OSPF is an interior gateway protocol (IGP) that routes packets within a single autonomous
system (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 or area that contain information about that router’s attached interfaces and routing
metrics. Each router uses the information in these link-state advertisements to calculate
the least cost path to each network and create a routing table for the protocol.
Junos OS supports OSPF version 2 (OSPFv2) and OSPF version 3 (OSPFv3), including
virtual links, stub areas, and for OSPFv2, authentication. Junos OS 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.
NOTE: On SRX Series devices, when only one link-protection is configured
under the OSPF interface, the device does not install an alternative route in
the forwarding table. When the per-packet load-balancing is enabled as a
workaround, the device does not observe both the OSPF metric and sending
the traffic through both the interfaces.
An OSPF AS can consist of a single area, or it can be subdivided into multiple areas. In a
single-area OSPF network topology, each router maintains a database that describes
the topology of the AS. Link-state information for each router is flooded throughout the
AS. In a multiarea OSPF topology, each router maintains a database that describes the
topology of its area, and link-state information for each router is flooded throughout that
area. All routers maintain summarized topologies of other areas within an AS. Within
each area, OSPF routers have identical topological databases. When the AS or area
topology changes, OSPF ensures that the contents of all routers’ topological databases
converge quickly.
All OSPFv2 protocol exchanges can be authenticated. OSPFv3 relies on IPsec to provide
this functionality. 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.
543Copyright © 2017, Juniper Networks, Inc.
Chapter 18: Configuring Routing Protocols
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