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1IXP/IXP Workshops © 1999, Cisco Systems, Inc.

Introduction toRouting in the Internet

Introduction toIntroduction toRouting in the InternetRouting in the Internet

ISP/IXP WorkshopsISP/IXP WorkshopsISP/IXP Workshops


Network Topologiesand Definitions

Network TopologiesNetwork Topologiesand Definitionsand Definitions

2ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com


Network Topology andNetwork Topology andDefinitionsDefinitions

• Definitions and icons

• Network topologies

• PoP topologies

• Interconnections and IXPs


Some IconsSome Icons

Router (layer 3, IP datagram forwarding)

ATM or Frame Relay switch (layer 2, frame or cell forwarding)

Network Cloud

Ethernet switch (layer 2, packet forwarding)


DefinitionsDefinitions

• PoP - Point of Presence

physical location of ISP’s equipment

• vPoP - virtual PoP

apparent ISP location

in reality a back hauled access point

used mainly for dial access networks

• Hub - large central PoP

links to many PoPs


Network TopologiesNetwork Topologies

Routed backbone• Routers are the

infrastructure

• HDLC or PPP linksbetween routers

• Easier routingconfiguration anddebugging


Network TopologiesNetwork Topologies

Switched backbone• frame relay or ATM

switches in the core

surrounded by routers

• more complex routingand debugging

• traffic management


PoPPoP Topologies Topologies

• Core routers - high speed trunk connections

• Distribution routers and Access routers -high port density

• Border routers - connections to otherproviders

• Service routers - hosting and servers

• Some functions might be handled by asingle router


Pure routedPure routed PoPs PoPs

Core Routers

AccessRouters

DistributionRouters

BorderRouters

to other provideror interconnectsother

PoPs

Customer Premises Routers


DefinitionsDefinitionsDefinitions

• Transit - carrying traffic across anetwork, usually for a fee

• Peering - exchanging routing informationand traffic

• Default - where to send traffic when thereis no explicit match is in the routing table


Peering and Transit examplePeering and Transit example

provider A

provider C

provider B

Backbone Provider D

IXP-East

IXP-West

A and B can peer, but need transitarrangements with D to get packets to/from C


Private InterconnectPrivate Interconnect

network A

network B

Autonomous System 99

Autonomous System 334

border border


PublicPublic Interconnect Points Interconnect Points

• IXP - Internet eXchange Point

• NAP - Network Access Point

• local IXPspeering point for a group of local/regional providers

• transit IXPs

connects local providers to backbone (transit) providers

• hybrid IXPs

combines the function of local and transit


Public Interconnect PointPublic Interconnect Point

• Centralised (in one facility)

• Distributed (connected via WAN links)

• Shared, switched or routed interconnect

Router, FDDI, Ethernet, ATM, Frame relay,SMDS, etc.

• Each provider establishes relationship withother provider at IXP

ISP border router peers with all other providerborder routers


Public interconnectPublic interconnect

Network 2

Network 1

Network 6

Network 4

Network 3

Network 5

each of these represents a border router in a different autonomous system


Route ServerRoute Server

• Advantages:

reduces resource burden on border routers (CPU,memory, configuration complexity)

reduces administrative burden on providers

• Disadvantages:

must rely on a third party (for management,configuration, software updates, maintenance,etc)


Route ServerRoute Server

Route Server

Network 2

Network 1

Network 6

Network 4

Network 3

Network 5


Default Free ZoneDefault Free ZoneDefault Free Zone

The default free zone is made upof Internet routers which have

explicit routing information aboutthe rest of the Internet, and

therefore do not need to use adefault route.


High Level View of the GlobalHigh Level View of the GlobalInternetInternet

Local NAP or IXPR4

Default Free Zone

BackboneProvider 1

AccessProviders 1

Customer Networks

AccessProviders 2

BackboneProvider 2


Categorizing ISPsCategorizing ISPsCategorizing ISPs

Tier 1 NSP

Tier 1 NSP Tier 1 NSP

Tier 1 NSP

$$$$$$$$$$$$$$$

Tier 2 ISP

IXP

Tier 3 ISP

Tier 2 ISP Tier 2 ISP

Tier 2 ISP

IXP

Tier 3 ISP

Tier 3 ISP Tier 3 ISP

Tier 3 ISP

Tier 3 ISP


Internet Topology andInternet Topology andArchitectureArchitecture

• Rapidly increasing complexitymore providers and locationsincreased meshing

• Emergence of global providers

capital, regulatory, and technicalreasons limit the scope of coverage of asingle provider

• Many new interconnect points (IXPs)


Routing BasicsRouting BasicsRouting Basics



Routing ConceptsRouting Concepts

• Routing

• Forwarding

• Some definitions

• Policy options

• Addressing

• Routing Protocols


What does a router do?What does a router do?

?


