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tech:network:cisco:multicast:l2:igmp:introduction:introduction

Multicast IGMP Introduction

IGMPv1

IP Protocol number 2.

  1. Packet Format
    1. Version 4 bit: 1
    2. Type 4 bit
      1. Membership Query 0x1
      2. Membership Report 0x2
    3. Ununsed 8 bit set to 0. ignored
    4. Checksum 16 bit
    5. Group address 32 bit:
      1. Membership Query: 0 (0.0.0.0)
      2. Membership Report: Multicast Group Address
  2. IGMP Join Message
    1. type 0x2 Membership Report
    2. IGMP Table
    3. Multicast Routing Table

Multicast Router send membership query to 224.0.0.1.

Multicast receiver do nothing for IGMPv1 when leave multicast group.

IGMP Membership Query response - IGMP Membership Report - not observed in 180sec,
Multicast Router remove the IGMP table entry.

  • Verify IGMPv1 effect to multicast routing table ''show ip mroute''
  • Verify IGMPv1 table ''show ip igmp groups''

IGMPv2

IP Protocol number 2.

IGMPv2 has compatiblity to IGMPv1.

  1. Packet Format
    1. Type 8 bit
      1. 0x11 Membership Query
      2. 0x12 v1 Membership Report
      3. 0x16 v2 Membership Report
      4. 0x17 Leave Group
    2. Max response time 8bit: 100 ms in 1: Default to 100 = 10 sec
    3. Checksum 16 bit
    4. Group Address 32 bit
      1. General Query: 0.0.0.0
      2. Groupspecific Query or v1/v2 Membership Report: Multicast Address
  2. Membership Query
    1. General Query
    2. Group Specific Query
  1. Querier: a router that send query
  2. IGMPv2 select Querier
  3. when a subnet has multiple routers in same subnet, minimum IP address router will be the querier.
  1. 224.0.0.2 same subnet all router
  • Verify IGMPv2 effect to multicast routing table ''show ip mroute''
  • Verify IGMPv2 table ''show ip igmp groups''

IGMPv3

IP Protocol number 2.

  • Verify IGMPv3 effect to multicast routing table ''show ip mroute''
  • Verify IGMPv3 table ''show ip igmp groups''

Membership Query

  1. Packet Format
    1. Type 8 bit
      1. Membership Query 0x11
    2. Max response time 8 bit: 100 ms in 1: Default to 100 = 10 sec
    3. Checksum 16 bit
    4. Group Address 32 bit
      1. General Query: 0.0.0.0
      2. Group Specific Query, Group Source Specific Query: Multicast Address
    5. Resv 4 bit: set to 0 when send. will be ignored when receive
    6. S 1 bit: Do not execute normal timer update when query message received.
    7. QRV 3 bit: Querier Robustness Value.
    8. QQIC 16 bit: Querier's Query Interval Code.
    9. Number of Source IPs 16 bit
    10. Source IP adress 32 bit

Membership Report

  1. Packet Format
    1. Type 8 bit
      1. Membership Report 0x22
    2. Reserved 8 bit: set to 0 on send time and ignored on receive time
    3. Checksum 16 bit
    4. Reserved 16 bit: set to 0 on send time and ignored on receive time
    5. Number of group records 16 bit
    6. Group Record
      1. Record Type 8 bit: 1 MODE_IS_INCLUDE, 2: MODE_IS_EXCLUDE
      2. supplemental data length 8 bit: set to 0 on send time and ignored on receive time
      3. number of sources 16 bit
      4. multicast address 32 bit: multicast address related to the group record
      5. Source IP address
      6. supplemental data: additional data related to group record

How it works

  • IGMPv3 Membership report can specify Sender address, so this can prevent to receive traffic from unauthorized sender
    • This behavior called Source Filter
      • Filter Mode
        • INCLUDE Mode receive multicast packets from specified address only
  • EXCLUDE Mode receive multicast pakcets from addresses other than specified address

IGMP Snooping

  • Snoop IGMP Join Message, so can forward only appropriate switch port multicast grop receiver
  • IGMP Snooping disabled switch will flood all switch ports.

References

tech/network/cisco/multicast/l2/igmp/introduction/introduction.txt · Last modified: 2021/07/16 08:04 by wnoguchi