lec-31 - What's Next Network Layer Network layer service...

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4 Nov 2008  BITS C481 (Computer Networks) 1 What’s Next… Network Layer Network layer service models (Done!!!) Forwarding versus routing (Done!!!) How a router works (Done!!!) Internet protocol (Done!!!) Routing (path selection) Algorithms Dijkstra’s algorithm Distance Vector Hierarchical Routing Routing Protocols RIP OSPF BGP (Today!!!) Multicast Routing (Today!!!)
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4 Nov 2008  BITS C481 (Computer Networks) 2 Internet inter-AS routing: BGP BGP (Border Gateway Protocol): the de facto standard BGP provides each AS a means to: 1. Obtain subnet reachability information from neighboring ASs. 2. Propagate reachability information to all AS-internal routers. 3. Determine “good” routes to subnets based on reachability information and policy. Allows subnet to advertise its existence to rest of Internet: “I am here”
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4 Nov 2008  BITS C481 (Computer Networks) 3 BGP basics Pairs of routers (BGP peers) exchange routing info over semi- permanent TCP connections: BGP sessions BGP sessions need not correspond to physical links. When AS2 advertises a prefix to AS1, AS2 is promising it will forward any datagrams destined to that prefix towards the prefix. AS2 can aggregate prefixes in its advertisement 3b 1d 3a 1c 2a AS3 AS1 AS2 1a 2c 2b 1b 3c eBGP session iBGP session
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4 Nov 2008  BITS C481 (Computer Networks) 4 Distributing reachability info With eBGP session between 3a and 1c, AS3 sends prefix reachability info to AS1. 1c can then use iBGP do distribute this new prefix reach info to all routers in AS1 1b can then re-advertise new reachability info to AS2 over 1b-to-2a eBGP session When router learns of new prefix, creates entry for prefix in its forwarding table. 3b 1d 3a 1c 2a AS3 AS1 AS2 1a 2c 2b 1b 3c eBGP session iBGP session
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4 Nov 2008  BITS C481 (Computer Networks) 5 When advertising a prefix, advertisement includes BGP attributes. prefix + attributes = “route”
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This note was uploaded on 07/20/2009 for the course CSIS BITS C481 taught by Professor Prof.virendersinghshekhawat during the Fall '08 term at Birla Institute of Technology & Science, Pilani - Hyderabad.

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lec-31 - What's Next Network Layer Network layer service...

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