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New Member

IGMP Joins over GRE Tunnel between sites



SIP phones residing on LAN1 are configured in software to use mcast address, when the first SIP phone registers it sends an igmp join message which creates a group on the L2 switch, each subsequent SIP phone joins the group when they register thus creating a "room" function whereby each SIP devices "listens" sends and receives notification messages on its status to the same multicast group address, similar to a presence function.

(The multicast group is created when the first SIP device registers itself on the LAN it isn't created by a host sender always.)

A remote WAN site which uses the same SIP devices wishes to send and receive to the same multicast address thus joining the same igmp group created on the LAN1 switch infrastructure.

The environment within the L3 core is PIM-Sparse using particular RP's which cannot be changed due to the nature of the business also requiring the use of real-time data.

I've tried to configure a GRE tunnel between the LAN1 next hop router and the router at the remote site and to then use the IGMP Join-group command on the remote router, the tunnel works fine but the Room/Presence feature does not work.

Due to it being a production network i can only attempt the work out of hours so cannot gather logs etc...

Any thoughts or advice would be appreciated.

Dave Stanbridge

Cisco Employee

IGMP Joins over GRE Tunnel between sites


If I understand you correctly then you have a number of SIP phones at different locations that use the multicast group for an application similar to room/presence feature, and you want these two locations to interoperate.

I have a couple of questions:

  1. Is the network between the main site and the remote site capable of native multicast routing, or is it necessary to tunnel the multicast traffic?
  2. If you are tunneling the multicast traffic, have you manually modified the multicast routing table to expect the multicast senders via the tunnel interface (the RPF check)?
  3. On which interface did you use the ip igmp join-group command?
  4. Even though you indicated that the RP routers cannot be changed, you should be aware that you can configure separate RPs for every multicast group. You may configure different RP router for and use the existing RP for all other groups. Is it then possible for the group to define your own RPs if the situation requires it?

Looking forward to reading from you soon.

Best regards,


New Member

IGMP Joins over GRE Tunnel between sites

Hi Peter,

Firstly, thank you for the swift response.

In answer to your questions:

1. Yes it is possible to use native multicasting routing, we currently use PIM-SM for multicast routing for real-time data and use the core routers as rp's for certain multicast traffic (in terms of the scenario I wish to tunnel as I'd prefer not to make too many changes to the core multicast config currently in place), when testing the deployment of the scenario we've used the core for the rp which is fine... however because SIP Devices in both the local and remote location send and receive on the same address it will be whichever SIP device sends data to the rp point that will determine the multicast route, so when we've tested using the rp as the core of the network only the presence feature for the SIP devices in one location will actually work...

2. Yes

3. Tunnel0 interface

4. I guess this relates to what i've written in point one.

The difficulty arises due to two different sites and hosts sending and receiving on the same multicast address.

I think I may have to look into configuring some sort of L2TP tunnel and to then extend the Local VLAN which the SIP Devices reside upon to the remote site and run IGMP over the tunnel



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