TCP IP Illustrated

The subnet mask for each entry applies to the

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Unformatted text preview: main. The route tag exists to support exterior gateway protocols. It carries an autonomous system number for EGP and BGP. The subnet mask for each entry applies to the corresponding IP address. The next-hop IP address is where packets to the corresponding destination IP address should be sent. A value of 0 in this field means packets to the destination should be sent to the system sending the RIP message. Figure 10.10 Format of a RIP-2 message. A simple authentication scheme is provided with RIP-2. The first 20-byte entry in a RIP message can specify an address family of 0xffff, with a route tag value of 2. The remaining 16 bytes of the entry contain a cleartext password. Finally, RIP-2 supports multicasting in addition to broadcasting (Chapter 12). This can reduce the load on hosts that are not listening for RIP-2 messages. 10.6 OSPF: Open Shortest Path First OSPF is a newer alternative to RIP as an interior gateway protocol. It overcomes all the limitations of RIP. OSPF Version 2 is described in RFC 1247 [Moy 1991]. OSPF is a link-state protocol, as opposed to RIP, which is a distance-vector protocol. The term distance-vector means the messages sent by RIP contain a vector of distances (hop counts). Each router updates its routing table based on the vector of these distances that it receives from its neighbors. In a link-state protocol a router does not exchange distances with its neighbors. Instead each router actively tests the status of its link to each of its neighbors, sends this information to its other neighbors, which then propagate it throughout the autonomous system. Each router takes this link-state information and builds a complete routing table. From a practical perspective, the important difference is that a link-state protocol will always converge faster than a distance-vector protocol. By converge we mean stabilizing after something changes, such as a router going down or a link going down. Section 9.3 of file:///D|/Documents%20and%20Settings/bigini/Docu.../homenet2run/tcpip/tcp-ip-illustrated/dynamic.htm (12 of 17) [12/09/2001 14.46.53] Ch...
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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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