Subject: Multicast in L3 VPN

From: "Leymann, Nicolai" <[email-masked]>
Date: Mon, 1 Nov 2004 17:24:31 +0100

Dear all,

As mentioned earlier on the list we are currently looking into deployment 
scenarios for IP-Multicast in VPN environments, with the main focus on
a single provider implementation (up til now no multidomain multicast VPNs
spanning several ISPs).

Our main scenario at the moment consists of only a small number of VPNs -
something in the range between one and five - and a limited number of sites 
per mVPN as well (about 15-20). This is a rather small deployment scenario
where most of the problems which are described later do not occur.

In order to fulfil our basic requirements (which means: we do need a 
solution which we can use and deploy within the next few weeks/month) 
draft-rosen-vpn-mcast-06.txt is the only possibility which is available 
now. This draft provides a solution for a limited deployment of 
IP-Multicast in L3 VPNs and also provides some interoperability between 
different vendors. In our case draft-rosen-vpn-mcast-06.txt works; PIM-SM
as routing protocol in the core is fine. 

I noticed that there was a long discussion about supporting PIM-SSM as a
core routing protocol. I think there are two different aspects which has
to be considered. One is related to the number of multicast states in 
the core and the other to the addressing mechanisms. 
>From the operational point of view the handling of multicast addresses
can be quite painful (e.g. if you have multicast traffic in the core
which is not related to mVPN). Hence the use of PIM-SSM as a core routing
protocol can be useful even if the number of states rises. PIM-SSM removes 
the risk of address collisions. This results in a more stable core and
eases the address management.

The general problem I see is the limited scalability of the current 
solutions and the fact, that MPLS transport is not supported. The 
scaling problems do occur in several different domains depending of 
the amount of traffic, number of mVPNs and number of sites per mVPN. 
If multi-provider scenarios are also taken into account scaling (and/or addressing) gets worse.

For mid-term deployment the goal should be a interoperable and scalable
solution which enables the use of MPLS features like TE and the transport
of IP-Multicast over MPLS (no GRE/Multicast encapsulation/routing in the 
provider core). Replication in the core should be done as late as possible 
(no ingress replication), therefor the support of point to multipoint LSPs 
should be preferred. It is also important, that there is agreement on one
solution which is supported by several vendors.

   best regards

     Nic

--
Nicolai Leymann                          EMail: [email-masked]                    
T-Systems System Integration, Berlin     Tel/Fax: ++49-30-3497-3570/-3571
ET52                                     Mobil: ++49-170-2275345
Senior Experte IP-Technologien           Goslarer Ufer 35, 10589 Berlin, Germany