OSPF Protocol Overview

OSPF is an IGP that routes packets within a single AS. OSPF uses link-state information to make routing decisions, making route calculations using the shortest path first (SPF) algorithm (also referred to as the Dijkstra algorithm). Each router running OSPF floods link-state advertisements throughout the AS that contain information about that router’s attached interfaces and routing metrics. Each router takes the information in these link-state advertisements and creates a complete routing table for the network.

The JUNOS software supports OSPF version 2, including virtual links, stub areas, and authentication. The JUNOS software does not support type-of-service (ToS) routing.

OSPF was designed for the Transmission Control Protocol/Internet Protocol (TCP/IP) environment and as a result explicitly supports IP subnetting and the tagging of externally derived routing information. OSPF also provides for the authentication of routing updates.

OSPF routes IP packets based solely on the destination IP address contained in the IP packet header. OSPF quickly detects topological changes, such as when router interfaces become unavailable, and calculates new loop-free routes quickly and with a minimum of routing overhead traffic.

Each interface running OSPF is assigned a cost, which is a unitless number based on factors such as throughput, round-trip time, and reliability, which are used to determine how easy or difficult it is to reach a destination. If two or more routes to a destination have the same cost, OSPF distributes traffic equally among the routes, a process that is called load balancing.

Each router maintains a database that describes the topology of the AS. Each OSPF router has an identical topological database so that all routers in the area have a consistent view of the network. All routers maintain summarized topologies of other areas within an AS. Each router distributes information about its local state by flooding link-state advertisements throughout the AS. When the AS topology changes, OSPF ensures that the contents of all routers’ topological databases converge quickly.

All OSPF protocol exchanges can be authenticated. This means that only trusted routers can participate in the AS’s routing. A variety of authentication schemes can be used; a single authentication scheme is configured for each area, which enables some areas to use stricter authentication than others.

Externally derived routing data (for example, routes learned from BGP) is passed transparently throughout the AS. This externally derived data is kept separate from the OSPF link-state data. Each external route can be tagged by the advertising router, enabling the passing of additional information between routers on the boundaries of the AS.

Note: By default, the JUNOS software is compatible with RFC 1583, OSPF Version 2. Beginning with JUNOS 8.5, you can disable compatibility with RFC 1583 by including the no-rfc-1583 statement.

OSPF Version 3

OSPFv3 is a modified version of OSPF that supports Internet Protocol version 6 (IPv6) addressing. OSPFv3 differs from OSPFv2 in the following ways:

• All neighbor ID information is based on a 32-bit router ID.
• The protocol runs per link rather than per subnet.
• Router and network link-state advertisements (LSAs) do not carry prefix information.
• Two new LSA types are included: link-LSA and intra-area-prefix-LSA.
• Flooding scopes are as follows:
  • Link-local
  • Area
  • AS
• Link-local addresses are used for all neighbor exchanges except virtual links.
• Authentication is removed; the IPv6 authentication header relies on the IP layer.
• The packet format has changed as follows:
  • Version number 2 is now version number 3.
  • The db option field has been expanded to 24 bits.
  • Authentication information has been removed.
  • Hello messages do not have address information.
  • Two new option bits are included: R and V6.
• Type 3 summary LSAs have been renamed inter-area-prefix-LSAs.
• Type 4 summary LSAs have been renamed inter-area-router-LSAs.

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