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netb has been configured to consider all the hosts on the subnet 140.252.13 as directly
connected to it - that is, netb knows nothing about which hosts are actually on the
140.252.13 subnet. Since there is only one connection point to the 140.252.13 subnet,
advertising different metrics for each host makes little practical sense. Figure 10.5 Two routers netb and gateway that we'll query for their routing tables.
Figure 10.6 shows the packet exchange using tcpdump. We specify the SLIP interface with
the -i sl0 option.
sun % tcpdump s600 i sl0
1 0.0 2 5.014702 (5.0147) 3 5.560427 (0.5457) 4 5.710251 (0.1498) sun.2879 >
12: netb.route: rip-poll
> sun.2879: rip-resp
> sun.2879: rip-resp Figure 10.6 tcpdump output while running ripquery program.
The first request issued is the RIP poll command (line 1). This times out after 5 seconds and
a normal RIP request is issued (line 2). The number 24 at the end of lines 1 and 2 is the size
of the request packets in bytes: the 4-byte RIP header (with the command and version)
followed by a single 20-byte address and metric.
Line 3 is the first reply message. The number 25 at the end indicates that 25 address and file:///D|/Documents%20and%20Settings/bigini/Docu.../homenet2run/tcpip/tcp-ip-illustrated/dynamic.htm (7 of 17) [12/09/2001 14.46.53] Chapter 10. Dynamic Routing Protocols metric pairs are in the message, which we calculated earlier to be 504 bytes. This is what
ripquery printed above. We specified the -s600 option to tcpdump telling it to read 600
bytes from the network. This allows it to receive the entire UDP datagram (instead of just the
first portion of it) and it then prints the contents of the RIP response. We've omitted that
Line 4 is the second response message from the router, with the next 12 address and metric
pairs. We can calculate the size of this message to be 12 x 20 + 4 = 244, which is what
ripquery printed earlier.
If we go one r...
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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