[c-nsp] multicast on transit LAN

ML ml at kenweb.org
Tue Feb 23 08:43:13 EST 2010

On 2/18/2010 5:29 AM, Marco Regini wrote:
> Hi,
> i did some progress on this topic, with the help of "ip igmp helper
> address".
> At L3 my network lab is like this, the vlan/network between 3560 and
> 3750 is the vlan 100.
> Customers_cpe--Cisco3560-|
> Customers_cpe--Cisco3560-|
> Customers_cpe--Cisco3560-|
> ........................-|-----------Cisco3750---Core
> Customers_cpe--Cisco3560-|
> 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.
> Questions:
>  Why a need "igmp helper address" hack?
>  Is anyone of you using "igmp helper address" in a production
> environment? 

If I understand you correctly you have two pim speakers which
communicate over VLAN100.  When two PIM neighbors traverse an L2 VLAN
IGMP snooping has no effect.  Your L2 switch gear cannot tell where
multicast streams are supposed to go therefor every group gets flooded
to all ports that have PIM speakers attached.

PIM snooping is what you are looking for.  I don't if that feature
exists on your platforms.

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