IPTV and ASM
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
wrote:
> 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
maintained):
http://www.cisco.com/en/US/docs/ios/12_3t/12_3t2/feature/guide/gtssmma.html#wp1119180
> 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
downstream.
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!
Cheers,
Mark.
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