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A set of
routing

protocols that are used within an autonomous system are referred to as
interior gateway protocols

(
IGP
).

In contrast an exterior gateway protocol is for determining network
reachability between
autonomous systems (AS) and make use of IGPs to resolve route within an AS.

The interior gateway protocols can be divided into two categories: 1) Distance
-
vector
routing protocol and 2) Link
-
state routing protocol.

Types of Interior ga
teway protocols

Distance
-
vector routing protocol

They use the Bellman
-
Ford algorithm to calculate paths. In Distance
-
vector routing
protocol
s each router does not posses information about the full network topology. It
advertises its distances from other routers and receives similar advertisements from other
routers. Using these routing advertisements each router populates its routing table. In

the
next advertisement cycle, a router advertises updated information from its routing table.
This process continues until the routing tables of each router converge to stable values.

This set of protocols has the disadvantage of slow convergence, however
, they are
usually simple to handle and are well suited for use with small networks. Some examples
of distance
-
vector routing protocols are:

1.

Routing Information Protocol (RIP)

2.

Interior Gateway Routing Protocol (IGRP)

[
edit
]

Link
-
state routing protocol

I
n the case of Link
-
state routing protocols, each node possesses information about the
complete network topology. Each node then independently calculates the best next hop
from it for every possible destination in the network using local information of the
topology. The collection of best next hops forms the routing table for the node.

This contrasts with distance
-
vector routing protocols, which work by having each node
share its routing table with its neighbors. In a link
-
state protocol, the only informatio
n
passed between the nodes is information used to construct the connectivity maps.

Example of Link
-
state routing protocols are:

1.

Open Shortest Path First (OSPF)

2.

Intermediate system to intermediate system (IS
-
IS)


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Open Shortest Path First


The
Open Shortes
t Path First

(
OSPF
) protocol is a link
-
state, hierarchical
interior
gateway protocol

(IGP) for network routing. Dijkstra's algorithm is used to calculate the
shortest path tree. It uses
cost

as its routing metric. A link state database is constructed of
th
e network topology which is identical on all routers in the area.

OSPF is perhaps the most widely used IGP in large networks. It can operate securely,
using MD5 to authenticate peers before forming adjacencies, and before accepting link
-
state advertisement
s (LSA). A natural successor to the
Routing Information Protocol

(RIP), it was VLSM
-
capable or
classless

from its inception. A newer version of OSPF
(OSPFv3) now supports IPv6 as well. Multicast extensions to OSPF, the
Multicast Open
Shortest Path First

(M
OSPF) protocols, have been defined, but these are not widely used
at present. OSPF can "tag" routes, and propagate the tags along with the routes.

An OSPF network can be broken up into smaller networks. A special area called the
backbone area

forms the cor
e of the network, and other areas are connected to it. Inter
-
area routing goes via the backbone. All areas must connect to the backbone; if no direct
connection is possible, a
virtual link

may be established.

Routers in the same broadcast domain or at each

end of a point
-
to
-
point
telecommunications link form
adjacencies

when they have detected each other. This
detection occurs when a router "sees" itself in a hello packet. This is called a two way
state and is the most basic relationship. The routers elect
a
designated router

(DR) and a
backup designated router

(BDR) which act as a hub to reduce traffic between routers.
OSPF uses both
unicast

and multicast to send "hello packets" and link stat
e updates.
Multicast addresses 224.0.0.5 and 224.0.0.6 are reserved for OSPF. In contrast to the
Routing Information Protocol

(RIP) or the
Border Gateway Protocol

(BGP), OSPF does
not use TCP or UDP but uses IP directly, via IP protocol 89.

Background

Ope
n Shortest Path First (OSPF)

is a routing protocol developed for Internet Protocol
(IP) networks by the Interior Gateway Protocol (IGP) working group of the Internet
Engineering Task Force (IETF). The working group was formed in 1988 to design an
IGP based

on the Shortest Path First (SPF) algorithm for use in the Internet. Similar to
the Interior Gateway Routing Protocol (IGRP), OSPF was created because in the mid
-
1980s, the Routing Information Protocol (RIP) was increasingly incapable of serving
large, het
erogeneous internetworks. This chapter examines the OSPF routing
environment, underlying routing algorithm, and general protocol components.

