Transcript Routing Protocal

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
congested.
 Routers need the routing protocol to
exchange routing information about
the topology and delays of the
internet

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
System
Internet Construction

Internet construct with many autonomous
systems
 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
protocol

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
Internet
 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
Exchange

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)
Protocol

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