The need for BGP

1. The need for BGP AfNOG Workshops Philip Smith

2. “Keeping Local Traffic Local”

3. IGP • Interior Gateway Protocol • Within a network/autonomous system • Carries information about internal infrastructure prefixes • Examples – OSPF, ISIS, EIGRP

4. Why do we need an IGP? • ISP Backbone Scaling • Hierarchy • Modular infrastructure • Limiting scope of failure • Healing of faults with fast convergence

5. EGP • Exterior Gateway Protocol • Used to convey routing information between networks/ASes • De-coupled from the IGP • Current EGP is BGP4

6. Why Do We Need an EGP? • Scaling to large network • Hierarchy • Limit scope of failure • Define administrative boundary • Policy • Control reachability of prefixes

7. Interior Automatic neighbour discovery Generally trust your IGP routers Routes go to all IGP routers Binds routers in an AS together Exterior Specifically configured peers Connecting with outside networks Set administrative boundaries Binds ASes together Interior vs. Exterior Routing Protocols

8. Interior Carries ISP infrastructure addresses only ISPs aim to keep IGPs small for scalability and efficiency Exterior Carries customer prefixes Carries Internet prefixes EGPs are independent of ISP network topology Interior vs. Exterior Routing Protocols

9. Overview • Typical small ISP • Direct connections with other ISP’s • Routing protocol requirements • Scaling things up

10. Typical Small ISP • Local network • May have multiple POPs • Line to Internet • International line providing transit connectivity • Very, very expensive

11. Typical Small ISP Upstream ISP Static routes Static and/or default routes Small ISP

12. 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

13. Other ISP in Country Europe Upstream ISP Small ISP Small ISP Africa

14. 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 • better performance, cheaper • No need to send traffic to other ISP down the street via New York!

15. Keeping Local Traffic Local Europe Upstream ISP Small ISP Small ISP Africa

16. 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 through network of other ISP to other networks • … getting connectivity to rest of world (or part thereof) • Usually at cost (customer-provider relationship)

17. Large ISP in Country • Large ISP multi-homes to two or more upstream providers • multiple connections • to achieve: • redundancy • connection diversity • increased speeds

18. Large ISP in Country Upstream ISP Upstream ISP USA Europe Large ISP Africa

19. 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/peers • Need to make sure the peer ISP doesn’t use us for transit • Need to control what networks to announce, what network announcements to accept to upstreams and peers

20. Not using static routes • ip route <their_network> <their_gateway> • Does not scale • The more external networks you connect to, the more complex and harder to maintain this becomes • No failover in case of link failure • Have to manually reconfigure • Often needs assistance of upstream too

21. Not using IGP (OSPF) • Using an IGP gives serious operational consequences: • If the other ISP has a routing problem, you will have problems too • Instability in their backbone becomes an instability in your backbone • Leaked prefixes into their backbone become leakages into your backbone • Won’t scale… • Etc… • Very hard to filter routes so that we don’t inadvertently give transit

22. Using BGP instead • BGP = Border Gateway Protocol • BGP is an Exterior Gateway 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

23. Autonomous Systems • Autonomous systems is a misnomer • Nothing to do with freedom, independence, … • Just a handle for a group of networks that is under the same administrative control • Uniquely identified by an AS number • 16 bit integer • Assigned by registries, just like IP addresses

24. Autonomous System(AS)... • Identified by ‘AS number’ • example: AS16907 (ISPKenya) • Public & Private AS numbers • Public (assigned by registries): 1-26623 • Private: 64512 – 65534 • AS 0 and 65535 are reserved • Typical applications: • Service provider • Multi-homed customers • Anyone needing policy discrimination

25. 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 • BGP actually does routing computation on IP numbers

26. Summary: Why do I need BGP? • Network scaling • Multi-homing – connecting to multiple providers • upstream providers • local networks – regional peering to get local traffic • Policy discrimination • controlling how traffic flows