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Figure 8.16. The IGP <strong>to</strong> ERP handoff.<br />

A host on Network A in AS 1 wants <strong>to</strong> deliver a packet <strong>to</strong> a host on<br />

Network 5 in AS 5. The destination address is outside <strong>the</strong> local<br />

<strong>network</strong>, so <strong>the</strong> datagram is forwarded <strong>to</strong> <strong>the</strong> gateway. The gateway<br />

has a route for <strong>the</strong> destination <strong>network</strong>, which is part of AS 5, and is<br />

reachable via AS 3, which is a multihomed transit AS. The datagram is<br />

<strong>the</strong>n forwarded across AS 3's transit path. Once it is forwarded <strong>to</strong> AS<br />

5's gateway, AS 5's IGP process determines <strong>the</strong> rest of <strong>the</strong> path. If AS<br />

5 were also available via AS 60, a policy would have been required on<br />

AS 1's and AS 5's routers <strong>to</strong> set <strong>the</strong> path preference.<br />

BGP Version 4<br />

The Border Gateway Pro<strong>to</strong>col (BGP-4) is <strong>the</strong> pro<strong>to</strong>col that is most<br />

commonly used <strong>to</strong> exchange routing information between ASs.<br />

According <strong>to</strong> RFC 1267:<br />

"The Border Gateway Pro<strong>to</strong>col is an inter-au<strong>to</strong>nomous system routing<br />

pro<strong>to</strong>col… The primary function of a BGP speaking system is <strong>to</strong><br />

exchange <strong>network</strong> reachability information with o<strong>the</strong>r BGP systems.<br />

This <strong>network</strong> reachability information includes information on <strong>the</strong> full<br />

path of ASs that traffic must transit <strong>to</strong> reach <strong>the</strong>se <strong>network</strong>s."

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