A day in a life of a routerA day in a life of a router

find path

forward packet, forward packet,forward packet, forward packet...

find alternate path

forward packet, forward packet,forward packet, forward packet…

repeat until powered off


Routing versus ForwardingRouting versus Forwarding

• Routing = buildingmaps and givingdirections

• Forwarding =moving packetsbetween interfacesaccording to the“directions”


IP Routing - finding the pathIP Routing - finding the path

• Path derived from information received froma routing protocol

• Several alternative paths may exist

best next hop stored in forwarding table

• Decisions are updated periodically or astopology changes (event driven)

• Decisions are based on:

topology, policies and metrics (hop count,filtering, delay, bandwidth, etc.)


IP route lookupIP route lookupIP route lookup

• Based on destination IP packet

• “longest match” routing

more specific prefix preferred over lessspecific prefix

example: packet with destination of10.1.1.1/32 is sent to the routerannouncing 10.1/16 rather than therouter announcing 10/8.


IP route lookupIP route lookup

R2

R3

R1 R4

All 10/8 except10.1/16

10.1/16


Packet: DestinationIP address: 10.1.1.1

10/8 -> R310.1/16 -> R420/8 -> R530/8 -> R6…..

R2’s IP routing table


IP route lookup: LongestIP route lookup: Longestmatch routingmatch routing

R2

R3

R1 R4


10.1/16


10/8 -> R310.1/16 -> R420/8 -> R530/8 -> R6…..


10.1.1.1 && FF.0.0.0 vs.10.0.0.0 && FF.0.0.0

Match!




R2

R3

R1 R4


10.1/16


10/8 -> R310.1/16 -> R420/8 -> R530/8 -> R6…..


10.1.1.1 && FF.FF.0.0 vs.10.1.0.0 && FF.FF.0.0

Match as well!




R2

R3

R1 R4


10.1/16


10/8 -> R310.1/16 -> R420/8 -> R530/8 -> R6…..


10.1.1.1 && FF.0.0.0 vs.20.0.0.0 && FF.0.0.0

Does not match!




R2

R3

R1 R4


10.1/16


10/8 -> R310.1/16 -> R420/8 -> R530/8 -> R6…..


10.1.1.1 && FF.0.0.0 vs.30.0.0.0 && FF.0.0.0

Does not match!




R2

R3

R1 R4


10.1/16


10/8 -> R310.1/16 -> R420/8 -> R530/8 -> R6…..



Longest match, 16 bit netmask


IP ForwardingIP Forwarding

• Router makes decision on which interface apacket is sent to

• Forwarding table populated by routingprocess

• Forwarding decisions:

destination address

class of service (fair queuing, precedence, others)

local requirements (packet filtering)

• Can be aided by special hardware


Explicit versus Default routingExplicit versus Default routing

• Default:simple, cheap (cycles, memory, bandwidth)

low granularity (metric games)

• Explicit (default free zone)high overhead, complex, high cost, high granularity

• Hybridminimise overhead

provide useful granularity

requires some filtering knowledge


Egress TrafficEgress Traffic

• How packets leave your network

• Egress traffic depends on:

route availability (what others send you)

route acceptance (what you accept fromothers)

policy and tuning (what you do withroutes from others)

Peering and transit agreements


Ingress TrafficIngress Traffic

• How packets get to your network andyour customers’ networks

• Ingress traffic depends on:

what information you send and to whom

based on your addressing and AS’s

based on others’ policy (what they acceptfrom you and what they do with it)


Autonomous System (AS)Autonomous System (AS)

• Collection of networks with same routing policy

• Single routing protocol

• Usually under single ownership, trust andadministrative control

AS 100AA


AS 100 AS 101

AS 102

DMZNetwork

AA

BB

CC

DD

EE

• Network shared between AS’s

Demarcation Zone (DMZ)Demarcation Zone (DMZ)


Definition of termsDefinition of termsDefinition of terms

• Neighbours - AS’s which directly exchange routinginformation

• Announce - send routing information to aneighbour

• Accept - receive and use routing information sentby a neighbour

• Originate - insert routing information into externalannouncements (usually as a result of the IGP)

• Peers - routers in neighbouring AS’s or within oneAS which exchange routing and policy information


Routing flow and packet flowRouting flow and packet flow

For networks in AS1 and AS2 to communicate:AS1 must announce to AS2

AS2 must accept from AS1

AS2 must announce to AS1

AS1 must accept from AS2

routing flowaccept

announce

announceacceptAS 1 AS 2

packet flow

packet flow


Routing flow and Traffic flowRouting flow and Traffic flow

• Traffic flow is always in the oppositedirection of the flow of routinginformation

filtering outgoing routing informationinhibits traffic flowing in

filtering incoming routing informationinhibits traffic flowing out


Routing policy limitationsRouting policy limitations

AS99 uses red link for traffic going to the red AS andgreen link for traffic going to the green AS

To implement this policy for AS99:

• accept routes originating in the red AS on the red link

• accept all other routes on the green link

red

green

AS99

packet flow

Internetred

green


Routing policy limitationsRouting policy limitations

For packets flowing toward AS 99:

Unless AS 22 and all other intermediate AS’s co-operate in pushing green traffic to the green link thensome reasonable policies can not be implemented.

packet flow

red

green

AS99Internetred

green

AS22


Routing policy with multipleRouting policy with multipleAS’sAS’s

For net N1 in AS1 to send traffic to net N16 in AS16:

• AS16 must originate and announce N16 to AS8.

