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2. LINK-STATE ROUTING PROTOCOLS AND AUTONOMOUS OSPF
ROUTING
Routing protocols are used by routers to create a map of the network, so
that the routers can determine how to forward packets from one point to
another. Link-state routing protocols create the map in three distinct
phases. First, each router meets its neighbors and learns about its local
neighborhood. Second, routers share that information with all other routers
on the network by exchanging information. During this phase, a router
learns about neighborhoods other than its local one. Finally, each router
combines the information about each individual neighbor to get a “map”of
the entire network by applying routing algorithms to come up with routing
tables.
OSPF (open shortest path first) [Moy 1998a], based on link-state and
SPF (shortest path first) technology, is classified as an interior gateway
protocol (IGP). This means that it distributes routing information between
routers belonging to a single autonomous system (AS) where a collection of
computer systems, routers, and other network devices is under one admin-istrative
domain.

The three stages of OSPF are as follows,
(1) Meeting the neighbors
The first step is the creation of adjacencies.Ahello protocol is defined
to establish and maintain the neighbor relationship. That is, every
OSPF router periodically sends hello packets to discover its neighboring
routers. Three components in the hello packet header keep information
about the status of routers. The hello interval indicates how frequently
the sender should retransmit its hello packets; the router dead interval
tells how long it takes to declare a router unavailable, and a list
describes the neighbors that the sender has already met. Once neigh-boring
routers have “met” via the hello protocol, they go through a
“database exchange process” to synchronize their databases.
(2) Share the information by flooding LSA
In the second step, the information about a router’s local neighborhood
is assembled into a link-state advertisement (LSA), and is broadcast (via
a reliable intelligent flooding scheme) to all other routers. The com-bined
information makes up the link state (LS) database for the
network. In a broadcasting network like Ethernet, a designated router
may be elected to advertise for the whole network.
(3) Calculate shortest routes based on LS database
Once all systems have an up-to-date link state (LS) database, each
router can use the Dijkstra algorithm to calculate a shortest-path tree
with the router itself as the root node and then form a complete picture
of routing in the network.

In a dynamic network, routers may restart, network cost metrics may
change, and links may fail. These three phases of the OSPF operation
ensure a functional routing environment.
Different from RIP, and many other routing protocols, the OSPF protocol
does not use UDP or TCP. It runs directly over IP, using IP protocol 89. IP
fragmentation and reassembly are used when fragmentation is necessary.
There are five types of OSPF packets. The hello packet is used by a router
to discover its neighbors. The database description and the link state
request packets are used to synchronize two routers’ databases when an
adjacency is being initialized. The link state update packets are used to
update link state database. The link state acknowledgment packets ensure
a reliable transmission by acknowledging flooded LSU packets.

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