IP Multicasting: Multicast Routing Protocols

1. IP Multicasting: Multicast Routing Protocols BSCI Module 7 Lesson 3

2. Objectives • Describe a multicast network in terms of the IP multicast routing protocols and processes used over various segments. • Describe multicast distribution trees including source trees and shared trees. • Describe the (S,G) and (*,G) multicast distribution trees entry formats. • Explain IP multicast routing. • Identify the characteristics of each of the PIM modes. • Describe the operation of PIM-DM, PIM-SM, and PIM sparse-dense modes.

3. Multicast Distribution Trees

4. Multicast Protocol Basics Types of multicast distribution trees: • Source distribution trees; also calledshortest pathtrees (SPTs) • Shared distribution trees; rooted at a meeting point in the network • A core router serves as a rendezvous point (RP)

5. Multicast Distribution Trees Shortest Path or Source Distribution Tree Source 1 Notation: (S, G) S = Source G = Group Source 2 A B F D C E Receiver 1 Receiver 2

6. Multicast Distribution Trees Shortest Path or Source Distribution Tree Source 1 Notation: (S, G) S = Source G = Group Source 2 A B F D C E Receiver 1 Receiver 2

7. Shared Tree Multicast Distribution Trees Shared Distribution Tree Notation: (*, G) * = All Sources G = Group A B F D (RP) (RP) PIM Rendezvous Point C E Receiver 1 Receiver 2

8. Source 1 Source 2 Source Tree Multicast Distribution Trees Shared Distribution Tree Notation: (*, G) * = All Sources G = Group A B F D (RP) (RP) PIM Rendezvous Point C E Shared Tree Receiver 1 Receiver 2

9. Multicast Distribution Tree Identification (S,G) entries • For this particular source sending to this particular group • Traffic is forwarded throughthe shortest path from the source (*,G) entries • For any (*) source sending to this group • Traffic is forwarded through a meeting point for this group

10. Multicast Distribution Trees Characteristics of Distribution Trees Source or Shortest Path trees • Uses more memory but optimal paths from source to all receivers; minimizes delay Shared trees • Uses less memory but sub-optimal paths from source to all receivers; may introduce extra delay

11. Self Check • What is an advantage of Shortest Path Trees compared to Shared Trees? • What does multicast use to make it’s forwarding decisions? • In the STP notation (S,G), what do each of the characters represent? • What is the root of the shared tree called? • Compare Shared Distribution trees and SPT trees in terms of router memory and packet delivery delay.

12. Multicast Routing

13. Protocols for IP Multicast Routing PIM is used between routers so that they can track which multicast packets to forward to each other and to their directly connected LANs.

14. Protocol-Independent Multicast (PIM) • PIM maintains the current IP multicast service mode of receiver-initiated membership. • PIM is not dependent on a specific unicast routing protocol. • With PIM, routers maintain forwarding tables to forward multicast datagrams. • PIM can operate in dense mode or sparse mode. • Dense mode protocols flood multicast traffic to all parts of the network and prune the flows where there are no receivers using a periodic flood-and-prune mechanism. • Sparse mode protocols use an explicit join mechanism where distribution trees are built on demand by explicit tree join messages sent by routers that have directly connected receivers.

15. Multicast Tree Creation PIM Join/Prune Control Messages • Used to create/remove Distribution Trees Shortest Path trees • PIM control messages are sent toward the Source Shared trees • PIM control messages are sent toward RP

16. Multicast Forwarding Multicast routing operation is the opposite of unicast routing. • Unicast routing is concerned with where the packet is going. • Multicast routing is concerned with where the packet comes from. Multicast routing uses Reverse Path Forwarding (RPF) to prevent forwarding loops.

17. Reverse Path Forwarding (RPF) The RPF Calculation • The multicast source address is checked against the unicast routing table. • This determines the interface and upstream router in the direction of the source to which PIM Joins are sent. • This interface becomes the “Incoming” or RPF interface. • A router forwards a multicast datagram only if received on the RPF interface.

18. Reverse Path Forwarding (RPF) 10.1.1.1 RPF Calculation • Based on Source Address. • Best path to source found in Unicast Route Table. • Determines where to send Joins. • Joins continue towards Source to build multicast tree. • Multicast data flows down tree. Join Join E0 E1 E2 Unicast Route Table Network Interface 10.1.0.0/24 E0

19. Reverse Path Forwarding (RPF) 10.1.1.1 RPF Calculation (cont.) • Repeat for other receivers… Join Join E0 E1 E2

20. Reverse Path Forwarding (RPF) 10.1.1.1 RPF Calculation • What if we have equal-cost paths? • We can’t use both. • Tie-Breaker • Use highest Next-Hop IP address. Join 1.1.1.1 1.1.2.1 E0 E1 E2 Unicast Route Table Network Intfc Nxt-Hop 10.1.0.0/24 E0 1.1.1.1 10.1.0.0/24 E1 1.1.2.1

21. Multicast Distribution Tree Creation Shared Tree Example

22. Self Check • Why is Protocol Independent Multicast called Independent? • Describe dense mode operation. • What does multicast routing use to prevent forwarding loops? • What is the RPF interface? • What if the RFP calculation finds 2 equal-cost paths?

