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Multicast PIM-SP

Hi all,

I have a very simple multicast set up. Site A has the source server which has a 2M leased line to the HQ. HQ is conneced to five sites with Motorola Canopy Access point (AP) and subscriber module (SM) where the end users are located. SM sites have single user connected to SM without any router.


Since there are only two routers, one at the source and one at the HQ. is there any need to create an RP? if i just enable multicast on the two routers will that be sufficient?



Pavel Bykov

No. You still need a multicast routing protocol. RP is required in PIM-SM setup, since register messages are sent to it (multicast encapsulated into unicast messages). PIM-DM does not require any additional settings, and although it is recommended for small networks like yours, it is not recommended for live environments.

Also remember, you have to enable PIM on EVERY interface where you want to receive multicasts, and where you want to route them.


Source Specific Multicast (SSM) does not require an RP. You can statically join a SSM group to the interface connected to the receiver and use SSM-mapping to map the group to a source, the router will then build the SPT direct to the router connected to the source.

Thanks guys. After doing some reading ....... I think I will use PIM-DM and then if the user base grows I will use PIM-SM and create a RP. I think I will also need to enable cgmp on the router port connecting to the switch. And enable cgmp on the switch as well.



Can you tell us what kind of switch do you have ?

Newer switches no longer support CGMP but instead they support IGMP snooping, which is a much better mechanism for controlling multicast traffic.

Hi Edison,

We may be able to use 1900 switch but since this is a new project, we could use new ones as well. The reason I though of using CGMP is because in a doc i read that L2 switch does not scall well with IGMP snooping. Then i read another doc after your post that says that IGMP snooping is ok on L2 switch, if IGMPv3 is uses. the same doc says that IGMPv3 is native on Win XP. could you please confirm this?

Now I have two options:

Option 1 - use C1841 with Cat 2940/2950 at both locations.

Option 2 - use L3 switch at both location. The link between source and the HQ is a wireless link (point to point back haul) so an ethernet interface is fine to connect the BH wireless.

Could you please suggest which option is better. And what L3 swtich will be required. I also have to choose the most cost effective opotion.



I recommend going with the latest and greatest equipment when deploying a new environment. You will get better support if something goes wrong with the hardware or software.

IGMP Snooping scales better than CGMP because the traffic control decision is made by the switch, internally, instead of the router.

WindowsXP comes with IGMPv3 enabled by default,

I suggest you go with the Layer3 option along with IGMP snooping.