[c-nsp] multicast on transit LAN

Marco Regini marco.regini at ascotlc.it
Thu Feb 18 05:29:34 EST 2010

i did some progress on this topic, with the help of "ip igmp helper
At L3 my network lab is like this, the vlan/network between 3560 and
3750 is the vlan 100.


Al L1 is simply a daisy-chain on the gigabit interface with a trunk that
carry only the vlan100.

Well, "IGMP snooping, CGMP, RGMP" do not limit the multicast packet on
the vlan 100, I do not know why. Perhaps this is because all apparatus
are routing and switching the vlan 100: on cisco doc I see dedicated L2
only switch connecting customers cpe and provider router. But this is
only an ipotesis, I need to capture some traffic to understand.

The workaround I have found is to put on the customer interface "ip igmp
helper address", in this way the multicast join/leave of the
customers cpe "are forwarded" by the 3560 to the Cisco3750. 
This has 2 nice effect:

1)   IGMP snooping start working on Vlan100.
2)   "show ip igmp groups" on the 4006 show me multicast group
registration on all the 3560.


 Why a need "igmp helper address" hack?
 Is anyone of you using "igmp helper address" in a production

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