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Lab 12, you have multicast running at R3 interface S1/2 so R3 can read this multicast packet of 225.25.25.25 but same interace on R3 interface S1/3 is not part of multicast. So thats why once you get multicast feed from VLAN17 (225.25.25.25), you will need to convert it into broadcast to be sent out on S1/3 on R3. These broadcast packets than reaches R2 on interface S0/1, here you will need to convert them back to multicast packets thats why you see this helper command configured on this interface.
You could also put ip multicast helper-address broadcast 225.25.25.25 111 on Fa0/0 on R1 and it should work but knowing R3 is a sort of multicast boundry router its better do it on R3. I hope this clear your confusion.
Later Omair > Subject: Mulitcast helper map> Date: Sat, 29 Mar 2008 15:13:48 -0500> From: DAthaide@eprod.com> To: ccielab@groupstudy.com> > I am trying to make sense of Internetwork Expert lab 12 Task 6.2 which> is Multicast Distribution. How is it different from LAB 8 Task 5.5> > > > In lab 12> > Why is the “ip multicast helper-address broadcast 225.25.25.25 111″> configured in the Solution guide on the S0/1 port. Shouldn’t it be on> the f0/0 port.> > Also shouldn’t the “ip multicast helper-map 225.25.25.25 129.1.17.255> 111 ” be on the S1/3 on R3 and the ip directed broadcast on S1/2 of R3.> > > > Please someone help me understand multicast helper-map. The DOC CD is> confusing> > thanks> >