• AS8 must accept N16 from AS16.

• AS8 must announce N16 to AS1 or AS34.

• AS1 must accept N16 from AS8 or AS34.

For two-way packet flow, similar policies must exist for N1.

AS 1

AS 8

AS 34

AS16

N16

N1


Routing policy with multipleRouting policy with multipleAS’sAS’s

As multiple paths between sites areimplemented it is easy to see howpolicies can become quitecomplex.

AS 1

AS 8

AS 34

AS16

N16

N1


Granularity of routing policyGranularity of routing policy

• What to announce/accept

• Preferences between multiple accepts

single route

routes originated by single AS

routes originated by a group of AS’s

routes traversing specific path

routes traversing specific AS

routes belonging to other groupings(including combinations)


Routing Policy IssuesRouting Policy Issues

• 77000+ prefixes (not realistic to setpolicy on all of them individually)

• 7000+ origin AS’s (too many)

• routes tied to a specific AS or path maybe unstable regardless of connectivity

• groups of AS’s are a natural abstractionfor filtering purposes


Routing Policy IssuesRouting Policy Issues

• Destination based limitations

• Global topology not known (andconstantly changing)

• Route groupings are not known

AS membership or AS groups

• Set of all routes in the Internet is notknown


IP AddressingIP AddressingIP Addressing

• Internet is classless

• Concept of Class A, class B or classC is no more

engineers talk in terms of prefix length,for example the class B 158.43 is nowcalled 158.43/16.

• All routers must be CIDR capable

Classless InterDomain Routing


IP AddressingIP Addressing

• IP Address space is a resource sharedamongst all Internet users

Regional Internet Registries delegatedallocation responsibility by the IANA

RIRs allocate address space to ISPs andLocal Internet Registries

ISPs/LIRs assign address space to endcustomers or other ISPs

• 61% of available address space allocated


Geographical and Provideraddressing

Geographical and ProviderGeographical and Provideraddressingaddressing

• Geographical addressingARIN - APNIC - RIPE NCC (the 3 IRs)

APNIC serves the Asia Pacific region

• Provider-based addressing

Addresses assigned by upstream provider

Local Internet Registries


Geographical addressingGeographical addressingGeographical addressing

• Advantages:

Not tied to local backbone provider

Part of the regional registry process

• Disadvantages:

Increases size of global routing table

More difficult to get started


Provider based addressingProvider based addressingProvider based addressing

• Advantages:

Easy to get started

No increase in size of global routing table

• Disadvantages:

Must renumber when changing providers

May fragment provider address block whenmultihoming

No part in the regional registry process


What Is an IGP?What Is an IGP?

• Interior Gateway Protocol

• Within an Autonomous System

• Carries information aboutinternal infrastructure prefixes

• Examples - OSPF, ISIS, EIGRP…


Why Do We Need an IGP?Why Do We Need an IGP?Why Do We Need an IGP?

• ISP backbone scaling

Hierarchy

Modular infrastructure construction

Limiting scope of failure

Healing of infrastructure faults usingdynamic routing with fast convergence


What Is an EGP?What Is an EGP?

• Exterior Gateway Protocol

• Used to convey routing informationbetween Autonomous Systems

• De-coupled from the IGP

• Current EGP is BGP


Why Do We Need an EGP?Why Do We Need an EGP?

• Scaling to large network

Hierarchy

Limit scope of failure

• Policy

Control reachability to prefixes

Merge separate organizations

Connect multiple IGPs


Interior versus ExteriorRouting Protocols

Interior versus ExteriorInterior versus ExteriorRouting ProtocolsRouting Protocols

• Interior

automatic neighbourdiscovery

generally trust yourIGP routers

prefixes go to allIGP routers

binds routers in oneAS together

• Exterior

specificallyconfigured peers

connecting withoutside networks

set administrativeboundaries

binds AS’s together


Interior versus ExteriorRouting Protocols

Interior versus ExteriorInterior versus ExteriorRouting ProtocolsRouting Protocols

• Interior

Carries ISPinfrastructureaddresses only

ISPs aim to keepthe IGP small forefficiency andscalability

• Exterior

Carries customerprefixes

Carries Internetprefixes

EGPs areindependent of ISPnetwork topology


Hierarchy of RoutingHierarchy of RoutingProtocolsProtocols

BGP4and OSPF/ISIS

FDDI

Other ISPs

CustomersLocalNAP

BGP4 Static/BGP4

BGP4


Default Administrative Distances

Connected Interface Connected Interface 00Static RouteStatic Route 11Enhanced IGRP Summary RouteEnhanced IGRP Summary Route 55External BGPExternal BGP 2020Internal Enhanced IGRPInternal Enhanced IGRP 9090IGRPIGRP 100100OSPFOSPF 110110IS-IS IS-IS 115115RIP RIP 120120EGPEGP 140140External Enhanced IGRPExternal Enhanced IGRP 170170Internal BGPInternal BGP 200200UnknownUnknown 255255

Route SourceRoute Source Default DistanceDefault Distance

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