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Multicast PIM-DM Introduction


  1. Protocol-Independent Multicast Dense Mode
  2. using Source Tree
  3. Flood & Prune
  4. Flood multicast traffic to PIM neighbor
  5. Multicast router has no PIM neighbor and no any multicast group member, sends Prune message.
  6. The router that receive Prune message stop to forward multicast traffic.
  7. PIM-DM periodically (every 180 sec) sends Floods & Prune.
  8. Pruned interface, Receiver connected, router receive IGMP Membership Report, send PIM Graft Message to upstream PIM neighbor, so clear Prune stae on upstream PIM neighbor. So can routing multicast packet without wait 180 sec.

(S, G) and (*, G)

  1. Sender sends multicast packet to first hop router.
  2. First hop router create reperesents (S, G) source tree.
  3. Cisco router creates (*, G) entry for (S, G) entry template.
  4. (S, G), (*, G) includes following information.
    1. RPF (Reverse Path Forwarding): Incoming Interface, RPF is decided from unicast routing table.
    2. OIL (Outgoing Interface List): Flood multicast packet from OIL listed interface

Distribution Tree can verify

show ip mroute
  • Verify RPF, OIL information using ''show ip mroute''

PIM-DM neighbor detection

  1. Router enables PIM-DM on an interface, the interface sends PIM Hello Message.
  2. ALL-PIM-Routers
  3. PIM Hello Message is send every 30 sec.
  4. Multi-access network(e.g. LAN) elect PIM DR
  5. PIM DR has no mean on PIM-DM
  6. PIM-SM only has mean PIM DR
  7. PIM DR how election
    1. Greatest PIM DR priority router
    2. Greatest IP address router
  8. PIM must be enabled all multicast interfaces that not only inter router and Sender, Receiver connected interfaces.
  9. So all multicast packet forward path interfaces enabled PIM even if currently no Sender, Receiver does not exist.
  10. Multicast possiblitiy interfaces

PIM-DM (*, G)

  • use (*, G) as (S, G) template.
  • RPF Information
    • Incoming Interface: Null
    • RPF nbr:
  • OIL Information
    • PIM Neighbor interface
    • Receiver exist interface
    • ip igmp join-group, ip igmp static-group configured interface

PIM-DM (*, G)

  • RPF Information
    • Incoming INterface: Shortest Path Interface
    • RPF nbr: Direction Incoming Interface PIM neighbor address
  • OIL Information
    • exclude RPF I/F following interface
    • PIM Neighbor
    • Receiver exist interface
    • ip igmp join-group, ip igmp static-group configured interface
    • OIL Interface status: Forward or Prune
      • Initial status: Forward
      • If Prune message received, it goes Prune state.
      • This will happen every 180 seconds.
  • Ditsribution Tree , Multicast Routing Table
  • Omitted Information
    • Windows PC UPnP (Universal Plug and Play)
    • PIM-SM Auto RP Multicast Address (Cisco equipment is Receiver)


  1. D Dense: (*, G) entry means PIM-DM template.
  2. C Connected: Router connected receiver exist.
  3. L Local: ip igmp join-group etc. Router interface itself is Receiver.
  4. P Pruned: OIL all interfaces Prune state.
  5. T SPT-bit set: (S, G) multicast packet will be forwarded

Distribution Tree Creation Flow

  1. Receiver sends IGMP Join Message (IGMP Membership Report)
  2. R2 Receive IGMP Membership Report
  3. R2 Create (*, entry
  4. Sender sends multicast packets
  5. R1 receive multicast packet
  6. R1 create (*,, and (, entry
  7. R1 forward multicast packet outgoing interface
  8. R2 receive multicast packet from R1
  9. R2 create (, entry from (*, template
    1. notice R2 (, entry RPF nbr indicate PIM neighbor.

PIM Assert

  1. Prevent multicat packet duplication
  2. PIM Assert Message
    1. Sender IP address
    2. Multicast Group Address
    3. distance and metric for reaching Sender IP address
  3. Assert Winner
    1. Compare distance and metric lower value router
    2. If the distance and metric is same compared Incoming Interface and most largest one winner
    3. Assert Winner Forward Multicast packet
  4. Assert Loser: not Assert Winner
    1. Assert Loser set the outgoing interface state to Pruned, so do not forward multicast packet
  5. as a result, Receiver can receive not duplicate packets.

PIM State Refresh

  1. This feature prevent PIM-DM every 180 sec. multicast flooding.
  2. This feature enabled by default.
  3. Sender connected router sends PIM State Refresh Message every 60 sec.
  4. Every PIM router reset following Expire timers
    1. (S, G) entry itself Expire timer (180 sec.)
    2. (S, G) entry Prune state Outgoing Interface Expire timer (180 sec.)
  5. State Refresh require Sender mutlicast active
  6. New Receiver connected, PIM Graft Message will sends. Upstream PIM neighbor I/F lift Prune state.


tech/network/cisco/multicast/routing/pim/pim-dm/pim-dm.txt · Last modified: 2021/05/22 14:19 by wnoguchi