11 routing protocol

8/3/2019 11 Routing Protocol

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Amity School of Engineering

B.Tech., CS(6th Sem.)

Advanced Networking

Topic: ROUTING PROTOCOLS

ANIL SAROLIYA



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Although it is desirable for routers to exchange routing

information, it is impractical for all routers in an arbitrarilylarge internet to participate in a single routing update

protocol. So, working of routers must be divided into groups

General Principle for Routing



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Autonomous System



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A portion of a large internetwork that is under a given administrative

authority.

AS can be under the authority of a particular corporation or institution, or

each one can be defined by intra AS routing protocol such as OSPF or RIP.

AS's are part of the routing infrastructure of a large internetwork and can be

subdivided into routing domains.

The Internet is the prime example of a large internetwork divided into different

autonomous systems.

These AS's are connected with backbone routers that use the Border

Gateway Protocol (BGP) for communication among them. Each AS is represented by a 16-bit integer assigned by Internet Network

Information Center (InterNIC).

BGP used above 16-bit AS number to avoid routing loops and implement

policy-based routing on the Internet backbone.

Autonoums System (AS): Group of networks under one administrative authority



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Modern Internet Architecture



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ROUTING PROTOCOLSInter Autonomous Systems



EGP’s: Exterior Gateway Protocols

A single protocol for communicating routes between two autonomous

systems

Solves two problems

– Allows router outside a group to advertise networks hidden in another

autonomous system

– Allows router outside a group to learn destinations in the group

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Border Gateway Protocol (BGP)

The most popular EGP in use in the Internet

Current version is BGP-4

Allows two autonomous systems to communicate routing information

Supports Classless Inter Domain Routing (CIDR) Each AS designates a border router to speak on its behalf

Two border routers become BGP peers

BGP



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Key Characteristics of BGP

Provides inter-autonomous system communication

Follows next-hop paradigm

Provides support for policies

Sends path information

Permits incremental updates

Allows route aggregation

Allows authentication

Additional BGP Facts

Uses reliable transport (i.e., TCP)

– Unusual: most routing update protocols use connectionless transport (e.g., UDP)

Sends keep-alive messages so other end knows that connection is valid

– if no new routing information is needed



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BGP Message Types (Four Types)

Type Code Message Type Description

1 OPEN Initialize communication

2 UPDATE Advertise or withdraw routes

3 NOTIFICATION Response to an incorrect message

4 KEEPALIVE Actively test peer connectivity



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BGP Open Message Format

Used to start a connection

HOLD TIME specifies max time that can elapse between BGP messages



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BGP Update Message

Sender can advertise new routes or withdraw old routes



Internet is too large for all routers to participate in one routing update

protocol

Group of networks and routers under one administrative authority is called

Autonomous System (AS)

Each AS chooses its own interior routing update protocol

Exterior Gateway Protocol (EGP) is used to communicate routing

information between two autonomous systems

Current exterior protocol is Border Gateway Protocol version 4, BGP-4

BGP (Summary)



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ROUTING PROTOCOLSIntra Autonomous Systems



Static vs. Dynamic Interior Routes

Static Dynamic

Fix route is initialized at startup on

routers

Only traffic handling is initialized at

startup on routers

Route is fixed, never change Route can be change, updated by route

propagation protocols

Complexity increases at host side, less

burden on router

Complexity increases at router, less

burden on host



Illustration of Topology in which Static Routing is Optimal

Only one route exists for each destination



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Illustration of Topology in which Dynamic Routing is Needed

Additional router introduces multiple paths



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Exchanging Routing Information in an “Autonomous System”

Mechanisms called Interior Gateway Protocols (IGPs)

Choice of IGP is made by autonomous system

Note: If AS connects to rest of the world, a router in the AS must use an EGP to

advertise network reachability to other AS’s.



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RIPRouting Information Protocol



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RIP - Routing Information Protocol

Originally developed as part of BSD UNIX

RIP uses a distance-vector algorithm

Best route is the one with least hops (distance)

Maximum distance supported is 15 hops

Does not factor in link speed and other metrics

RIP takes time to converge (join)

– 3 minutes typical

– 7 minutes worst case

Uses split horizon and poison reverse techniques to solve inconsistencies

RIP v2 added network masks for CIDR

Protocol is fairly simple

Each router broadcasts an advertisement every 30 seconds or when a receivedadvertisement changes the routing table

Each advertisement includes the cost of reaching each network through that router

<network-address[, mask], distance> pairs

Each router increments the cost as it propagates the reachability information

Note: In networking metrics is a

measure used in calculating the

next host to route a packet.



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RIP1 Update Format

COMMAND

REQUEST - Request either a partial or full table update from another RIP router.

RESPONSE - A response to a request. All route updates use this command in the command field.



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RIP2 Update Format: RIP2 can be broadcast



Initial routing table for Router A:Router A routing Table:

Destination Next Hop Interface Hops

10.1.0.0 0 1 1

10.2.0.0 0 2 110.3.0.0 0 3 1

and, Router B only knew of its direct networks

and router C’s

Destination Hops

10.2.0.0 1

10.4.0.0 1

10.6.0.0 2

Routing Information Protocol (RIP) Working



AfterRouter B’s

advertisement received:Router A Routing Table:

Routing Information Protocol (RIP) Working

Destination Next Hop Interface Hops

10.1.0.0 0 1 1

10.2.0.0 0 2 110.3.0.0 0 3 1

10.4.0.0 B 2 2

10.6.0.0 B 2 3

10.5.0.0 D 3 2

10.7.0.0 D 3 3



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OSPFOpen Shortest Path First



OSPF (Open Shortest Path First)

• Here “Open” means publicly available

• It uses Link State algorithm

– LS packet distribution

– Topology map at each node

– Route computation using Dijkstra’s algorithm

• OSPF advertisement (updating messages) carries one entry

per neighbor router

• Advertisements disseminated to entire AS (via flooding)

– Carried in OSPF messages directly over IP



OSPF “advanced” features (not in RIP)

• Security: all OSPF messages authenticated (to prevent

malicious intrusion)

• Multiple same-cost paths allowed (only one path in RIP)

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OSPF integrated with uni- and multicast support: – Multicast OSPF (MOSPF) uses same topology data base as OSPF

• Hierarchical OSPF in large domains.



Hierarchical OSPF



Hierarchical OSPF (continued...)

• Two-level hierarchy: Area(local area), backbone.

– Link-state advertisements only in area

– each nodes has detailed area topology;

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only know direction (shortest path) to nets in other areas.

– Area border routers: “summarize” distances to nets in

own area, advertise to other Area Border routers.

–

Backbone routers: run OSPF routing limited to backbone. – Boundary routers: connect to other AS’s.



Difference between RIP & OSPF

S.no. RIP OSPF

01 RIP eats lots of bandwidth (all

broadcast traffic) on large

networks. It send entire routing

table in every 30 seconds.

After initialization, OSPF only sends

changes.

02 RIP takes 30 – 60 seconds to

converge (noticeable downtime)

OSPF converges almost immediately,

even in large networks.

03 uses hop counts for link status

(distance vector protocol)

link state protocol

04 Normally, wont send subnet maskupdates

will send subnet mask updates



Thanks

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Amity School of Engineering

B.Tech., CS(6th Sem.)

11 routing protocol

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