Routing Protocal - Anvari.Net

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29 Οκτ 2013 (πριν από 4 χρόνια και 8 μήνες)

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Routing Protocols

COSC 541

Data Commun. System & Networks

Yue Dou

Router Function

Routers responsible for receiving
and forwarding packets through the
interconnected set of networks

Each router makes routing decision
based on knowledge of the topology
and conditions of the internet

Routing Protocal

Routers need to make dynamic
routing decisions to avoid portions
of the network that have failed or

Routers need the routing protocol to
exchange routing information about
the topology and delays of the

Autonomous Systems ( AS )

An AS consists of a group of routers
exchanging information via an interior
router protocol

An AS is a set of routers and networks
managed by a single organization ( such
as an university )

Except failure, there is a path between any
two pair of routers in AS

An Example of Autonomous

Internet Construction

Internet construct with many autonomous

Routing protocol divides into interior and
exterior router protocol

Interior router protocol ( IRP ) passes
routing information between routers
within an autonomous system

Exterior router protocol ( ERP ) passes
routing information between routers in
different autonomous system

Interior & Exterior router

An interior router protocol needs to build
up a rather detailed model of the
interconnection of routers within an AS in
order to calculate the least
cost path from
a give router to any network within the AS.

An exterior router protocol supports the
exchange of summary reachability
information between separately
administered ASs.

Border Gateway Protocol ( BGP )

Preferred exterior router protocol for the

BGP allow gateway routers in different
autonomous systems (ASs ) to cooperate
in the exchange of routing information

BGP operates in terms of 4 messages:

Open, Update, Keepalive, Notification

BGP 3 Functional Procedures

Neighbor acquisition: one router sends an
open message to the other, which may
either return a keepalive message or
refuse the offer

Neighbor reachability: the two routers
periodically issue Keepalive messages to
each other

Network reachability: when a router
change its routing information, it will
broadcast an Update message

BGP Routing Information

Border router in one AS can issue an
Update message to the border router in
another AS

The Update message could include:

AS_Path: the identity of AS(s)

Next_Hop: the IP address of border router

NLRI: a list of all of the networks in AS(s)

Open Shortest Path First (OSPF)

Widely used as the interior router protocol
in TCP/IP networks

Each router maintains a routing table that
reflects the known topology of the
autonomous system of which it is a part

OSPF computes a route through the

internet that incurs the least cost based on
a user
configurable metric of cost ( like
data rate, dollar cost etc.)

An Example of Routing Table

Destination Next Hop Distance


N1 R3 10

N7 R10 12

R7 R10 8


Destination: LAN or border router

Next Hop: Forwarding router

Distance: Sum of cost on the whole route