Open Shortest Path First (OSPF) is a hierarchical link state protocol. OSPF is an interior gateway protocol (IGP) used within large autonomous systems (ASs). OSPF routers exchange state, cost, and other relevant interface information with neighbors. The information exchange enables all participating routers to establish a network topology map. Each router applies the Dijkstra algorithm to calculate the shortest path to each destination in the network. The resulting OSPF forwarding table is submitted to the routing table manager to calculate the routing table.
When a router is started with OSPF configured, OSPF, along with the routing-protocol data structures, is initialized and waits for indications from lower-layer protocols that its interfaces are functional. Nokia’s implementation of OSPF conforms to OSPF Version 2 specifications presented in RFC 2328, OSPF Version 2 and OSPF Version 3 specifications presented in RFC 2740, OSPF for IPv6. Routers running OSPF can be enabled with minimal configuration. All default and command parameters can be modified.
Changes between OSPF for IPv4 and OSPF3 for IPv6 include the following:
Addressing semantics have been removed from OSPF packets and the basic link-state advertisements (LSAs). New LSAs have been created to carry IPv6 addresses and prefixes.
OSPF3 runs on a per-link basis, instead of on a per-IP-subnet basis.
Flooding scope for LSAs has been generalized.
Unlike OSPFv2, OSPFv3 authentication relies on IPV6's authentication header and encapsulating security payload.
Most packets in OSPF for IPv6 are almost as compact as those in OSPF for IPv4, even with the larger IPv6 addresses.
Most field and packet-size limitations present in OSPF for IPv4 have been relaxed.
Option handling has been made more flexible.
Key OSPF features are:
backbone areas
stub areas
not-so-stubby areas (NSSAs)
virtual links
authentication
route redistribution
routing interface parameters
OSPF-TE extensions (Nokia’s implementation allows MPLS fast reroute)