multicast helper map and ip directed-broadcast

Hello Experts,

I have a question regarding directed broadcast in multicast helper map: Why
"ip directed-broadcast" is needed on R4 int F0/0? It does not work without
it.
For those who use Narbik workbooks it is about Vol2, Multicast, Lab 10.

Topology:

R1(F0/0)---(F0/0)R2--frame--R3--frame--R4(F0/0)---(F0/0)R5

Multicast runs on R2, R3, R4
R1 is configured to send broadcast updates.

When I debug ip packets on R5 I see that R5 receives RIP updates from R1
with destination address of 255.255.255.255. There is no 10.1.45.255 in the
debug:

*Mar 1 10:31:30.717: IP: s=10.1.12.1 (FastEthernet0/0), d=255.255.255.255,
len 52, rcvd 2

Why then ip directed-broadcast is needed in R4 F0/0 ? What it does ?

Thanks,

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Received on Tue Oct 12 2010 - 10:23:23 ART