Open Shortest Path First (OSPF)

How OSPF works

An OSPF installation consists of one or more areas. An OSPF area is typically divided into logical areas linked by Area Border Routers. A group of contiguous networks form an area. An Area Border Router (ABR) links one or more areas to the OSPF network backbone (area ID 0). See Dynamic Routing Overview on page 284.

OSPF is an interior routing protocol. It includes a backbone AS, and possibly additional ASes. The DR and BDR are elected from potential routers with the highest priorities. The DR handles much of the administration to lower the network traffic required. New routers are discovered through hello packets sent from the DR using the multicast address of 224.0.0.5. If the DR goes offline at any time, the BDR has a complete table of routes that is uses when it takes over as the DR router.

OSPF does not use UDP or TCP, but is encapsulated directly in IP datagrams as protocol 89. This is in contrast to RIP, or BGP. OSPF handles its own error detection and correction functions.

The OSPF protocol, when running on IPv4, can operate securely between routers, optionally using a variety of authentication methods to allow only trusted routers to participate in routing. OSPFv3, running on IPv6, no longer supports protocol-internal authentication. Instead, it relies on IPv6 protocol security (IPsec).

Other important parts of how OSPF works includes:

  • OSPF router discovery
  • How OSPF works on FortiGate units
  • External routes
  • Link-state Database (LSDB) and route updates
  • OSPF packets

 

OSPF router discovery

OSPF-enabled routers generate Link-State Advertisements (LSA) and send them to their neighbors whenever the status of a neighbor changes or a new neighbor comes online. As long as the OSPF network is stable, LSAs between OSPF neighbors do not occur. An LSA identifies the interfaces of all OSPF-enabled routers in an area, and provides information that enables OSPF-enabled routers to select the shortest path to a destination. All LSA exchanges between OSPF-enabled routers are authenticated.

When a network of OSPF routers comes online, the follow steps occur.

1. When OSPF routers come online, they send out Hello packets to find other OSPF routers on their network segment.

2. When they discover other routers on their network segment, generally they become adjacent. Adjacent routers can exchange routing updates. See Adjacency on page 379.

3. A DR and BDR are elected from the available routers using priority settings, and router ID. See Designated router (DR) and backup router (BDR) on page 380, and OSPF Background and concepts on page 377.

4. Link state updates are sent between adjacent routers to map the topology of the OSPF area.

5. Once complete, the DR floods the network with the updates to ensure all OSPF routers in the area have the same OSPF route database. After the initial update, there are very few required updates if the network is stable.


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