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The need for BGP

The need for BGP. Keeping local traffic local. IGP. Interior Gateway Protocol Within a network/autonomous system Carries information about internal prefixes Examples - OSPF, ISIS, EIGRP. EGP. Exterior Gateway Protocol Used to convey routing information between networks/ASes

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The need for BGP

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  1. The need for BGP Keeping local traffic local

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

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

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

  5. Interior Automatic discovery Generally trust your IGP routers Routes go to all IGP routers Exterior Specifically configured peers Connecting with outside networks Set administrative boundaries Interior vs. Exterior Routing Protocols

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

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

  8. Typical Small ISP Upstream ISP Static, default routes or IGP Static, default routes or IGP Small ISP

  9. 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

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

  11. 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!

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

  13. 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 (customer-provider relationship)

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

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

  16. 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

  17. Not using static routes • ip route their_network their_gateway • Does not scale

  18. 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

  19. 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

  20. 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 • Identified by an AS number

  21. Autonomous System(AS)... • Identified by ‘AS number’ • example: AS16907 (ISPKenya) • Public & Private AS numbers • Public (assigned by registries): 1-22527 • Private: 64512 - 65534 • Examples: • Service provider • Multi-homed customers • Anyone needing policy discrimination

  22. 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

  23. 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

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

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