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Lecture 18

Lecture 18. Internet Routing: The goal is to find any route that is loop free-Global optimization is a distant dream depending on economic and political drivers Homework: 4.1-35, 37-45. Distance-vector routing: A’s routing table just after link A-E failure. A.

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Lecture 18

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  1. Lecture 18 Internet Routing: The goal is to find any route that is loop free-Global optimization is a distant dream depending on economic and political drivers Homework: 4.1-35, 37-45

  2. Distance-vector routing: A’s routing table just after link A-E failure

  3. A What happens when G advertises its table to D? How does D get the message that A-E has failed? F G

  4. A advertises [E, ¥] to B and B advertises [E, 2] to A (they cross intransit) A advertises [E,3] and B advertises [E, ¥] etc. B resets [E, ¥, -] (since A is next hop) A updates [E, 3, B] since 3<¥ B updates [E, 4,A] and A resets [E, ¥, -] since B is next hop etc. Loops -A Split horizon: B should not advertise a route [E, 2] it got from A (in the first step)

  5. A advertises [E, ¥] to B & C Advertisement to C is delayed B may advertise [E, ¥] to C C advertises [E, 2] to B A’s advertisement [E, ¥] arrives at C B advertises [E,3] to A C advertises [E, ¥] to B A advertises [E,4] to C B advertises [E, ¥] to A C advertises [E,5] to B B resets [E, ¥, -] since A is next hop C will not update (why) B updates [E, 3, C] since 3<¥ C updates [E, ¥, -] since A is next hop A updates [E,4, B] since 4<¥ B updates [E, ¥, -] since C is next hop C updates [E, 5, A] since 5<¥ A updates [E, ¥, -] since B is next hop B updates [E, 6, A] since 6<¥ Loops -B Split horizon will not solve this problem

  6. Link State Routing You have a global view: routing table is spanning tree as seen from root node

  7. The Internet circa 1990: A hierarchical collection of autonomous systems (AS)

  8. Today’s multiple backbone Stub AS Multi-homed AS Transit AS

  9. Inefficiency of IP address classes • If you have 257 end users, you need class B and then you have 16K addresses. We need finer distinctions. • Two issues: • How do you give different network addresses to physical networks within 1 class A, B or C network---subnetting • How do you aggregate networks within an domain to simplifier routing outside the domain--supernetting

  10. Forwarding Table of R1

  11. More Subnetting • You can break the same physical network into subnets—forcing hosts to speak through a router • Each host has it’s own subnet in the new engineering network • Subnet mask=IP address---non-contiguous 1’s and 0’s

  12. Classless Interdomain Routing (CIDR) • We can get better utilization if we hand out Class C addresses • This would increase the size of forwarding tables • We aggregate contiguous class C blocks

  13. Aggregation of 16 Class C network addresses into a single 20 bit CIDER address

  14. Intradomain Routing Border Gateway Routers—Default router for outbound traffic

  15. Intradomain routing issues • Scale—100,000 network addresses • Autonomy • Trust • Flexibility—hot potato routing

  16. BGP example 1 BGP speaker/AS: Advertise routes-prevents looping Withdrawn broken routes

  17. Routing Areas in OSPF

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