Shortest Path First daemon
is an Open Shortest Path First (OSPF) daemon
which manages routing tables. This implementation supports OSPF version 2,
thus it is only capable of maintaining IPv4 routing tables.
OSPF is an interior gateway protocol designed to supersede RIP. It has several
advantages over RIP. For instance, every router has an understanding of the
complete network topology. Response to changes in the network is faster.
Furthermore, failure detection is improved.
The OSPF daemon maintains a Link State Database (LSDB) containing information
about routers and networks within an Autonomous System (AS).
Dijkstra's shortest path first algorithm is used to compute a Routing
Information Base (RIB) using the LSDB as input. The Forwarding Information
Base (FIB), a.k.a. the kernel routing table, is updated with information from
OSPF routers discover one another automatically via OSPF hello packets. OSPF
routers communicate via two multicast groups: 126.96.36.199 (all Shortest Path
First routers) and 188.8.131.52 (all Designated Routers). OSPF runs directly on
top of IP and uses neither TCP nor UDP. IP protocol number 89 is reserved for
All routers in an OSPF network spend most of their time keeping each others'
LSDBs in sync. All routers must have the same information in the LSDB at all
times. Every time the LSDB is updated the RIB is updated; if needed the FIB is
In a multi-access network such as Ethernet, it is unfeasible for all routers to
synchronize their LSDB with all other routers in the network. In such networks
a Designated Router (DR) and a Backup Designated Router (BDR) are elected. The
DR's responsibility is to synchronize with all routers; the BDR will not do
much until the DR fails. The first router in a network is automatically
elected DR, the second router BDR. All routers have a FULL adjacency with the
DR and the BDR. Routers with FULL adjacency exchange information about their
LSDBs. A router not elected either DR or BDR will have 2-WAY adjacency with
all routers but the DR and BDR. Routers with 2-WAY adjacency recognize that
they know each other, but do not exchange information about their LSDBs. If a
DR or BDR fails another router is elected DR or BDR and all routers form FULL
adjacencies with the newly elected DR or BDR.
When routers are connected via point-to-point links, DR and BDR election is
skipped since only two routers are connected to the link.
To limit the impact changes in the network have on the LSDB it is possible to
segment an OSPF network into areas. Area 0.0.0.0 (a.k.a. the backbone area)
must always be present. Routers can be configured as Area Border Router (ABR),
being part of multiple areas. Every area must have direct access to the
backbone area. ABRs not directly connected to the backbone area need to
establish a virtual link to a router in the backbone area.
AS Border Routers (ASBR) are connected to an OSPF network and other external
networks via BGP, RIP, or static routing, and provide connectivity to networks
outside the AS.
is usually started at boot time, and can be
enabled by setting the following in
for more information
on the boot process and enabling daemons.
A running ospfd
can be controlled with the
The options are as follows:
- Define macro to be set to
value on the command line. Overrides the
definition of macro in the configuration
- Do not daemonize. If this option is specified,
ospfd will run in the foreground and log to
- Specify an alternative configuration file.
- Configtest mode. Only check the configuration file for
- Use an alternate location for the default control
- Produce more verbose output.
- Default ospfd configuration
- UNIX-domain socket used for
OSPF Version 2, RFC 2328,
L. Nguyen, R. White,
A. Zinin, and D. McPherson,
OSPF Stub Router Advertisement, RFC
3137, June 2001.
program first appeared in
Virtual links are currently not available in