OSPF was derived from several research efforts, including Bolt, Beranek, and Newman's
(BBN's) SPF algorithm develo
ped in 1978 for the ARPANET (a landmark packet
-
switching network developed in the early 1970s by BBN), Dr. Radia Perlman's research
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on fault
-
tolerant broadcasting of routing information (1988), BBN's work on area routing
(1986), and an early version of OSI
's Intermediate System
-
to
-
Intermediate System (IS
-
IS)
routing protocol.

OSPF has two primary characteristics. The first is that the protocol is open, which means
that its specification is in the public domain. The OSPF specification is published as
Reques
t For Comments (RFC) 1247. The second principal characteristic is that OSPF is
based on the SPF algorithm, which sometimes is referred to as the Dijkstra algorithm,
named for the person credited with its creation.

OSPF is a link
-
state routing protocol tha
t calls for the sending of link
-
state
advertisements (LSAs) to all other routers within the same hierarchical area. Information
on attached interfaces, metrics used, and other variables is included in OSPF LSAs. As
OSPF routers accumulate link
-
state inform
ation, they use the SPF algorithm to calculate
the shortest path to each node.

As a link
-
state routing protocol, OSPF contrasts with RIP and IGRP, which are distance
-
vector routing protocols. Routers running the distance
-
vector algorithm send all or a
por
tion of their routing tables in routing
-
update messages to their neighbors.

Routing Hierarchy

Unlike RIP, OSPF can operate within a hierarchy. The largest entity within the hierarchy
is the autonomous system (AS), which is a collection of networks under
a common
administration that share a common routing strategy. OSPF is an intra
-
AS (interior
gateway) routing protocol, although it is capable of receiving routes from and sending
routes to other ASs.

An AS can be divided into a number of areas, which are
groups of contiguous networks
and attached hosts. Routers with multiple interfaces can participate in multiple areas.
These routers, which are called Area Border Routers, maintain separate topological
databases for each area.

A topological database is ess
entially an overall picture of networks in relationship 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 to
pological databases.

The term
domain

sometimes is used to describe a portion of the network in which all
routers have identical topological databases. Domain is frequently used interchangeably
with AS.

An area's topology is invisible to entities outside
the area. By keeping area topologies
separate, OSPF passes less routing traffic than it would if the AS were not partitioned.

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Area partitioning creates two different types of OSPF routing, depending on whether the
source and the destination are in the sam
e or different areas. Intra
-
area routing occurs
when the source and destination are in the same area; interarea routing occurs when they
are in different areas.

An OSPF backbone is responsible for distributing routing information between areas. It
consist
s of all Area Border Routers, networks not wholly contained in any area, and their
attached routers. Figure 46
-
1 shows an example of an internetwork with several areas.

In the figure, routers 4, 5, 6, 10, 11, and 12 make up the backbone. If Host H1 in Are
a 3
wants to send a packet to Host H2 in Area 2, the packet is sent to Router 13, which
forwards the packet to Router 12, which sends the packet to Router 11. Router 11 then
forwards the packet along the backbone to Area Border Router 10, which sends the
p
acket through two intra
-
area routers (Router 9 and Router 7) to be forwarded to Host
H2.

The backbone itself is an OSPF area, so all backbone routers use the same procedures and
algorithms to maintain routing information within the backbone that any area
router
would. The backbone topology is invisible to all intra
-
area routers, as are individual area
topologies to the backbone.

Areas can be defined in such a way that the backbone is not contiguous. In this case,
backbone connectivity must be restored thr
ough virtual links. Virtual links are configured
between any backbone routers that share a link to a nonbackbone area and function as if
they were direct links.

Figure

46
-
1 An OSPF AS Consists of Multiple Areas Linked by Routers

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AS border routers running OSPF learn about exterior routes through exterior gateway
protocols (EGPs), such as Exterior Gateway Protocol (EGP) or Border Gateway Protocol
(BGP), or throug
h configuration information. For more information about these
protocols, see Chapter 39, "Border Gateway Protocol."

SPF Algorithm

The
Shortest Path First (SPF)

routing algorithm is the basis for OSPF operations. When
an SPF router is powered up, it initi
alizes its routing
-
protocol data structures and then
waits for indications from lower
-
layer protocols that its interfaces are functional.

After a router is assured that its interfaces are functioning, it uses the OSPF Hello
protocol to acquire neighbors,
which are routers with interfaces to a common network.
The router sends hello packets to its neighbors and receives their hello packets. In
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addition to helping acquire neighbors, hello packets also act as keepalives to let routers
know that other routers a
re still functional.

