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IEG 3090 Tutorial 3 Border Gateway Protocol

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IEG 3090 Tutorial 3 Border Gateway Protocol

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    1. IEG 3090 Tutorial 3 Border Gateway Protocol Fong Chi Hang, Bosco 1

    2. Outline Introduction to BGP BGP Attributes in Update Message Routing Filtering eBGP vs iBGP More in iBGP 2

    3. Introduction to BGP Policy-based Inter-domain routing Protocol All the networks on the same AS would share the same set of routing policy 3

    4. Goal of BGP Find loop-free paths that Support routing policy established as part of peering relationship Support traffic engineering to minimize (monetary) cost Optimizing performance is only another goal (not the only goal as in Intra-domain routing) 4

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    7. Four Types of messages Update – Exchange route information Network prefix Announcements or withdrawals Route-associated BGP attributes. Open - establish the BGP sessions. Notification Indicate an error during the BGP session. The TCP connection will be closed immediately afterwards. Keepalive – To confirm the connection is still active 7

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    9. How routes are advertised? 9

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    12. BGP Attributes Each prefix advertised together with its associated attributes BGP router may change the attributes before re-advertising the prefix to other peers If there are more than 1 route to the same prefix, the attributes are used find out which route is used. 12

    13. BGP Attributes Attributes commonly used for comparing different routes LOCAL_PREF Used on multiple routes learnt from different AS AS-PATH Stores the sequence of AS that the route has gone through Used to prevent routing loop Multi-Exit-Discriminator (MED) Used on multiple links between a single pair of AS 13

    14. BGP Attributes Other important attributes COMMUNITY An id used to tell neighbor AS how to set local pref NEXT-HOP the IP address of the router that advertised the route. ORIGIN how the route was learned (IGP, EGP, Incomplete) 14

    15. How BGP attributes affect ISP Policies Loop Prevention ? Use AS_PATH Use LOCAL_PREF to differentiate different relationships “Hot Potato” routing V.S. “Cold Potato” routing “Cold Potato” routing based on MED attribute Multi-homing: backup routes, and load balancing Use AS_PATH pre-pending method. Use COMMUNITIES to alter provider’s local preference. 15

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    17. Local-Pref 17

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    20. AS-PATH 20

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    27. Multi-Exit Discriminator 27

    28. Route Filtering AS_PATH is not the primary basis for BGP routing. Primary factor is the peering agreements between ISPs. Peering agreements define which neighbor(s) will provide transit for what traffic (from what source, and to what destination) 28

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    40. eBGP and iBGP Two BGP routers from different AS – EBGP Peers. Two BGP routers from the same AS – IBGP Peers. Both EBGP and IBGP follow the same BGP protocol 40

    41. eBGP and iBGP Essential difference between EBGP and IBGP: AS Path information in EBGP. IBGP “session” is fully meshed. EBGP peers must be directly connected. IBGP peers can be hops away within the AS (given that IGP has built up the connectivity) 41

    42. More in IBGP Loop-back address BGP extensions to make iBGP scalable 42

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    45. IBGP – Scalability One of the requirements of IBGP is to maintain a fully meshed graph. Why ? Prefixes learned from an EBGP neighbor can be advertised to an i-BGP neighbor, vice versa. However, prefixes learned from an IBGP neighbor cannot be advertised to another IBGP neighbor. Results: IBGP is not scalable. Solutions: Route reflector Confederation 45

    46. IBGP Reflector Introduce hierarchy to iBGP Route reflector Configured to have a number of clients Maintains full mesh with other route reflectors configured to re-advertise routes to its clients Route reflector client behaves as regular iBGP Only maintain a session with its route reflector Cluster Each route reflector and its clients form a cluster Has a cluster ID (set to route reflector’s router ID) 46

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    49. ~ The End ~ Thank you very much ! 49

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