Computer Networks II

Transcription

1 Computer Networks II Inter-domain routing with BGP4 (3/4) Giorgio Ventre COMICS LAB Dipartimento di Informatica e Sistemistica Università di Napoli Federico II Nota di Copyright Quest insieme di trasparenze è stato ideato e realizzato dai ricercatori del Gruppo di icerca sull Informatica Distribuita del Dipartimento di Informatica e Sistemistica dell Università di Napoli e del Laboratorio Nazionale per la Informatica e la Telematica Multimediali. Esse possono essere impiegate liberamente per fini didattici esclusivamente senza fini di lucro, a meno di un esplicito consenso scritto degli Autori. Nell uso dovrà essere esplicitamente riportata la fonte e gli Autori. Gli Autori non sono responsabili per eventuali imprecisioni contenute in tali trasparenze né per eventuali problemi, danni o malfunzionamenti derivanti dal loro uso o applicazione. 1

2 Outline Organization of the global Internet BGP basics BGP in large networks The needs for Confederations and oute eflectors The dynamics of BGP Inter-domain traffic engineering with BGP How to scale in large domains? Confederations Divide the large domain in smaller sub-domains Use full mesh inside each sub-domain Use between sub-domains Confederation : Member-AS AS65002 Member-AS AS65001 Each router is configured with two AS numbers Its confederation AS number Its Member-AS AS number Usually, a single IGP covers the whole domain session session 2

3 Confederations : example UPDATE (via ) 1 AS AS65021 On the session between and, belongs to On the session between 5 and, 5 belongs to On the session between 1 and 6, 1 belongs to AS65020 and 6 belongs to AS65021 Confederations : example (2) UPDATE (via ) 1 AS65020 UPDATE (via ) ASPath: [AS65020]: 5 6 AS65021 When propagating an UPDATE via to another router of the same confederation, 1 inserts its Member-AS number in the AS_PATH 3

4 Confederations : example (3) When propagating an UPDATE via to a router outside its confederation, 5 removes the internal path from the AS_Path and inserts its Confederation AS number in the AS_PATH UPDATE (via ) ASPath: [AS65020]: 6 AS65021UPDATE (via ) 1 ASPath: : AS In practice, BGP confederations are particularly useful when two companies or two distinct ASes from the same company must be merged in a single AS oute reflectors An alternative to confederations oute reflectors (FC 2796) A route reflector is a special router that is allowed to propagate the routes learned over sessions on other sessions Normal full mesh with one route reflector 1 oute eflector 4

5 Behavior of a oute eflector Two types of peers of a route reflector... 1 N clients peers ( do not participate in full mesh) Z Non-clients peers (participate in full mesh) Behavior of a oute eflector oute received from an session or a client peer Select best path Advertise to clients peers... 1 N All client peers All non-client peers oute received from non-client peer Select best path Advertise to All client peers Non-clients peers Z 5

6 Fault tolerance of route reflectors How to avoid having the as a single point of failure? Solution Allow each client peer to be connected at 2 s clients peers... 1 N Issue 1 Configuration errors may cause redistribution loops OIGINATO_ID used to carry router ID of originator of route CLUSTE_LIST contains the list of that sent the UPDATE message inside the current AS oute reflectors : an example UPDATE (via ) 6 UPDATE (via ) Z 1 5 and are clients of oute eflector 1 1 and 6 are in full mesh 5 is client of oute eflector 6 6

7 oute reflectors : an example (2) UPDATE (via ) Nexthop: 6 1 Z UPDATE (via ) Nexthop:Z 5 1 will select its best path towards /8 and will re-advertise it by adding the OIGINATO_ID and the CLUSTEID oute reflectors : an example (3) Z UPDATE (via ) Nexthop: OIGINATO_ID: CLUSTE_ID:1 UPDATE (via ) Nexthop: OIGINATO_ID: CLUSTE_ID: prefers the path to /8 via - 1 advertises this path to its client peer () the path is not advertised to since already received it 1 advertises this path to its non-client peer (6) 7

8 oute reflectors : an example (4) 6 UPDATE (via ) Nexthop: OIGINATO_ID: CLUSTE_ID:1:6 1 Z 5 6 advertises the path to /8 via - to its client peer 5 5 will remove OIGINATO_ID and CLUSTE_ID before advertising the path to via Hierarchy of route reflectors In large domains, a hierarchy of route reflectors can be built 1 1 session 4 A B 5 4 C 6 5 1, and are clients of route reflectors 1 and 1 and are clients of route reflectors A and B 4 and 5 are clients of route reflector A 6 is client of route reflectors 4 and 5 A, B and C are in full mesh 8

9 Confederations versus oute reflectors Confederations Solves scaling edundancy with full-mesh inside each MemberAS Possible to run one IGP per Member AS equires manual router configuration Can be used when merging domains Can lead to some routing oscillations oute reflectors Solves scaling edundancy by using edundant s Usually a single IGP for the whole AS equires manual router configuration Can lead to some routing oscillations 9