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IP Multicasting: Explaining Multicast

IP Multicasting: Explaining Multicast

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IP Multicasting: Explaining Multicast

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  1. IP Multicasting: Explaining Multicast BSCI Module 7 Lesson 1

  2. Objectives • Describe the IP multicast group. • Compare and contrast Unicast packets and multicast packets. • List the advantages and disadvantages of multicast traffic. • Discuss two types of multicast applications. • Describe the types of IP multicast addresses. • Describe how receivers can learn about a scheduled multicast session.

  3. Multicast Overview

  4. IP Multicast Distribute information to large audiences over an IP network

  5. Why Multicast? • Used when sending same data to multiple receivers • Better bandwidth utilization • Less host/router processing • Used when addresses of receivers unknown • Used when simultaneous delivery for a group of receivers is required (simulcast)

  6. Unicast vs. Multicast

  7. Multicast Advantages Enhancedefficiency: Controls network traffic and reduces server and CPU loads Optimizedperformance: Eliminates traffic redundancy Distributedapplications: Makes multipoint applications possible

  8. Types of Multicast Applications One-to-many • A single host sending to two or more (n) receivers Many-to-many • Any number of hosts sending to the same multicast group; hosts are also members of the group (sender = receiver) Many-to-one • Any number of receivers sending data back to a source (via unicast or multicast)

  9. IP Multicast Applications Corporate Broadcasts Live TV and Radio Broadcast to the Desktop Distance Learning Multicast File Transfer Data and File Replication Training Videoconferencing Video-on-Demand Whiteboard/Collaboration Real-Time Data Delivery—Financial

  10. Self Check • List some advantages of multicast transmission over unicast transmission. • How does the best effort delivery nature of UDP impact multicast transmissions? • What are the 3 basic types of multicast applications? • Give examples of one-to-many. • What model is used when a host can be a sender as well as a receiver simultaneously?

  11. Multicast Addressing

  12. IP Multicast Address Structure IP group addresses: • Class D address (high-order three bits are set) • Range from through

  13. Source - (Class A, B, C) Destination - (Class D)Multicast Group Address Range Multicast Addressing IPv4 Header Version IHL Type of Service Total Length Source Address can never be Class D Multicast Group Address Identification Flags Fragment Offset Time to Live Protocol Header Checksum Source Address Source Destination Address Destination Options Padding

  14. IP Multicast Address Groups • Local scope addresses • to • Global scope addresses • to • Administratively scoped addresses (private) • to

  15. Local Scope Addresses Well-known addresses assigned by IANA • Reserved use: through • (all multicast clients on subnet) • (all routers on subnet) • (all DVMRP routers) • (all PIMv2 routers) •,,, and used by unicast routing protocols

  16. 00000001:00000000:01011110:00000000:00000000:00000000 IANA Ethernet MAC Address Range Available range of MAC addresses for IP multicast 01-00-5e-00-00-00 through 01-00-5e-7f-ff-ff Because the 7th hexadecimal character is 0 – 7, the 25th bit will always be 0.

  17. IANA Ethernet MAC Address Range Available range of MAC addresses for IP multicast • Within this range, these MAC addresses have the first 25 bits in common. • The remaining 23 bits are available for mapping to the lower 23 bits of the IP multicast group address. 00000001:00000000:01011110:00000000:00000000:00000000 through 00000001:00000000:01011110:01111111:11111111:11111111

  18. Multicast IP Address to MAC Address

  19. Multicast Addressing Since the only 23 bits of the multicast IP address are used and the first four bits are always 1110, bits 5 – 8 (five bits) are always ignored! This means that there are 32 IP addresses that match any one multicast MAC address!!! . . . 1 - Multicast MAC Address (FDDI and Ethernet) 0x0100.5E01.0101 11100000.00000001.00000001.00000001 =

  20. 32 IP Address to 1 Multicast MAC Address

  21. Summary • IP multicast is a much more efficient means of delivering content where a single sender needs to deliver the content to multiple receivers. This task may be achieved through the use of multicast groups. • IP multicasts are designated by the use of a specific Class D IP address range. This is achieved through global scope addresses, which are assigned dynamically, and administratively scoped, which are assigned locally and are reserved for use inside private domains.

  22. Resources • Wikipedia IP Multicast article • • Webopedia Mbone article •

  23. IP Multicasting: IGMP and Layer 2 Issues BSCI Module 7 Lesson 2

  24. Objectives • Explain the operations of IGMPv2 and how IGMPv2 utilizes Join Group and Leave Group messages. • Explain the operations of IGMPv3 and how IGMPv2 and IGMPv3 interoperate. • Describe the methods used to deal with multicast in a Layer 2 switching environment.

  25. IGMP Overview

  26. Internet Group Management Protocol (IGMP) How hosts tell routers about group membership • Routers solicit group membership from directly connected hosts • RFC 1112 specifies IGMPv1 • Supported on Windows 95 • RFC 2236 specifies IGMPv2 • Supported on latest service pack for Windows and most UNIX systems • RFC 3376 specifies IGMPv3 • Supported in Window XP and various UNIX systems

  27. IGMPv2 RFC 2236 • Group-specific query • Router sends query membership message to a single group rather than all hosts (reduces traffic). • Leave group message • Host sends leave message if it leaves the group and is the last member (reduces leave latency in comparison to v1). • Query-interval response time • The Query router sets the maximum Query-Response time (controls burstiness and fine-tunes leave latencies). • Querier election process • IGMPv2 routers can elect the Query Router without relying on the multicast routing protocol.

