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Open Shortest Path First is a link-state routing protocol. You can think of a link as an interface
on a router. The state of the link is a description of that interface and of its relationship to its
neighboring routers. A description of the interface would include, for example, the IP address
140 Chapter 4: Single-Area OSPF Implementation
of the interface, the subnet mask, the type of network to which it is connected, the routers that are
connected to that network, and so on. The collection of all of these link states forms a link-state
database.
A router sends link-state advertisement (LSA) packets to advertise its state periodically (every 30
minutes) and immediately when the router state changes. Information about attached interfaces,
metrics used, and other variables is included in OSPF LSAs. As OSPF routers accumulate linkstate
information, they use the shortest path first (SPF) algorithm to calculate the shortest path to
each node.
A topological (link-state) database is, essentially, an overall picture of networks in relation to
routers. The topological database contains the collection of LSAs received from all routers in the
same area. Because routers within the same area share the same information, they have identical
topological databases.
OSPF can operate within a hierarchy. The largest entity within the hierarchy is the autonomous
system, which is a collection of networks under a common administration that share a common
routing strategy. An autonomous system can be divided into a number of areas, which are groups
of contiguous networks and attached hosts. Figure 4-1 shows an example of an OSPF hierarchy.
Figure 4-1 OSPF Hierarchy
I A B
D E
F G H
C
Area 1 Area 2 Area 3
External
Routing
Domain
Autonomous System
Backbone Area
Introducing OSPF 141
OSPF uses a two-layer network hierarchy that has two primary elements:
■ Autonomous system: An autonomous system consists of a collection of networks under a
common administration that share a common routing strategy. An autonomous system,
sometimes called a domain, can be logically subdivided into multiple areas.
■ Area: An area is a grouping of contiguous networks. Areas are logical subdivisions of the
autonomous system.
Within each autonomous system, a contiguous backbone area must be defined. All other
nonbackbone areas are connected off the backbone area. The backbone area is the transition area
because all other areas communicate through it. For OSPF, the nonbackbone areas can be
additionally configured as stub areas, totally stubby areas, or not-so-stubby areas (NSSA) to help
reduce the link-state database and routing table size.
OSPF special areas such as NSSAs, totally stubby, and stub areas are beyond the scope of this text.
Routers that operate within the two-layer network hierarchy have different routing entities and
different functions in OSPF. The following are some examples based on Figure 4-1:
■ Router B is the backbone router. The backbone router provides connectivity between different
areas.
■ Routers C, D, and E are area border routers (ABR). ABRs attach to multiple areas, maintain
separate link-state databases for each area to which they are connected, and route traffic
destined for or arriving from other areas.
■ Routers F, G, and H are nonbackbone, internal routers. Nonbackbone, internal routers are
aware of the topology within their respective areas and maintain identical link-state databases
about the areas.
■ Depending on the configuration of the OSPF nonbackbone area (stub area, totally stubby area,
or NSSA) the ABR advertises a default route to the nonbackbone, internal, router. The
nonbackbone, internal router uses the default route to forward all interarea or interdomain
traffic to the ABR router.
■ Router A is the autonomous system boundary router (ASBR) that connects to an external
routing domain, or autonomous system.
■ Router I is a router that belongs to another routing domain, or autonomous system.
Establishing OSPF Neighbor Adjacencies
Neighbor OSPF routers must recognize each other on the network before they can share
information because OSPF routing depends on the status of the link between two routers. This
process is done using the Hello protocol. The Hello protocol establishes and maintains neighbor
142 Chapter 4: Single-Area OSPF Implementation
relationships by ensuring bidirectional (two-way) communication between neighbors.
Bidirectional communication occurs when a router recognizes itself listed in the hello packet
received from a neighbor. Figure 4-2 illustrates the hello packet.
Figure 4-2 OSPF Hello
Each interface that is participating in OSPF uses IP multicast address 224.0.0.5 to periodically
send hello packets. A hello packet contains the following information:
■ Router ID: The router ID is a 32-bit number that uniquely identifies the router. The highest
IP address on an active interface is chosen by default, unless a loopback interface or the router
ID is configured; for example, IP address 172.16.12.1 would be chosen over 172.16.1.1. This
identification is important in establishing and troubleshooting neighbor relationships and
coordinating route exchanges.
■ Hello and dead intervals: The hello interval specifies the frequency in seconds at which a
router sends hello packets. The default hello interval on multiaccess networks is 10 seconds.
The dead interval is the time in seconds that a router waits to hear from a neighbor before
declaring the neighboring router out of service. By default, the dead interval is four times the
hello interval. These timers must be the same on neighboring routers; otherwise, an adjacency
will not be established.
■ Neighbors: The Neighbors field lists the adjacent routers with established bidirectional
communication. This bidirectional communication is indicated when the router recognizes
itself listed in the Neighbors field of the hello packet from the neighbor.
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39547439070713
D E
B A C
Hello
Hello
Router ID
Hello and Dead Intervals*
Neighbors
Area ID*
Router Priority
DR IP Address
BDR IP Address
Authentication Pasword*
Stub Area Flag*
*Entry must match on neighboring routers.
Introducing OSPF 143
■ Area ID: To communicate, two routers must share a common segment, and their interfaces
must belong to the same OSPF area on that segment. The neighbors must also share the same
subnet and mask. All these routers will have the same link-state information.
■ Router priority: The router priority is an 8-bit number that indicates the priority of a router.
OSPF uses the priority to select a designated router (DR) and a backup DR (BDR).
■ DR and BDR IP addresses: These are the IP addresses of the DR and BDR for the specific
network, if they are known.
■ Authentication password: If router authentication is enabled, two routers must exchange the
same password. OSPF has three types of authentication: Null (no authentication), simple
(plain-text passwords), and MD5. Authentication is not required, but if it is enabled, all peer
routers must have the same password.
■ Stub area flag: A stub area is a special area. Designating a stub area is a technique that
reduces routing updates by replacing them with a default route. Two routers must agree on the
stub area flag in the hello packets.
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