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OSPF routers establish relationships, or
states, with their neighbors for efficiently sharing link-state
information. In contrast, distance vector routing protocols, such as
RIP, blindly broadcast or multicast their complete routing table out
every interface, hoping that a router is out there to receive it.
Every 30 seconds, by default, RIP routers send only one kind of
message. This message is their complete routing table. OSPF
routers, on the other hand, rely on five different kinds of packets to
identify their neighbors and to update link-state routing information.

These five packet types make OSPF capable
of sophisticated and complex communications. These packet types will
be discussed in more detail later in the module. At this point, become
familiar with the different relationships, or states, possible between
OSPF routers, the different OSPF network types, and the OSPF Hello
protocol.
OSPF States
The key to effectively designing and troubleshooting OSPF networks
is to understand the relationships, or states, that develop between
OSPF routers. OSPF interfaces can be in one of seven states. OSPF
neighbor relationships progress through these states, one at a time,
in the order presented in Figure
.
- Down State
In the Down state, the OSPF process has not exchanged information
with any neighbor. OSPF is waiting to enter the next state, which is
the Init state.
- Init State
OSPF routers send Type 1 packets, or Hello packets, at regular intervals to
establish a relationship with neighbor routers. These intervals are
usually ten seconds. When an interface receives its first Hello
packet, the router enters the Init state. This means the router
knows a neighbor is out there and is waiting to take the
relationship to the next step.
The two kinds of relationships are
the two-way state and adjacency. A router must receive a Hello from
a neighbor before it can establish any relationship.
- Two-Way State
Using Hello packets, every OSPF router tries to establish a
two-way state, or bidirectional communication, with every neighbor
router on the same IP network. Among other things, Hello packets
include a list of the sender's known OSPF neighbors. A router enters
the two-way state when it sees itself in a neighbor's Hello. When RTB learns that RTA knows about RTB, RTB declares a two-way state to
exist with RTA.

The two-way state is the most basic
relationship that OSPF neighbors can have, but routing information
is not shared between routers in this relationship. To learn about
the link states of other routers and eventually build a routing table,
every OSPF router must form at least one adjacency. An adjacency is
an advanced relationship between OSPF routers that involves a series
of progressive states that rely not just on Hellos, but also on the
other four types of OSPF packets. Routers attempting to become
adjacent to one another exchange routing information even before the
adjacency is fully established. The first step toward full adjacency
is the ExStart state, which is described next.
- ExStart State
Technically, when a router and its neighbor enter the ExStart state,
their conversation is characterized as an adjacency, but they have
not become fully adjacent. ExStart is established using Type 2
database description (DBD) packets, also known as DDPs. The two
neighbor routers use Hello packets to negotiate who is the "master"
and who is the "slave" in their relationship and use DBD packets to
exchange databases.

The router with the highest OSPF router
ID "wins" and becomes the master. The OSPF router ID is discussed later
in this module. When the neighbors establish their roles as master
and slave, they enter the Exchange state and begin sending routing
information.
- Exchange State
In the Exchange state, neighbor routers use Type 2 DBD packets
to send each other their link-state information.
In other
words, the routers describe their link-state databases to each
other. The routers compare what they learn with their existing
link-state databases. If either of the routers receives information
about a link that is not already in its database, the router
requests a complete update from its neighbor. Complete routing
information is exchanged in the Loading state.
- Loading State
After the databases have been described to each router, they may
request information that is more complete by using Type 3 packets,
link-state requests (LSRs). When a router receives an LSR, it
responds with an update by using a Type 4 link-state update (LSU)
packet. These Type 4 LSU packets contain the actual link-state
advertisements (LSAs), which are the heart of link-state routing
protocols.
Type 4
LSUs are acknowledged using Type 5 packets, called link-state
acknowledgments (LSAcks).
- Full Adjacency
With the Loading state complete, the routers are fully adjacent.
Each router keeps a list of adjacent neighbors, called the adjacency
database. Do not confuse the adjacency database with the link-state
database or the forwarding database.

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Interactive Media Activity
Drag and Drop: OSPF Packet Types
Upon completion of this activity, the student will be able to
identify the different OSPF packet types.

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Interactive Media Activity
Drag and Drop: OSPF States
Upon completion of this activity, the student will be able to
list in order the different states of OSPF.

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