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Multicast Routing Introduction

Distribution Tree

Distribution Tree Notation

  1. S: Source
  2. G: Multicast Group
  3. represent tree with (S, G) notation

Source Tree (SPT: Shortest Path Tree)

  1. Source Tree: Create individual distribution tree
    1. SPT: Shortest Path Tree
    2. Pros
      1. Create shortest path
      2. Minimize traffic delay
    3. Cons
      1. Increase more router resource when multiple sender, multicast group, distributed receiver
    4. e.g. SPT has following tree
      1. (,
      2. (,
      3. (,

Shared Tree

  1. Shared Tree: Create shared Distribution tree by RP with multiple sender
    1. RP (Rendezvous Point) centric single tree
    2. Multiple sender has common distribution tree
    3. Sender sends multicast packet to RP.
    4. Next, RP forward multicast packet to receiver by shared tree.
    5. If Receiver present between Sender and RP.
    6. Pros
      1. Needs only Single Shared Tree
      2. Minimize router resource consumption
    7. Cons
      1. Path from Sender to Rceiver is not shortest path by RP
      2. So, if sender far from RP, it has potential transport delay.
    8. (S, G) representation:
      1. all sender use common single tree, so sender represent '*' means any.
      2. e.g.
        1. (*,
        2. (*,
        3. (*,

Which tree to use?

Multicast routing protocol mode affect that which tree to use.

  1. Multicast Routing Protocol, Dnese Mode: Source Tree(SPT)
  2. Multicast Routing Protocol, Sparse Mode: Combination of Source Tree(SPT) and Shared Tree

Multicast Routing Protocol

  1. Dense Mode
  2. Sparse Mode
  3. Which Mode to use?: following factor is important to making decision
    1. Sender location
    2. Receiver location
    3. Network Bandwidth

Dense Mode

  1. Sender, Receiver located 1 site, and consists of GigabitEthernet LAN.
  2. However, if network has voice system, may be used Sparse Mode.
  3. Multicast Routing Protocol:
    1. PIM-DM (typical)
    2. DVMRP
    3. MOSPF
  4. Dense Mode flooding multicast traffic periodically
  5. Flood & Prune: Flooding first, if no reciver exist, that port prune

Sparse Mode

  1. Sender, Receiver located via WAN.
  2. Narrow bandwidth. e.g. exists WAN
  3. First, Create Empty Tree
  4. Explicit Join Request, add the router to tree
  5. Tree has the router called RP (Rendezvous Point).
  6. Receiver exists router registered to RP.
  7. Sender sends multicast packets to RP, forward to Receiver, optimze path information.
  8. Sparse Mode is Explicit Join Model.
  9. Multicast Routing Protocol:
    1. PIM-SM (typical)
    2. CBT

RPF Check

  1. RPF (Reverse path Forwarding) check
  2. check shortest path route
  3. reference unicast routing table, interface match
  4. if RPF check fail, packet drops
  5. prevent duplicate packet, routing loop
  6. RPF check enabled by default on multicast router
  7. where packet comes from? - Reverse Path Forwarding
  8. RPF check successful interface is called RPF interface.
  9. RPF interface is interface that receive multicast packet.
  10. unicast routing use dentination address.
  11. multicast routing check source address first.

RPF check case study: multiple path exists for destination network

  1. RPF interface is always 1 interface.
  2. In this case, RPF interface is interface that has maximum next hop address.


tech/network/cisco/multicast/routing/introduction/introduction.txt · Last modified: 2021/05/21 09:43 by wnoguchi