Multicast - Implementing Multicast (MCAST V1.0) - IGMP (2)

Um Traffic einer Multicast-Gruppe zu bekommen muss man am Next-Hop-Router per IGMP (Internet Group Message Protocol) der Gruppe joinen...


IGMP - Internet Group Message Protocol (RFC1112, RFC2236 (v2))
the way hosts tell routers about membership to a multicast group


IGMP v1

• Membership Query (TTL=1, sent by Router)
• joining a group : IGMP v1 Type 0x2 Report (by a Host/PC)
• periodic router queries to 224.0.0.1 (all-hosts-multicast-address)
• leaving group silently, group membership times out

IGMP v2

• Group specific query - Router sends Group Specific Query to make sure no listener active
• Leave Group Message - hosts can leave a group (reduce leave latency)
• Query election - lowest IP, only router who sends queries
• Query interval response time - time within a host must respond

IGMP v2 Packet

• Type 8bit - 0x11 Membership Query, 0x12 Version 1 Membership Report, 0x16 Version 2 Membership Report, 0x17 Leave Group
• Max. Response Time : Def:10 sec (in 1/10 sec)
• Checksum 16bit
• Group Address 32bit

IGMPv2 Joining a group
Hosts sends a Membership Report


IGMPv2 Querier Election
all routers send query, lowest IP wins


IGMPv2 Maintaining group membership
only one host per group per subnet reports the membership (other host suppress) -> countdown timer)

IGMPv2 Leave Group
one host leaves group (Leave Message)
Router sends Group-specific Query
remaining host sends Report
Group remains active
last router leaves
Router stops forwarding traffic for that group/multicast address


If IGMPv2/v1 router in same segment "ip igmp version 1" must be configured