The need for BGP

Keeping local stuff local

Overview

Typical small ISP

Direct connections with other ISP’s

Routing protocol requirements

Scaling things up

Typical Small ISP

Local network

May have multiple POPs

Line to Internet

International line providing transit connectivity

Very, very expensive

Other ISP in Country

Similar setup

Traffic between you and them goes over

Your expensive line

Their expensive line

Traffic can be significant

Same language/culture

Traffic between your and their customers

Bringing down costs

Local (national) links much cheaper than international ones

Might be interesting to get direct link between you and them

Saving traffic on expensive lines

No need to send traffic to other ISP down the street via New York!

Terminology: peer and transit

Peer: getting connectivity to network of other ISP

… and just that network, no other networks

Frequently at zero cost (zero-settlement)

Transit: getting connectivity though network of other ISP to other networks

… getting connectivity to rest of world (or part thereof)

Usually at cost (client-provider relationship)

Making it work

Just getting direct line is not enough

Need to work out how to do routing

Need to get local traffic between ISP’s

Need to make sure the other ISP doesn’t use us for transit

Need to control what networks to announce, what network announcements to accept

Not using static routes

ip route their_network their_gateway

Does not scale

Not using IGP (OSPF)

Serious operational consequences:

If the other ISP has a routing problem, you will have problems too

Very hard to filter routes so that we don’t inadvertently give transit

Using BGP instead

BGP = Border Gateway protocol

BGP is an EGP routing protocol

Focus on routing policy, not topology

BGP can make ‘groups’ of networks (Autonomous Systems)

Good route filtering capabilities

Ability to isolate from other’s problems

Autonomous Systems

Autonomous systems is a misnomer

Nothing to do with freedom, independence, …

Just a handle for a group of networks that should be routed in a similar way

Identified by an AS number

Autonomous System numbers

16-bit number, 1-65534

Assigned by registry, just like IP numbers

AS 0 and AS 65535 are reserved

Top 1024 AS numbers (AS64512-AS65534) are private numbers

see RFC1930 for details

Using AS numbers

BGP can filter on AS numbers

Get all networks of the other ISP using one handle

Include future new networks without having to change routing filters

AS number for new network will be same

Can use AS numbers in filters with regular expressions

Terminology: AUP

Acceptable Use Policy

List of rules defining what types of traffic are allowed on a network

Typical example: no commercial traffic allowed on government-sponsored educational network

Terminology: DMZ

DeMilitarized Zone

AUP-free zone

Network without AUP restrictions

Scaling inter-provider direct connections

2 providers need 1 direct serial line

3 providers need 3 direct serial lines

4 providers need 6 direct serial lines

5 providers need …?

Direct lines do not scale

Building an exchange point

Exchange point is nothing more than a DMZ that connects ISP’s so they can exchange traffic

Topology of an ethernet ‘bus’

Usually implemented as a hub/switch in a neutral location, with each provider installing a serial line & router to that location

Many countries have (at least) one

Exchange point rules

People are free to decide whether or not they want to peer

Setting static routes is explicitely disallowed

Transit traffic usually not allowed

The need for BGP

ymbk

2000-05-02T09:40:05