This section provides definitions for terms that have a specific
meaning to the OSPF protocol and that are used throughout the
text. The reader unfamiliar with the Internet Protocol Suite is
referred to [RS-85-153] for an introduction to IP.
A level three Internet Protocol packet switch. Formerly
called a gateway in much of the IP literature.
- Autonomous System
A group of routers exchanging routing information via a
common routing protocol. Abbreviated as AS.
- Interior Gateway Protocol
The routing protocol spoken by the routers belonging to an
Autonomous system. Abbreviated as IGP. Each Autonomous
System has a single IGP. Separate Autonomous Systems may be
running different IGPs.
- Router ID
A 32-bit number assigned to each router running the OSPF
protocol. This number uniquely identifies the router within
an Autonomous System.
In this memo, an IP network/subnet/supernet. It is possible
for one physical network to be assigned multiple IP
network/subnet numbers. We consider these to be separate
networks. Point-to-point physical networks are an exception
- they are considered a single network no matter how many
(if any at all) IP network/subnet numbers are assigned to
- Network mask
A 32-bit number indicating the range of IP addresses
residing on a single IP network/subnet/supernet. This
specification displays network masks as hexadecimal numbers.
For example, the network mask for a class C IP network is
displayed as 0xffffff00. Such a mask is often displayed
elsewhere in the literature as 255.255.255.0.
- Multi-access networks
Those physical networks that support the attachment of
multiple (more than two) routers. Each pair of routers on
such a network is assumed to be able to communicate directly
(e.g., multi-drop networks are excluded).
The connection between a router and one of its attached
networks. An interface has state information associated
with it, which is obtained from the underlying lower level
protocols and the routing protocol itself. An interface to
a network has associated with it a single IP address and
mask (unless the network is an unnumbered point-to-point
network). An interface is sometimes also referred to as a
- Neighboring routers
Two routers that have interfaces to a common network. On
multi-access networks, neighbors are dynamically discovered
by OSPF's Hello Protocol.
A relationship formed between selected neighboring routers
for the purpose of exchanging routing information. Not
every pair of neighboring routers become adjacent.
- Link state advertisement
Describes the local state of a router or network. This
includes the state of the router's interfaces and
adjacencies. Each link state advertisement is flooded
throughout the routing domain. The collected link state
advertisements of all routers and networks forms the
protocol's topological database.
- Hello Protocol
The part of the OSPF protocol used to establish and maintain
neighbor relationships. On multi-access networks the Hello
Protocol can also dynamically discover neighboring routers.
- Designated Router
Each multi-access network that has at least two attached
routers has a Designated Router. The Designated Router
generates a link state advertisement for the multi-access
network and has other special responsibilities in the
running of the protocol. The Designated Router is elected
by the Hello Protocol.
The Designated Router concept enables a reduction in the
number of adjacencies required on a multi-access network.
This in turn reduces the amount of routing protocol traffic
and the size of the topological database.
- Lower-level protocols
The underlying network access protocols that provide
services to the Internet Protocol and in turn the OSPF
protocol. Examples of these are the X.25 packet and frame
levels for X.25 PDNs, and the ethernet data link layer for