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BSCI Module 7

Lesson 2 1

BSCI Module 7 Lesson 2

IP Multicasting:

IGMP and Layer 2Issues

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BSCI Module 7

Lesson 2 2

Objectives

Explain the operations of IGMPv2 and how IGMPv2utilizes Join Group and Leave Group messages.

Explain the operations of IGMPv3 and how IGMPv2and IGMPv3 interoperate.

Describe the methods used to deal with multicast in aLayer 2 switching environment.

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BSCI Module 7

Lesson 2 3

IGMP Overview

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BSCI Module 7

Lesson 2 4

Internet Group Management Protocol(IGMP)

How hosts tell routers about group membership

Routers solicit group membership from directlyconnected hosts

RFC 1112 specifies IGMPv1Supported on Windows 95

RFC 2236 specifies IGMPv2

Supported on latest service pack for Windows and mostUNIX systems

RFC 3376 specifies IGMPv3

Supported in Window XP and various UNIX systems

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BSCI Module 7

Lesson 2 5

IGMPv2RFC 2236

Group-specific query

Router sends query membership message to a single group rather than all hosts (reduces traffic).

Leave group messageHost sends leave message if it leaves the group and is the lastmember (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 onthe multicast routing protocol.

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BSCI Module 7

Lesson 2 6

IGMPv2²Joining a Group

224.1.1.1

Join Group

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BSCI Module 7

Lesson 2 7

IGMPv2²Leaving a Group

IGMPv2 has explicit Leave Group messages, whichreduces overall leave latency.

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BSCI Module 7

Lesson 2 8

IGMPv2²Leaving a Group (Cont.)

Hosts H2 and H3 are members of group 224.1.1.1.

1. H2 sends a leave message.

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BSCI Module 7

Lesson 2 9

IGMPv2²Leaving a Group (Cont.)

2. Router sends group-specific query.

KA3

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Slide 9

KA3 In the original slide the screen of computer H3 was a different color to bring it to the attention of the learner. Please change the scree

color of computer H3 from the light teal color to a white.K aren  A lderson, 10/19/2006

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BSCI Module 7

Lesson 2 10

IGMPv2²Leaving a Group (Cont.)

3. A remaining member host sends report, so group remains active.

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BSCI Module 7

Lesson 2 11

IGMPv2²Leaving a Group (Cont.)

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BSCI Module 7

Lesson 2 12

IGMPv2²Leaving a Group (Cont.)

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BSCI Module 7

Lesson 2 13

IGMPv3²Joining a Group

Joining member sends IGMPv3 report to 224.0.0.22immediately upon joining.

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BSCI Module 7

Lesson 2 14

IGMPv3²Joining Specific Source(s)

IGMPv3 Report contains desired sources in theInclude list. Only ³Included´ sources are joined.

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BSCI Module 7

Lesson 2 15

IGMPv3²Maintaining State

Router sends periodic queries:

 All IGMPv3 members respond.

Reports contain multiple group state records.

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BSCI Module 7

Lesson 2 16

Self Check

1. What is the primary pur pose of IGMP?

2. When 2 IGMP routers are located on the sameEthernet segment, which router will be the designated

querier?

3. What does the ICMPv2 Query router do when itreceives a Leave Message?

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BSCI Module 7

Lesson 2 17

IGMP Layer 2Issues

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BSCI Module 7

Lesson 2 18

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 1.1.1.1, subnet mask is 255.255.255.0

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 1.1.1.1 (this system)

IGMP querying router is 1.1.1.1 (this system)

 Multicast groups joined: 224.0.1.40 224.2.127.254

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BSCI Module 7

Lesson 2 19

Layer 2 Multicast Frame Switching

Problem: Layer 2 flooding of multicast

frames

Typical Layer 2 switches treatmulticast traffic as unknown or broadcast and must flood the frameto every port (in VLAN).

Static entries may sometimes be setto specify which ports receive which

groups of multicast traffic.

Dynamic configuration of theseentries may reduce administration.

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BSCI Module 7

Lesson 2 20

Layer 2 Multicast Switching Solutions

Cisco Group Management Protocol (CGMP): Simple,proprietary; routers and switches

IGMP snooping: Complex, standardized, proprietary

implementations; switches only

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BSCI Module 7

Lesson 2 21

Layer 2 Multicast Frame SwitchingCGMP

Solution 1: CGMP

Runs on switches and routers.

CGMP packets sent by routers toswitches at the CGMP multicast MACaddress 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 toadd or remove an entry for a particular multicast MAC address.

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BSCI Module 7

Lesson 2 22

IGMP Snooping

Solution 2: IGMP snooping

Switches become IGMP-aware.

IGMP packets are intercepted by the CPU or byspecial hardware ASICs.

Switch examines contents of IGMP messages to learnwhich ports want what traffic.

Effect on switch without Layer 3-awareHardware/ASICs

Must process all Layer 2 multicast packets

 Administration load increased with multicast trafficload

Effect on switch with Layer 3-aware Hardware/ASICs

Maintain high-throughput performance but cost of 

switch increases

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BSCI Module 7

Lesson 2 23

Impact of IGMPv3 on IGMP Snooping

IGMPv3 Reports are sent to a separate group (224.0.0.22)reduces load on switch CPU

No Report Suppression in IGMPv3

IGMP Snooping should not cause a seriousperformance problem once IGMPv3 is implemented.

IGMPv3 and IGMP Snooping

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BSCI Module 7

Lesson 2 24

Self Check

1. What command is used to determine the version of IGMP active on an interface?

2. How does a typical layer 2 switch treat multicast

traffic?

3. What is CGMP?

4. What type of switch is recommending for use withIGMP snooping?

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BSCI Module 7

Lesson 2 25

Summary

IGMPv2 is a protocol used by multicast clients to join amulticast group.

IGMPv3 allows a receiver to specify a source.

If controls such as CGMP and IGMP snooping are notadded at the Ethernet switching level, all multicastframes are flooded.

CGMP is a Cisco proprietary protocol used toimplement multicast efficiently.

IGMP snooping is a standard protocol that has afunction similar to CGMP.

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BSCI Module 7

Lesson 2 26

Q and A

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BSCI Module 7

Lesson 2 27

Resources

Wikipedia IGMP article

http://en.wikipedia.org/wiki/IGMP

Multicast in a Campus Network: CGMP and IGMPSnooping

http://www.cisco.com/war p/public/473/22.html

IP Multicast Glossary of Terms

http://www.cisco.com/en/US/tech/tk828/tech_brief0900aecd801

bca26.html

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BSCI Module 7

Lesson 2 28