree. Routes are continuously recalculated to capture ongoing topology changes.
ONS 15327s use the OSPF protocol in internal ONS 15327 networks for node discovery, circuit routing,
and node management. You can enable OSPF on the ONS 15327s so that the ONS 15327 topology is
sent to OSPF routers on a LAN. Advertising the ONS 15327 network topology to LAN routers eliminates
the need to manually enter static routes for ONS 15327 subnetworks.
Figure 4-7 shows the same network
enabled for OSPF. Figure 4-8 shows the same network without OSPF. Static routes must be manually
added to the router in order for CTC computers on LAN A to communicate with ONS 15327 #2 and #3
because these nodes reside on different subnets.
OSPF divides networks into smaller regions, called areas. An area is a collection of networked end
systems, routers, and transmission facilities organized by traffic patterns. Each OSPF area has a unique
ID number, known as the area ID, that can range from 0 to 4,294,967,295. Every OSPF network has one
backbone area called “area 0.” All other OSPF areas must connect to area 0.
When you enable ONS 15327 OSPF topology for advertising to an OSPF network, you must assign an
OSPF area ID to the ONS 15327 network. Coordinate the area ID number assignment with your LAN
administrator. In general, all DCC-connected ONS 15327s are assigned the same OSPF area ID.