Converting broadcast to multicast and back to broadcast

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Jul 15th, 2009
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I'm attempting to convert broadcast traffic on one lan segment into multicast, route it to another segment and then convert it back to broadcast again.


My problem occurs in the last step when the multicast traffic is not converted back to broadcast. I can see the multicast traffic makes it to the destination lan segment when I use wireshark. The topology is below and configs attached.


b'cast-src(10.71.10.2/24) -> Router1 -> Router2 -> dest-lan(192.168.10.0/24)


Any help would be greatly appreciated.



Correct Answer by Giuseppe Larosa about 7 years 8 months ago

Hello Robert,

on router2 remove


ip igmp join-group 239.3.5.35

under interface f0/1

otherwise traffic is sent to cpu (process switched)


being in pim sparse-dense-mode it should not be necessary


Notice also that the original TTL of the broadcast packet has to be > 1 as explained in usage guidelines of command

ip multicast helper-map

otherwise traffic can be dropped for TTL expired.

you can use the ttl option on the first-hop router to increase the ttl of multicast packets.

This can be helpful but I didn't try this option.


Hope to help

Giuseppe


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Correct Answer
Giuseppe Larosa Fri, 07/17/2009 - 00:49
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Hello Robert,

on router2 remove


ip igmp join-group 239.3.5.35

under interface f0/1

otherwise traffic is sent to cpu (process switched)


being in pim sparse-dense-mode it should not be necessary


Notice also that the original TTL of the broadcast packet has to be > 1 as explained in usage guidelines of command

ip multicast helper-map

otherwise traffic can be dropped for TTL expired.

you can use the ttl option on the first-hop router to increase the ttl of multicast packets.

This can be helpful but I didn't try this option.


Hope to help

Giuseppe


Marwan ALshawi Fri, 07/17/2009 - 03:25
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Hi robert

you ned to do the following to get it working

first on both routers

add the follwoing command


ip forward-protocol eq 65535


* In order for incoming traffic to hit the multicast helper-map it must be

processed switched. In order to process switch traffic on a per port basis use

the ip forward-protocol udp


Next:

As R2 has the recieving said

remove the map from the 192.168 interface and keep the ip directed broadcast


apply your helper map in R@ to the interface connected to R1 to make it work as intcoming map



In order to convert a UDP broadcast to a multicast for transit, and then back to a

broadcast for final delivery, as in this scenario, the helper-map must be

configured on both the ingress device and egress device for the traffic flow.


R1 incoming from the source

and R2 incoming from R1


good luck if worked let me know


HTH

ROBERT THOMSON Fri, 07/17/2009 - 23:20
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Thank you. Putting the multicast helper-map command on the input interface of r2 worked.

Marwan ALshawi Sat, 07/18/2009 - 03:44
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i am glad its working now :)


please rate the helpful post


Thank you

Giuseppe Larosa Sat, 07/18/2009 - 04:41
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Hello Marwan,

you got it the right logic for double conversion is the one you have described (rated as it deserves)


Best Regards

Giuseppe


Marwan ALshawi Sat, 07/18/2009 - 16:06
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Thank you Giuseppe :)


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