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Introduction to BGP

Introduction to BGP. Lixin Gao ECE Dept. UMASS, Amherst. Border Gateway Protocol (BGP). Dynamic Inter-AS Routing Protocol Goals: Exchange reachability information between ASes Apply local policies for selecting route receiving and announcing route. BGP Sessions. eBGP.

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Introduction to BGP

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  1. Introduction to BGP Lixin Gao ECE Dept. UMASS, Amherst ECE697F: Special Topics-Internet Routing

  2. Border Gateway Protocol (BGP) • Dynamic Inter-AS Routing Protocol • Goals: • Exchange reachability information between ASes • Apply local policies for • selecting route • receiving and announcing route ECE697F: Special Topics-Internet Routing

  3. BGP Sessions eBGP • Used for Exchange Reachability Information • iBGP : fully meshed, might not directly connected, • Alternative to fully meshed: confederation, route reflector • eBGP iBGP AS701 MCI AS7018 AS1 AT&T BBN ECE697F: Special Topics-Internet Routing

  4. Route Announcement • Local Policies Route Information Exchange 1.2.0.0/16 (1) 1.2.0.0/16 (7018,1) eBGP iBGP AS701 1.2.0.0/16 (1) 1.2.0.0/16 (1) 1.2.0.0/16 (7018,1) 1.2.0.0/16 (7018,1) MCI 1.2.0.0/16 ( ) AS7018 AS1 1.2.0.0/16 (1) 1.2.0.0/16 (1) 1.2.0.0/16 AT&T BBN ECE697F: Special Topics-Internet Routing

  5. Route Announcement • Network Prefix: 12.0.0.0/8 • Attributes: for reaching network prefix • AS path • Next hop : next hop IP addr • Local Preference • Multiple Exit Discriminator (MED) • Community Set • Origin code • Atomic Aggregate • Aggregator • ….. ECE697F: Special Topics-Internet Routing

  6. AS Path • Sequence of ASes traversed • Loop avoidance • Path selection: Path Length • Policy: AS7018 in AS path is more preferred ECE697F: Special Topics-Internet Routing

  7. Next Hop • Next hop IP address to reach network prefix • EBGP: next hop IP addr of announcer • iBGP • From same AS:IP address of announcer • From diff AS: next hop unchanged AS7018 AS1 2.3.4.1 3.4.5.2 1.2.0.0/16 (1) 2.3.4.1 1.2.0.0/16 (1) 2.3.4.1 1.2.0.0/16 1.2.0.0/16 ( ) 3.4.5.2 ECE697F: Special Topics-Internet Routing

  8. Next hop • Determine next hop in forwarding table • Combing Inter-AS and Intra-AS routing table • Recursive route lookup • BGP abstract topology in AS level ECE697F: Special Topics-Internet Routing

  9. Local Preference eBGP • Local to AS • carry in iBGP session but not eBGP session • Used to influence path selection • Select the highest Local Pref path iBGP AS701 1.2.0.0/16 (1) 100 1.2.0.0/16 (1) 100 1.2.0.0/16 (7018,1) 200 1.2.0.0/16 (7018,1) 200 MCI 1.2.0.0/16 ( ) 300 AS7018 AS1 1.2.0.0/16 (1) 90 1.2.0.0/16 (1) 90 1.2.0.0/16 AT&T BBN ECE697F: Special Topics-Internet Routing

  10. Multiple Exit Discriminator (MED) • Used for path selection • select an entry point among several to an AS (smaller MED is preferred) • meaningful among paths from the same AS • may use Intra-AS routing cost metrics • Non-transitive AS701 1.2.0.0/16 (1) 200 1.2.0.0/16 (1) 100 AS1 1.2.0.0/16 ECE697F: Special Topics-Internet Routing

  11. Community Set • Used for grouping destination • Simplify policy setting AS701 1.2.0.0/16 (1) 1000 1.2.0.0/16 (1) 1000 3.0.0.0/18 (7018) 1000 3.0.0.0/16 (7018) 1000 MCI 3.0.0.0/8 AS7018 AS1 1.2.0.0/16 AT&T BBN ECE697F: Special Topics-Internet Routing

  12. Origin Code • IGP • interior to the originating AS. • EGP • learned via EGP (Exterior Gateway Protocol) • Incomplete • unknown ECE697F: Special Topics-Internet Routing

  13. Atomic Aggregate • Aggregation that loses information ECE697F: Special Topics-Internet Routing

  14. Aggregator • Optional • For route that is an aggregation ECE697F: Special Topics-Internet Routing

  15. Announcement Processing • Input Policy Engine • Receive from neigboring Ases new announcements • Attribute Manipulation and Filter announcement • Store announcements (Incremental protocol) • Path Selection among announcements • Output Policy Engine • Attribute Manipulation • Send to neighboring Ases new announcements ECE697F: Special Topics-Internet Routing

  16. Input Policy Engine • Set local preference • Filter routes with the same AS number (loop avoidance) • Filter bogus routes ECE697F: Special Topics-Internet Routing

  17. Path Selection Process • For each prefix, • Prefer largest Local Preference • Prefer route this router originates • Prefer shorter AS path • prefer the lowest origin code (IGP<EGP<INCOMPLETE). • Prefer path with lowest MED • Prefer route with lowest IP addr ECE697F: Special Topics-Internet Routing

  18. Output Policy Engine • Send to neighbors only the best path • Send to neighbors only the best path changes • Set MED • Set Community set • Set Atomic Aggregate • Set next hop • Append AS number ECE697F: Special Topics-Internet Routing

  19. Routing Information Storage • Adj-RIBs-In • routes received • Input to route selection • Loc-RIB • Routes selected by selection process • Output of route selection • Adj-RIBs-Out • announcements to neighbor • Output of Output Policy Engine ECE697F: Special Topics-Internet Routing

  20. BGP Session Implementation • OPEN message • establish TCP session • KEEPALIVE message • keep session going • Update message • announcement or withdrawal • Notification message • error in announcement ECE697F: Special Topics-Internet Routing

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