[Xorp-users] GRE for multicast (but no PIM-SM on the tunnel)

[Vikram Kaul]

Folks

 A few question about the details of GRE tunnels between two disjoint 
multicast enabled (using PIM-SM) domains in XORP.

 Firstly, I understand that GRE tunnels can be created between two 
gateway routers (each one taking care of its own "domain"). I refer to
http://mailman.icsi.berkeley.edu/pipermail/xorp-users/2007-February/001660.html 

which dealt with a xorp router interacting with a cisco box with PIM-SM 
enabled on the tunnel on both ends so that the end-points see each other 
as PIM neighbors.
 Given that, the need for static rp-address configuration (same at both 
ends) pointing to the same RP and default mroute (on cisco) and 
mrib-route (on xorp)

 My situation is different. Those differences lie in:

 - I need to have individual separate RPs for each domain for the same 
group. The gateway routers could be the RPs.
    I am ok with static configurations on all routers on each "domain" 
pointing to the respective gateway router as the RP.
 - The above creates the need for disabling PIM-SM on  the tunnels.
    I want them to be pure tunnels for multicast traffic alone. No PIM 
hellos or Join/Prunes are exchanged between these
    RP/gateway routers
- I am ok with disabling RPT-to-SPT switchover so that the RP is always 
the root of the tree in each domain.
    I merely want to "tunnel" the data from this RP/gateway to the other 
side and be disseminated

      \     /                                                            
     \     /
       \   /                                                            
       \   /
        \ /                                                             
        \ /
         /                                                               
         /
        /                                                                
         /
       /                            GRE Tunnel                           /
   RP/GW----------------------------------------RP/GW
       \                                                                 
         \
        \                                                               
           \
         \                                                               
           \
   ------

First, I want to implement this purely for two gateway XORP routers. 
Essentially, the questions are :

 1. Can I have a multicast tunnel in XORP without enabling PIM on it ?
 2. How do I configure the an mrib-route like

/       mrib-route 224.0.0.0/4 {
//                 next-hop: 200.x.x.153 /* tunnel */
         }
/

    which acts in addition to the multicast forwarding caches that might 
have been established in the RP.

3. For PIM Register packets that arrive at the RP, I want them to be 
forwarded on the tunnel after they have been decapsulated. Does XORP 
configuration allow for forwarding on an interface which is not "PIM 
capable" ? Will "mrib-route" do the trick if the next hop is on an 
interface which is not PIM capable ?

Any pointers on feasibility/alternatives/gaping holes in approach are 
welcome.
Any ideas on if this were possible in XORP, how well could it fit into 
interoperability with a cisco box ?

regards..
Vikram