On multiaccess networks (networks supporting more than two routers), the Hello protocol
elects a designated router and a backup designated router. Among other things, the
designated router is responsible for generating LSAs for the en
tire multiaccess network.
Designated routers allow a reduction in network traffic and in the size of the topological
database.

When the link
-
state databases of two neighboring routers are synchronized, the routers
are said to be adjacent. On multiaccess n
etworks, the designated router determines which
routers should become adjacent. Topological databases are synchronized between pairs of
adjacent routers. Adjacencies control the distribution of routing
-
protocol packets, which
are sent and received only on
adjacencies.

Each router periodically sends an LSA to provide information on a router's adjacencies or
to inform others when a router's state changes. By comparing established adjacencies to
link states, failed routers can be detected quickly, and the net
work's topology can be
altered appropriately. From the topological database generated from LSAs, each router
calculates a shortest
-
path tree, with itself as root. The shortest
-
path tree, in turn, yields a
routing table.

Packet Format

All OSPF packets beg
in with a 24
-
byte header, as illustrated in Figure 46
-
2.

Figure

46
-
2 OSPF Packets Consist of Nine Fields


The following descriptions summarize the header field
s illustrated in Figure 46
-
2.


Version number

Identifies the OSPF version used.


Type

Identifies the OSPF packet type as one of the following:


Hello

Establishes and maintains neighbor relationships.


Database description

Describes the contents of the topological database. These
messages are exchanged when an adjacency is initialized.

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Link
-
state request

Requests pieces of the topological database from neighbor
routers. These messages are exchanged after a router discovers (by examining database
-
description packets) that parts of its topologic
al database are outdated.


Link
-
state update

Responds to a link
-
state request packet. These messages also are
used for the regular dispersal of LSAs. Several LSA
s can be included within a single
link
-
state update packet.


Link
-
state acknowledgment

Acknowledges link
-
state update packets.


Packet length

Specifies the packet length, including the OSPF header, in bytes.


Router ID

Identifi
es the source of the packet.


Area ID

Identifies the area to which the packet belongs. All OSPF packets are
associated with a single area.


Checksum

Checks the entire packet contents for any damage suffered in transit.


Authent
ication type

Contains the authentication type. All OSPF protocol
exchanges are authenticated. The authentication type is configurable on per
-
area basis.


Authent
ication

Contains authentication information.


Data

Contains encapsulated upper
-
layer information.

Additional OSPF Features

Additional OSPF features include equ
al
-
cost, multipath routing, and routing based on
upper
-
layer type
-
of
-
service (TOS) requests. TOS
-
based routing supports those upper
-
layer protocols that can specify particular types of service. An application, for example,
might specify that certain data i
s urgent. If OSPF has high
-
priority links at its disposal,
these can be used to transport the urgent datagram.

OSPF supports one or more metrics. If only one metric is used, it is considered to be
arbitrary, and TOS is not supported. If more than one metr
ic is used, TOS is optionally
supported through the use of a separate metric (and, therefore, a separate routing table)
for each of the eight combinations created by the three IP TOS bits (the delay,
throughput, and reliability bits). For example, if the I
P TOS bits specify low delay, low
throughput, and high reliability, OSPF calculates routes to all destinations based on this
TOS designation.

IP subnet masks are included with each advertised destination, enabling variable
-
length
subnet masks. With variab
le
-
length subnet masks, an IP network can be broken into
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many subnets of various sizes. This provides network administrators with extra network
-
configuration flexibility.

Review Questions

Q

When using OSPF, can you have two areas attached to each other w
here only one
AS has an interface in Area 0?


A

Yes, you can. This describes the use of a virtual path. One area has an interface in
Area 0 (legal), and the other AS is brought up and attached off an ABR in Area 1, so
we'll call it Area 2. Area 2 has no in
terface in Area 0, so it must have a virtual path to
Area 0 through Area 1. When this is in place, Area 2 looks like it is directly connected to
Area 0. When Area 1 wants to send packets to Area 2, it must send them to Area 0, which
in turn redirects them
back through Area 1 using the virtual path to Area 2.

Q

Area 0 contains five routers (A, B, C, D, and E), and Area 1 contains three routers

(R, S, and T). What routers does Router T know exists? Router S is the ABR.


A

Router T knows about routers R and
S only. Likewise, Router S only knows about R
and T, as well as routers to the ABR in Area 0. The AS's separate the areas so that router
updates contain only information needed for that AS.