23. PIM Dense Mode Operation

24. PIM-DM Flood and Prune Initial Flooding

25. PIM-DM Flood and Prune (Cont.)

26. PIM-DM Flood and Prune (Cont.) Results After Pruning

27. Self Check • What happens to packets arriving through the non-RPF interface? • When would prune messages be sent on the RPF interface? • How often do prune messages expire in PIM-DM? • What happens when the prune messages expire?

28. PIM Sparse Mode Operation

29. PIMSparse Mode • PIM-SM works with any of the underlying unicast routing protocols. • PIM-SMsupports both sourceandsharedtrees. • PIM-SMis based on an explicit pull model. • PIM-SMuses an RP. • Senders and receivers “meet each other.” • Senders are registered with RP by their first-hop router. • Receivers are joined to the shared tree (rooted at the RP) by their local DR.

30. (*, G) Join Shared Tree PIM-SM Shared Tree Join RP (*, G) State created only along the Shared Tree. Receiver

31. Source (S, G) Register (unicast) (S, G) Join Traffic Flow Source Tree PIM-SM Sender Registration RP (S, G) State created only along the Source Tree. Shared Tree Receiver

32. Source (S, G) Register (unicast) Traffic Flow Source Tree (S, G) Register-Stop (unicast) PIM-SM Sender Registration RP (S, G) traffic begins arriving at the RP through the Source tree. Shared Tree RP sends a Register-Stop back to the first-hop router to stop the Register process. Receiver

33. Source Traffic Flow Source Tree PIM-SM Sender Registration RP Source traffic flows nativelyalong SPT to RP. From RP, traffic flows downthe Shared Tree to Receivers. Shared Tree Receiver

34. Source (S, G) Join Traffic Flow Source Tree PIM-SM SPT Switchover RP Last-hop router joins the Source Tree. Shared Tree Additional (S, G) State is created along new part of the Source Tree. Receiver

35. Source Traffic Flow Source Tree (S, G)RP-bit Prune PIM-SM SPT Switchover RP Traffic begins flowing down the new branch of the Source Tree. Shared Tree Additional (S, G) State is created along along the Shared Tree to prune off (S, G) traffic. Receiver

36. Source Traffic Flow Source Tree PIM-SM SPT Switchover RP (S, G) Traffic flow is now pruned off of the Shared Tree and is flowing to the Receiver through the Source Tree. Shared Tree Receiver

37. Source Traffic Flow Source Tree (S, G) Prune PIM-SM SPT Switchover RP (S, G) traffic flow is no longer needed by the RP so it Prunes the flow of (S, G) traffic. Shared Tree Receiver

38. Source Traffic Flow Source Tree PIM-SM SPT Switchover RP (S, G) Traffic flow is now only flowing to the Receiver through a single branch of the Source Tree. Shared Tree Receiver

39. “The default behavior of PIM-SM is that routers with directly connected members will join the Shortest Path Tree as soon as they detect a new multicast source.” PIM-SM Frequently Forgotten Fact

40. PIM-SM Evaluation Effective for Sparse or Dense distribution of multicast receivers Advantages: • Traffic only sent down “joined” branches • Can switch to optimal source-trees for high traffic sources dynamically • Unicast routing protocol-independent • Basis for inter-domain multicast routing

41. Multiple RPs with Auto RP PIM Sparse-Dense-Mode

42. Self Check • What types of deployments is PIM-SM appropriate for? • When using PIM-SM, to what device does the receiver send a Join when wishing to receive multicast traffic? • How is a Register message used? • Explain the implications of the default value of the SPT-Threshold in Cisco routers. • Describe the potential issues with PIM-SM.

43. Summary • IP multicast requires multiple protocols and processes for proper packet forwarding. • Source and shared trees may be used to define multicast packet flows to group members. • Multicast routing utilizes the distribution trees for proper packet forwarding. • PIM is the routing protocol for multicast. • PIM-DM uses flood and prune. • PIM-SM uses less device and bandwidth resources and is typically chosen to implement multicast. • PIM sparse-dense mode is the recommended methodology for maximum efficiency in IP multicast.

44. Q and A

45. Resources • Internet Protocol IP Multicast Technology • http://www.cisco.com/en/US/tech/tk828/tech_brief09186a00800a4415.html • IP Multicast Deployment Fundamentals • http://www.cisco.com/en/US/tech/tk828/tech_brief09186a00800e9952.html • Cisco IOS Multicast Q&A • http://www.cisco.com/en/US/tech/tk828/technologies_q_and_a_item09186a00801bb25d.shtml