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We have shown the hardware addresses in these two lines, to verify that the Ethernet destination address is
the correct multicast address. We can also see that the source IP address is the one corresponding to sun, and file:///D|/Documents%20and%20Settings/bigini/Docum.../homenet2run/tcpip/tcp-ip-illustrated/igmp_int.htm (5 of 7) [12/09/2001 14.47.02] Chapter 13. IGMP: Internet Group Management Protocol the destination IP address is the multicast group address. We can also see that the reported address is that
same multicast group address.
Finally, we note that the TTL is 1, as specified, tcpdump prints the TTL in square brackets when its value is
0 or 1. This is because the TTL is normally greater than this. With multicasting, however, we expect to see
lots of IP datagrams with a TTL of 1.
Implied in this output is that a multicast router must receive all multicast datagrams on all its interfaces. The
router has no idea which multicast groups the hosts might join.
Multicast Router Example
Let's continue the previous example, but we'll also start a multicast routing daemon on the host sun. Our
interest here is not the details of multicast routing protocols, but to see the IGMP queries and reports that are
exchanged. Even though the multicast routing daemon is running on the only host that supports multicasting
(sun), all the queries and reports are multicast on the Ethernet, so we can see them on any other system on
Before starting the routing daemon we joined another multicast group: 126.96.36.199. Figure 13.5 shows the
0.00 ( 0.00)
5.10 ( 5.10)
5.22 ( 0.12)
7.90 ( 2.68)
8.50 ( 0.60)
11.70 ( 3.20)
125.70 ( 0.19)
128.50 ( 2.80)
129.10 ( 0.60)
248.09 ( 0.27)
248.69 ( 0.60)
255.29 ( 6.60) sun
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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