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Unformatted text preview: tm (2 of 17) [12/09/2001 14.46.53] Chapter 10. Dynamic Routing Protocols 10.5, OSPF in Section 10.6, and BGP in Section 10.7. 10.4 RIP: Routing Information Protocol
This section provides an overview of RIP, because it is the most widely used (and most often
maligned) routing protocol. The official specification for RIP is RFC 1058 [Hedrick 1988a],
but this RFC was written years after the protocol was widely implemented.
RIP messages are carried in UDP datagrams, as shown in Figure 10.2. (We talk more about
UDP in Chapter 11.) Figure 10.2 RIP message encapsulated within a UDP datagram.
Figure 10.3 shows the format of the RIP message, when used with IP addresses.
A command of 1 is a request, and 2 is a reply. There are two other obsolete commands (3 and
4), and two undocumented ones: poll (5) and poll-entry (6). A request asks the other system
to send all or part of its routing table. A reply contains all or part of the sender's routing table.
The version is normally 1, although RIP Version 2 (Section 10.5) sets this to 2.
The next 20 bytes specify the address family (which is always 2 for IP addresses), an IP
address, and an associated metric. We'll see later in this section that RIP metrics are hop
Up to 25 routes can be advertised in a RIP message using this 20-byte format. The limit of 25
is to keep the total size of the RIP message, 20 x 25+4 = 504, less than 512 bytes. With this
limit of 25 routes per message, multiple messages are often required to send an entire routing
table. file:///D|/Documents%20and%20Settings/bigini/Docu.../homenet2run/tcpip/tcp-ip-illustrated/dynamic.htm (3 of 17) [12/09/2001 14.46.53] Chapter 10. Dynamic Routing Protocols Figure 10.3 Format of a RIP message.
Let's look at the normal operation of routed, using RIP. The well-known port number for
RIP is UDP port 520.
q q q q Initialization. When the daemon starts it determines all the interfaces that are up and
sends a request packet out each interface, asking for the other router's complete
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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.
- Spring '12