Routing Protocal - Anvari.Net

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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
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