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ROUTING PROTOCOLS

Mario Baldiwww.baldi.info

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TWO PROTOCOL FAMILIES

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Two Kinds of Protocols

Interior Gateway Protocol (IGP)

Intra-domain routing

Exterior Gateway Protocol (EGP)

Inter-domain routing

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Different objectives

Different design criteria

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IGP Features

Distribute topological information

Choose routes based on topological information

Find “best” route

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Distribute Autonomous System information

Distribute administrative costs

Decide based on policies

Find “preferred” route

EGP Features

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IGP – Distance Vector

RIP: Routing Information Protocol

IGRP: Interior Gateway Routing Protocol

E-IGRP: Enhanced IGRP

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IGP – Link State

OSPF: Open Shortest Path First

Integrated IS-IS

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EGP

BGP: Border Gateway Protocol

IDRP: Inter Domain Routing Protocol

Static routing also a viable option

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INTERIOR GATEWAY PROTOCOLS

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RIP

Originally developed for another architecture

RFC 1O58 (1988) and RFC 1388 (1993)

Implemented also by Unix/Linux hosts

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Features

Hop count

At most 15 hops

Periodic update messages

Distance vectors

Every 3O s

Time-out based operation

Convergence: 3 min

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IGRP

Cisco Systems proprietary

It overcomes some of the shortcomings of RIP

For a while, the only alternative to RIP

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Features

Articulated metrics

Delay

Bandwidth

Reliability

Load

Maximum packet length

Multipath routing

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OSPF

RFC 1247 (1991) and RFC 1583 (1994)

Hierarchical routing

Routing domain divided in areas

Aggregation of information among areas

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Hierarchical RoutingRouters know all details of their

domain/zone/area

Routers have limited or no knowledge about the outside

Can be iterated

A2 B3

B2

Domain/

Zone/Area A

Domain/Zone

/Area B

A1

B1

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Strictly Hierarchical RoutingNo knowledge/visibility outside own area/domain/zone

When the destination of a packet is not in the area, it is forwarded towards an edge router

Maximum scale

Limited routing capability A2

Domain/

Zone/Area A

A1

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Strictly Hierarchical RoutingHigher layer routers have area/domain/zone level view

A2 B3

B2

Domain/Zone

/Area A

Domain/Zone

/Area B

A1

B1

A B

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Strictly Hierarchical RoutingSub-optimal paths

Loss of connectivity due to faults

B1 B2

A4 Domain A

Domain B

A1 A3A2

Edge router

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A2 B3

B2

Domain/Zone

/Area A

Domain/Zone

/Area B

A1

B1

Loosely Hierarchical RoutingRouters have high level knowledge of the outside

A2 B3

B2

A1 B1

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Loosely Hierarchical RoutingLess scalability

Routers have to store and exchange more information

Does not require strictly hierarchical addressing

All host in domain B do not need a common identifier

Prefix

Possible in IPv4

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Area 2

Area 1

Backbone routerBackbone area

AS boundary router

Areaborder router

Internal router

R2 R7

R4R8

R9

R3

R6

R1

R5

D1

OSPF Architecture

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Broadcast NetworksN routers = N2 adjacencies

N2 links

Dijkstra complexity is linear with number of links

H1

H2

R1 R2 R3 R4

Net1R1

R2 R3

R4

Net1

Link State Database

R1 R2/1 R3/1 R4/1 Net1/1

R2 R1/1 R3/1 R4/1 Net1/1

R3 R1/1 R2/1 R4/1 Net1/1

R4 R1/1 R2/1 R3/1 Net1/1

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Link State Database

R1 PN/1

R2 PN/1

R3 PN/1

R4 PN/1

PN R1/1 R2/1 R3/1

R4/1 Net1/1

Broadcast NetworksPseudo-node

Virtual node to transform full mesh into star

Role played by one of the routers (election)

Only for (proactive) routing

Packets delivered directly

R1

R2

R3

Net1

R4

R1

R2 R3

R4

Net1PN

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Integrated IS-IS

Extension of an OSI protocol

Hierarchical routing

Different level routers

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Widely deployed before OSPF was available

Large networks

ISPs

Still in use

Do not change what works

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EXTERIOR GATEWAY PROTOCOLS

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BGP

Currently Version 4

RFC 1654 (1994)

Path vector

AS sequence to destination

Rich set of attributes

Configurable route computation policy

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InterDomain Routing Protocol (IDRP)

Evolution of BGP for OSI

Ported back to TCP/IP

Supposed to be “the” choice for IPv6

Not used a lot