  28. IGMPv3 • IGMPv3 adds the ability to filter multicasts based on the multicast source. • IGMPv3 allows hosts to indicate that they want to receive traffic only from particular sources within a multicast group. • IGMPv3 is the industry-designated standard protocol for hosts to signal channel subscriptions in Source Specific Multicast (SSM).

  29. IGMPv3 (cont.) • IGMPv3 runs in two modes: INCLUDE and EXCLUDE • INCLUDE mode—In this mode, the receiver announces membership to a host group and provides a list of IP addresses (the INCLUDE list) from which it wants to receive traffic. • EXCLUDE mode—In this mode, the receiver announces membership to a host group and provides a list of IP addresses (the EXCLUDE list) from which it does not want to receive traffic. • This indicates that the host wants to receive traffic only from other sources whose IP addresses are not listed in the EXCLUDE list.

  30. Join Group IGMPv2—Joining a Group

  31. IGMPv2—Leaving a Group IGMPv2 has explicit Leave Group messages, which reduces overall leave latency.

  32. IGMPv2—Leaving a Group (Cont.) • Hosts H2 and H3 are members of group • H2 sends a leave message.

  33. IGMPv2—Leaving a Group (Cont.) • Router sends group-specific query.

  34. IGMPv2—Leaving a Group (Cont.) • A remaining member host sends report, so group remains active.

  35. IGMPv2—Leaving a Group (Cont.)

  36. IGMPv2—Leaving a Group (Cont.)

  37. Joining member sends IGMPv3 report to immediately upon joining. IGMPv3—Joining a Group

  38. IGMPv3 Report contains desired sources in the Include list. Only “Included” sources are joined. IGMPv3—Joining Specific Source(s)

  39. Router sends periodic queries: All IGMPv3 members respond. Reports contain multiple group state records. IGMPv3—Maintaining State

  40. Self Check • What is the primary purpose of IGMP? • When 2 IGMP routers are located on the same Ethernet segment, which router will be the designated querier? • What does the ICMPv2 Query router doe when it receives a Leave Message?

  41. IGMP Layer 2 Issues

  42. Determining IGMP Version Running Determining which IGMP version is running on an interface. rtr-a>show ip igmp interface e0 Ethernet0 is up, line protocol is up Internet address is, subnet mask is IGMP is enabled on interface Current IGMP version is 2 CGMP is disabled on interface IGMP query interval is 60 seconds IGMP querier timeout is 120 seconds IGMP max query response time is 10 seconds Inbound IGMP access group is not set Multicast routing is enabled on interface Multicast TTL threshold is 0 Multicast designated router (DR) is (this system) IGMP querying router is (this system) Multicast groups joined:

  43. Layer 2 Multicast Frame Switching Problem: Layer 2flooding of multicastframes • Typical Layer 2 switches treat multicast traffic as unknown or broadcast and must flood the frame to every port (in VLAN). • Static entries may sometimes be set to specify which ports receive which groups of multicast traffic. • Dynamic configuration of these entries may reduce administration.

  44. Layer 2 Multicast Switching Solutions • Cisco Group Management Protocol (CGMP):Simple, proprietary; routers and switches • IGMP snooping:Complex, standardized, proprietary implementations; switches only

  45. Layer 2 Multicast Frame Switching CGMP Solution 1:CGMP • Runs on switches and routers. • CGMP packetssent by routers to switches at the CGMP multicast MAC address of 0100.0cdd.dddd. • CGMP packet contains: • Type field: join or leave • MAC address of the IGMP client • Multicast MAC address of the group • Switch uses CGMP packet information to add or remove an entry for a particular multicast MAC address.

  46. IGMP Snooping Solution 2:IGMP snooping • Switches become IGMP-aware. • IGMP packets are intercepted by the CPU or by special hardware ASICs. • Switch examines contents of IGMP messages to learn which ports want what traffic. • Effect on switch without Layer 3-aware Hardware/ASICs • Must process all Layer 2 multicast packets • Administration load increased with multicast traffic load • Effect on switch with Layer 3-aware Hardware/ASICs • Maintain high-throughput performance but cost of switch increases

  47. Self Check • What command is used to determine the version of IGMP active on an interface? • How does a typical layer 2 switch treat multicast traffic? • What is CGMP? • What type of switch is recommending for use with IGMP snooping?

  48. Summary • IGMPv2 is a protocol used by multicast clients to join a multicast group. • IGMPv3 allows a receiver to specify a source. • If controls such as CGMP and IGMP snooping are not added at the Ethernet switching level, all multicast frames are flooded. • CGMP is a Cisco proprietary protocol used to implement multicast efficiently. • IGMP snooping is a standard protocol that has a function similar to CGMP.

  49. Resources • Wikipedia IGMP article • • Multicast in a Campus Network: CGMP and IGMP Snooping • • IP Multicast Glossary of Terms •

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