The need for BGP Keeping local stuff local

Overview zTypical small ISP zDirect connections with other ISP’s zRouting protocol requirements zScaling things up

Typical Small ISP zLocal network zMay have multiple POPs zLine to Internet yInternational line providing transit connectivity yVery, very expensive

Other ISP in Country zSimilar setup zTraffic between you and them goes over yYour expensive line yTheir expensive line zTraffic can be significant ySame language/culture yTraffic between your and their customers

Bringing down costs zLocal (national) links much cheaper than international ones zMight be interesting to get direct link between you and them ySaving traffic on expensive lines yNo need to send traffic to other ISP down the street via New York!

Terminology: peer and transit zPeer: getting connectivity to network of other ISP y… and just that network, no other networks yFrequently at zero cost (zero-settlement) zTransit: getting connectivity though network of other ISP to other networks y … getting connectivity to rest of world (or part thereof) yUsually at cost (client-provider relationship)

Making it work zJust getting direct line is not enough zNeed to work out how to do routing yNeed to get local traffic between ISP’s yNeed to make sure the other ISP doesn’t use us for transit yNeed to control what networks to announce, what network announcements to accept

Not using static routes  ip route their_network their_gateway zDoes not scale

Not using IGP (OSPF) zSerious operational consequences: yIf the other ISP has a routing problem, you will have problems too yVery hard to filter routes so that we don’t inadvertently give transit

Using BGP instead zBGP = Border Gateway protocol zBGP is an EGP routing protocol zFocus on routing policy, not topology zBGP can make ‘groups’ of networks (Autonomous Systems) zGood route filtering capabilities zAbility to isolate from other’s problems

Autonomous Systems zAutonomous systems is a misnomer yNothing to do with freedom, independence, … zJust a handle for a group of networks that should be routed in a similar way zIdentified by an AS number

Autonomous System numbers z16-bit number, zAssigned by registry, just like IP numbers zAS 0 and AS are reserved zTop 1024 AS numbers (AS AS65534) are private numbers ysee RFC1930 for details

Using AS numbers zBGP can filter on AS numbers yGet all networks of the other ISP using one handle yInclude future new networks without having to change routing filters xAS number for new network will be same yCan use AS numbers in filters with regular expressions

Terminology: AUP zAcceptable Use Policy zList of rules defining what types of traffic are allowed on a network yTypical example: no commercial traffic allowed on government-sponsored educational network

Terminology: DMZ zDeMilitarized Zone zAUP-free zone zNetwork without AUP restrictions

Scaling inter-provider direct connections z2 providers need 1 direct serial line z3 providers need 3 direct serial lines z4 providers need 6 direct serial lines z5 providers need …? zDirect lines do not scale

Building an exchange point zExchange point is nothing more than a DMZ that connects ISP’s so they can exchange traffic zTopology of an ethernet ‘bus’ zUsually implemented as a hub/switch in a neutral location, with each provider installing a serial line & router to that location zMany countries have (at least) one

Exchange point rules zPeople are free to decide whether or not they want to peer zSetting static routes is explicitely disallowed zTransit traffic usually not allowed