OSPF Terminology

OSPF Terminology
The most basic of terms that are related to OSPF are related to many routing protocols. We
begin by defining relationships among routers. From there, we will move on to defining terms
relating to OSPF operations.
Neighbor
A neighbor refers to a connected (physically adjacent) router that is running an OSPF
process with the adjoining interface assigned to the same area. Neighbors are found via Hello
packets. No routing information is exchanged with neighbors unless adjacencies are formed.
Adjacency
An adjacency refers to the logical connection between a router and its corresponding
designated routers and backup designated routers or its point-to-point neighbor. The formation
of this type of relationship depends heavily on the type of network that connects the
OSPF routers. On point-to-point connections, the two routers will form adjacencies with each
other without requiring a designated router. Not all neighbors become adjacent.
Link
In OSPF, a link refers to a network or router interface assigned to any given network.
Within OSPF, link is synonymous with interface.
Interface
The interface is a physical or logical interface on a router. When an interface is added
to the OSPF process, it is considered by OSPF as a link. If the interface is up, then the link is up.
OSPF uses this association to build its link database.
Link-state advertisement
Link-state advertisement (LSA)
is an OSPF data packet containing
link-state and routing information that is shared among OSPF routers. LSAs are covered in
detail in Chapter 6, “Interconnecting OSPF Areas.”
Designated router
A
designated router (DR)
is used only when the OSPF router is connected
to a broadcast (multi-access) network. To minimize the number of adjacencies formed, a DR is
chosen to disseminate/receive routing information to/from the remaining routers on the broadcast
network or link.
Backup designated router
A
backup designated router (BDR)
is a hot standby for the DR on
broadcast (multi-access) networks. The BDR receives all routing updates from OSPF adjacent
routers but does not flood LSA updates.
OSPF areas
OSPF areas
often map to network or subnet boundaries. Areas are used to establish
a hierarchical network. OSPF uses four types of areas, all of which are discussed later in this chapter.
Internal router
An internal router is a router that has all of its interfaces participating in one area.
Area border router
An
area border router (ABR)
is a router that has multiple area assignments.
An interface may belong to only one area. If a router has multiple interfaces and if any
of these interfaces belong to different areas, the router is considered an ABR.
Autonomous system boundary router
An
autonomous system boundary router (ASBR)
is a
router with an interface connected to an external network or to a different AS. An external network
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or autonomous system refers to an interface belonging to a different routing protocol such as
EIGRP. An ASBR is responsible for injecting route information learned by other routing protocols
into OSPF.
Non-broadcast multi-access
Non-broadcast multi-access (NBMA)
networks are networks
such as Frame Relay, X.25, and ATM. This type of network is one of two NBMA network
types, along with point-to-multipoint. NBMA networks allow for multi-access but have no
broadcast ability, unlike Ethernet. They require special OSPF configuration to function properly.
Specifically, you must manually define neighbors, due to the non-broadcast characteristic,
but a DR and a BDR will be elected, due to the multi-access nature of the network. In order for
such elections to work, however, the network must be arranged in a full-mesh configuration.
Broadcast (multi-access)
Networks such as Ethernet allow concurrent access, as well as provide
broadcast ability. A DR and BDR will be elected for multi-access networks, and neighbors
will be discovered automatically for broadcast networks. This network type is a Ciscoproprietary
implementation.
Be aware that Cisco often uses the stand-alone, standards-based term
broadcast
to refer to a
broadcast multi-access
network. It is imperative that you realize this
fact in order to understand the properties of the broadcast network type discussed
here and later in this chapter, because some of the characteristics are due
to the multi-access—not broadcast—nature of these networks. For example, the
ability to automatically discover neighbors and the propensity to multicast Hellos
and updates ties to the broadcast properties, but it is the multi-access characteristic
that leads to the election of a DR. This latter behavior exists for nonbroadcast
multi-access (NBMA) networks, as well. Note that the two network
types share the multi-access characteristic, not the ability to send broadcasts.
Point-to-point
Leased-line circuits are examples of OSPF point-to-point networks, by default.
For NBMA networks, this type of network connection consists of a Cisco-proprietary configuration.
The network can be configured on Frame Relay and ATM circuits to allow point-topoint
connectivity. This configuration eliminates the need for a DR and BDR.
Point-to-multipoint
This type of connection is the other type of NBMA network and treats
each of the router interconnections as point-to-point links, not electing a DR and BDR and not
requiring a full-mesh configuration. Cisco offers both a proprietary broadcast and standardsbased
non-broadcast option for this type of network. As a result, automatic neighbor detection
relies on which of these you choose. Alternatively, Inverse ARP may be used for neighbor discovery,
which is outside of the responsibility of OSPF.
Router ID
The router ID is an IP address that is used to identify the router. Cisco chooses
the configured router ID, if one is configured. If a router ID is not configured, the router ID
will be the highest IP address of all configured loopback interfaces. If no loopback addresses
are configured, OSPF will choose the highest IP address of all configured physical interfaces
on the router.
Open Shortest Path First
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All of these terms play an important part in understanding the operation of OSPF. You must
know and understand each of these terms. As you read through this chapter, you will be able
to place the terms in their proper context.