TCP IP Illustrated

Policy decisions are not part of the protocol but

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Unformatted text preview: licy-based routing. Policies are determined by the AS administrator and specified to BGP in configuration files. Policy decisions are not part of the protocol, but policy specifications allow a BGP implementation to choose between paths when multiple alternatives exist and to control the redistribution of information. Routing policies are related to political, security, or economic considerations. BGP is different from RIP and OSPF in that BGP uses TCP as its transport protocol. Two systems running BGP establish a TCP connection between themselves and then exchange the entire BGP routing table. From that point on, incremental updates are sent as the routing table changes. BGP is a distance vector protocol, but unlike RIP (which announces hops to a destination), BGP enumerates the route to each destination (the sequence of AS numbers to the destination). This removes some of the problems associated with distance-vector protocols. An AS is identified by a 16-bit number. BGP detects the failure of either the link or the host on the other end of the TCP connection by sending a keepalive message to its neighbor on a regular basis. The recommended time between these messages is 30 seconds. This application-level keepalive message is independent of the TCP keepalive option (Chapter 23). 10.8 CIDR: Classless Interdomain Routing file:///D|/Documents%20and%20Settings/bigini/Docu.../homenet2run/tcpip/tcp-ip-illustrated/dynamic.htm (14 of 17) [12/09/2001 14.46.53] Chapter 10. Dynamic Routing Protocols In Chapter 3 we said there is a shortage of class B addresses, requiring sites with multiple networks to now obtain multiple class C network IDs, instead of a single class B network ID. Although the allocation of these class C addresses solves one problem (running out of class B addresses) it introduces another problem: every class C network requires a routing table entry. Classless Interdomain Routing (CIDR) is a way to prevent this explosion in the size of the Internet routing tables. I...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.

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