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Published bySolomon Jason Campbell Modified over 9 years ago
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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
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Distance-vector routing: A’s routing table just after link A-E failure DCNH B B C C D C E - F1F G F
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DestCostNH B B C C D C E - F1F G F Dest.CostNHop A A B A C A D G E2A G G A F Dest.CostNHop A A B A C A D G E - G G Dest.CostNHop A F B F or C C D D D E3F F F G Dest.CostNHop A F B F C D D D E3F F F Dest.CostNHop A F B F C D D D E - F F What happens when G advertises its table to D? How does D get the message that A-E has failed?
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Loops -A 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. Split horizon: B should not advertise a route [E, 2] it got from A (in the first step )
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Loops -B 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, , 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< Split horizon will not solve this problem
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Link State Routing ABCD A B C D You have a global view: routing table is spanning tree as seen from root node
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The Internet circa 1990: A hierarchical collection of autonomous systems (AS)
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Today’s multiple backbone Stub AS Multi-homed AS Transit AS
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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
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Forwarding Table of R1 Subnet NumberSubnet MaskNextHop 128.96.34.0255.255.255.128Interface0 128.96.34.128255.255.255.128Interface 1 128.96.33.0255.255.255.0R2
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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
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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
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Aggregation of 16 Class C network addresses into a single 20 bit CIDER address Class C network numbers 192.4.00010000.XYZ 192.4.00010001.XYZ 192.4.00010010.XYZCIDR 20 bit network address 192.4.00010011.XYZ192.4.0001 =194.4.16/20 192.4.00010100.XYZ 192.4.00010101.XYZ 192.4.00010110.XYZ 192.4.00010111.XYZ 192.4.00011000.XYZ 192.4.00011001.XYZ 192.4.00011010.XYZ 192.4.00011011.XYZ 192.4.00011100.XYZ 192.4.00011101.XYZ 192.4.00011110.XYZ 192.4.00011111.XYZ
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Intradomain Routing Border Gateway Routers—Default router for outbound traffic
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Intradomain routing issues Scale—100,000 network addresses Autonomy Trust Flexibility—hot potato routing
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BGP example 1 BGP speaker/AS: Advertise routes-prevents looping Withdrawn broken routes
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Routing Areas in OSPF
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