Protocol Independent Multicast (PIM) Models

patelrasesh Multicast
Source Tree
Source Tree
Shared Distribution Tree
Shared Distribution Tree

PIM Dense Mode (PIM DM) – Push Model

  • Routers configured for PIM DM mode initially flood multicast traffic out of all interfaces.
  • Neighbor that do not have members send Prune messages toward the source to prune off the unwanted traffic. Prune messages timeout’s after three minutes and traffic is re-flooded to all the neighbors.
  • In PIM Dense mode, the control plane and the data plane are the same. Requires plentiful of bandwidth due to re-flooding.
  • Dense mode only has sources trees – no shared threes are used.

PIM Sparse Mode – Explicit join Model

  • Widely used for one-to-many application.
  • PIM Sparse mode uses the explicit join model where receivers send PIM Join messages to a designated RP.
  • Used shared tree from RP to the source and source tree from receiver to RP.
  • Last hop routers may be configured with an “SPT-Threshold” which, once exceeded, will cause the last hop router to join the “Shortest Path Tree” (SPT) up to the source. Default SPT-Threshold value = 0 (Cisco), if configured for infinity it will not use SPT.
  • SPT uses most optimal path to deliver the multicast traffic.
  • In networks with large numbers of senders, an increased amount of state must be kept in the routers. (Increase in memory usage / control plane)
  • Uses Reverse Path Forwarding (RPF) check to avoid multicast routing loops
  • MSDP is used for inter-domain multicast.

 Source-Specific Multicast

  • Utilizes all the benefits of spare mode protocols but eliminates shared trees.
  • Allows only one source per group.
  • No RP required. Receive is aware about multicast source. Work’s with IGMPv3 only.
  • Does not need MSDP for inter-domain multicast.

Bidirectional PIM

  • Widely used for many-to-many application.
  • Use the same shared tree for traffic from sources towards RP and from RP to receivers.
  • Less state in routers (Better performance / easy on control plane)
  • You can run into suboptimal multicast traffic delivery.
  • Violates RPF rules which can lead to multicast routing loops.
  • Only Phantom RP can be used for RP redundancy.

Leave a comment