16-dynamic-routing-BGP-v1

Packet should be sent the attributes can be compared

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Unformatted text preview: in the routing table ITM440/540 Carol Davids © 2008 22 The Border Gateway Protocol (BGP) • BGP is specified in RFC 1267, 1268 and 1467, published in 1991, 1991, and 1993 respectively. • BGP is an Exterior Gateway Protocol that enables routers on different Autonomous Systems to communicate with each other. ITM440/540 Carol Davids © 2008 23 BGP Protocol • BGP protocol messages are used to create sessions between routers • There are two types of sessions possible – Internal BGP and External BGP • Internal BGP sessions are established between routers in the same AS • External BGP sessions are established between speaker nodes on different AS’s. ITM440/540 Carol Davids © 2008 24 BGP Messages • BGP messages include the full path to a destination AS. • This full path is used to construct a graph showing how the AS’es are connected. ITM440/540 Carol Davids © 2008 25 BGP transport protocol • BGP uses TCP for its transport protocol. • TCP is the connection-oriented transport of the IP model. • The TCP port used by BGP is 179. ITM440/540 Carol Davids © 2008 26 BGP transport protocol • Two systems running BGP establish a TCP connection and then exchange the entire BGP routing table. • After this initial exchange, incremental updates are sent as the routing table changes. ITM440/540 Carol Davids © 2008 27 Routing IP datagrams using BGP • An IP datagram in an Autonomous System is categorized as carrying either local traffic or transit traffic. ITM440/540 Carol Davids © 2008 28 Routing IP datagrams using BGP • Local traffic either originates on the AS or terminates on the AS. • That is, either its source of its destination IP Address is a host on the AS. • Transit traffic neither originates nor terminates on the AS. • This means that transit traffic is “just passing through”. ITM440/540 Carol Davids © 2008 29 Categories of AS • Stub AS • Has only a single connection to one other AS. • Carries only local traffic ITM440/540 Carol Davids © 2008 30 Categories of AS • Multihomed AS • Has connections to more than one other AS. • Refuses to carry transit traffic. • Transit AS • Has connections to more than one other AS. • Accepts to carry both local and transit traffic. ITM440/540 Carol Davids © 2008 31 R2 N4 N3 N1 AS1 N8 AS2 N5 N2 AS2 has connections to more than one AS. ** The administrator can choose to make it a multihomed network. ** If AS2 is provisioned to be a multihomed AS it will refuse to carry transit traffic. ** Then it could not be in the path that routes datagrams between AS1 and AS4. R1 R3 N6 N8 N7 R4 AS4 N9 N10 N9 AS5 R5 N11 ITM440/540 AS3 Carol Davids © 2008 AS5 is a stub network because it is connected to only one other AS. 32 BGP required by Transit AS • A Transit AS must run a Border Gateway Protocol. ITM440/540 Carol Davids © 2008 33 BGP detection of link or host failure • BGP sends a regular keep-alive message at the application layer. • Normally the keep-alive is sent every 30 seconds. • If the f...
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