Mark Tinka mtinka at globaltransit.net
Thu Dec 29 09:42:02 UTC 2011

On Thursday, December 29, 2011 07:32:38 AM Jeff Tantsura 

> To my knowledge in most today's networks even if legacy
> equipment don't support IGMPv3 most likely 1st hop
> router does static translation and SSM upstream.

Yes, SSM Mapping allows for PIM-SSM to be used in a network 
where the receivers don't support IGMPv3. But it tends to be 
static in nature, although both Juniper and Cisco suggest 
that dynamically-configured options are possible.

I couldn't quite decode the Juniper dynamic method, but the 
Cisco one appears to be based on DNS. That should be 
interesting (and a colossal screw-up if things are poorly 


> The
> reason not to migrate to SSM is usually - ASM is already
> there and works just fine :)

This is our case.

> Cost to support RP
> infrastructure is usually the main non-technical factor
> to not to use ASM. Would be interested to hear from the
> SPs on the list.

For us, the cost of the RP isn't an issue. The Sender PE 
routers (in NG-MVPN speak, the ISP's routers that are 
connected toward the Source) are also the RP's.

But due to the use of NG-MVPN, and how we designed our 
Multicast backbone, there really isn't any need for the 
Receiver PE routers to contact the RP whenever a customer is 
joining a group.

BGP has been extended to handle PIM messages in NG-MVPN. 
When a Source is discovered by the Sender PE router, it 
generates a Type 5 SA-AD (Source Active, Auto-discovery)  
BGP update route which is sent to all Receiver PE routers 
participating in the MVPN. This Type 5 route is generated 
from the PIM Register state that is created by PIM running 
between the Sender PE and CE routers.

If the Receiver PE router is configured to operate the MVPN 
in the SPT-only mode, it generates a Type 7 (C-S,C-G) route 
for every Type 5 route it received, effectively creating the 
necessary state in the local Receiver PE router. Once 
customers send (*,G) IGMP reports requesting to join 
Multicast groups, that state is already present on the 
Receiver PE router, and traffic starts flowing immediately 

If the Receiver PE router is configured to operate the MVPN 
in RPT-SPT mode, it will follow regular PIM mecahnisms when 
users are trying to join groups, i.e., Join messages are 
forwarded toward the RP along the RPT, and then Multicast 
traffic forwarded along the SPT once the correct (C-S,C-G) 
state is created locally.

The above explanation is somewhat simplified, but represents 
the general architecture of how things work in NG-MVPN's.

For us, SPT-only mode makes sense because we have IPTv 
probes attached to all Receiver PE routers; and since 
they're collecting telemetry for all IPTv channels, no point 
running the RPT-SPT mode.

Please note that this whole setup doesn't require MSDP, 
which is nice!


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