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More on Internet Routing

More on Internet Routing. A large portion of this lecture material comes from BGP tutorial given by Philip Smith from Cisco (ftp://ftp-eng.cisco.com/pfs/seminars/APRICOT2004-BGP00.pdf). More on Internet Routing. What does a router do? Find path (routing)

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More on Internet Routing

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  1. More on Internet Routing • A large portion of this lecture material comes from BGP tutorial given by Philip Smith from Cisco (ftp://ftp-eng.cisco.com/pfs/seminars/APRICOT2004-BGP00.pdf)

  2. More on Internet Routing • What does a router do? • Find path (routing) • Forward packet from one interface to another interface (forwarding) • Routing versus forwarding • Routing: building maps and giving directions • Forwarding: moving packet based on the directions.

  3. IP Routing: • Path derived from information received from a routing protocol. • Several alternative paths may exist • Best next hop stored in forwarding table. • Decisions are updated periodically or as topology changes (event driven) • Decisions are based on • Topology, policies and metrics (hop count, filtering, delay, bandwidth)

  4. IP route lookup (part of forwarding): • Based on destination address • “longest match” routing • More specific prefix preferred over less specific prefix. • Example: packet with destination of 10.1.1.1/32 is sent to the router announcing 10.1/16 rather than the router announcing 10/8

  5. Explicit versus default routing: • Default: • Simple, cheap • Low granularity • Explicit • High overhead, complex, high cost, high granularity • Hybrid • Minimize overhead • Provide useful granularity

  6. Autonomous System (AS) • Collective of networks with same routing policy • Single routing protocol • Usually under single ownership and administrative control • Identified by the autonomous system number (ASN), which is a 16-bit number.

  7. Border Gateway Protocol (BGP) • Described in RFC 1771 • Routing protocol used to exchange routing information between networks • The AS is BGP’s fundamental operating unit • Used to identify networks with common routing polocy. • BGP-4 is the de facto standard in the Internet

  8. BGP terms: • Neighbours: AS’s which directly exchange routing information • Announce: send route information to the neighbor • Accept: receive and use the information sent by a neighbour • Originate: insert routing information into external announcement • Peers: routers that can directly exchange routing and policy information

  9. Routing flow and packet flow:

  10. BGP basic: • Runs over TCP – port 179 • Path vector protocol (distance vector algorithm + explicit AS path) • Two types of updates • (Route) announcement and withdraw • BGP update attributes • ASPATH and some other information

  11. NLRI=128.186.0.0/16 ASPATH=[0] NLRI=128.186.0.0/16 ASPATH=[10] NLRI=128.186.0.0/16 ASPATH=[10] NLRI=128.186.0.0/16 ASPATH=[210] NLRI=128.186.0.0/16 ASPATH=[610] NLRI=128.186.0.0/16 ASPATH=[610] NLRI=128.186.0.0/16 ASPATH=[210] NLRI=128.186.0.0/16 ASPATH=[7610] NLRI=128.186.0.0/16 ASPATH=[4210] NLRI=128.186.0.0/16 ASPATH=[3210] NLRI=128.186.0.0/16 ASPATH=[53210] An example [3210]* [4210] [7610] 128.186.0.0/16

  12. BGP general operation • Learn multiple paths via internal and external BGP speakers • Picks the best path and installs in the forwarding table • Best path is sent to external BGP neighbours • Policies applies by influencing the best path selection

  13. Policy based routing protocol • BGP route selection: • Prefer routes with highest local preference • Prefer routers with shortest AS path • Prefer routers with lowest MED value • Prefer EBGP routes over IBGP routes • Prefer routes via nearest IGP neighbor • Tie breaking: lowest router ID

  14. Policy disputes • Global routing system never converge

  15. Security: control plane security • problems • Anyone can claim ownership of NLRI • Anyone can claim en route to NLRI

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