Next Generation Multicast Default MDT: Profile 0

This post describes how Multicast packet traverses with the use of Multiprotocol Label Switching (MPLS) core in Next Generation Multicast.

Draft Rosen uses Generic Routing Encapsulation (GRE) as an overlay protocol. That means that all multicast packets are encapsulated inside GRE. A virtual LAN is emulated with all Provider Edge (PE) routers in the VPN join a multicast group. This is known as the default Multicast Distribution Tree (MDT). The default MDT is used for Protocol Independent Multicast (PIM) hello’s and other PIM signaling but also for data traffic. If the source sends a lot of traffic it is inefficient to use the default MDT and a data MDT can be created. The data MDT will only include PE’s that have receivers for the group in use.

Draft Rosen is fairly simple to deploy and works well but it has a few drawbacks. Let’s take a look at these:

Overhead – GRE adds 24 bytes of overhead to the packet. Compared to MPLS which typically adds 8 or 12 bytes there is 100% or more of overhead added to each packet.

PIM in the core – Draft Rosen requires that PIM is enabled in the core because the PE’s must join the default and or data MDT which is done through PIM signalling. If PIM ASM is used in the core, an RP is needed as well. If PIM SSM is run in the core, no RP is needed.

Core state – Unnecessary state is created in the core due to the PIM signalling from the PE’s. The core should have as little state as possible.

PIM adjacencies – The PE’s will become PIM neighbors with each other. If it’s a large VPN and a lot of PE’s, a lot of PIM adjacencies will be created. This generates a lot of hello’s and other signaling which adds to the burden of the router.

Unicast vs multicast – Unicast forwarding uses MPLS, multicast uses GRE. This adds complexity and means that unicast uses a different forwarding mechanism than multicast, which is not the optimal solution.

Inefficiency – The default MDT sends traffic to all PE’s in the VPN regardless if the PE has a receiver in the (*,G) or (S,G) for the group in use.

  • Default MDT will be used to connect multicast to all the PE in one VRF.
  • Default means it connects all the PE routers.
  • By default, it carries all the traffic.
  • All the PIM control traffic and the Data plane traffic. Eg (*,G) Traffic and the (S,G) traffic.
  • This represent multipoint to multipoint.
  • Anybody can send and everybody can receive from the tree.


For more details, please visit the below document written by Sabyasachi and Shashi:


Categories: General, IPv4, MPLS, Multicast

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