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FTOS Configuration Guide for the C-Series - Force10 Networks

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To receive multicast traffic from a particular source, a host must join <strong>the</strong> multicast group to which <strong>the</strong><br />

source is sending traffic. A host that is a member of a group is called a receiver. A host may join many<br />

groups, and may join or leave any group at any time. A host joins and leaves a multicast group by sending<br />

an IGMP message to its IGMP Querier. The querier is <strong>the</strong> router that surveys a subnet <strong>for</strong> multicast<br />

receivers, and processes survey responses to populate <strong>the</strong> multicast routing table.<br />

IGMP messages are encapsulated in IP packets, as shown in Figure 409.<br />

Figure 409 IGMP version 2 Packet Format<br />

Version<br />

(4)<br />

IHL TOS<br />

(0xc0)<br />

Preamble Start Frame<br />

Delimiter<br />

Total Length Flags Frag Offset<br />

Joining a Multicast Group<br />

There are two ways that a host may join a multicast group: it may respond to a general query from its<br />

querier, or it may send an unsolicited report to its querier.<br />

Responding to an IGMP Query<br />

Destination MAC Source MAC E<strong>the</strong>rnet Type IP Packet Padding<br />

1. One router on a subnet is elected as <strong>the</strong> querier. The querier periodically multicasts (to<br />

all-multicast-systems address 224.0.0.1) a general query to all hosts on <strong>the</strong> subnet.<br />

2. A host that wants to join a multicast group responds (to all-multicast-systems address 224.0.0.1) with<br />

an IGMP Membership Report that contains <strong>the</strong> multicast address of <strong>the</strong> group it wants to join. If<br />

multiple hosts want to join <strong>the</strong> same multicast group, only <strong>the</strong> report from <strong>the</strong> first host to respond<br />

reaches <strong>the</strong> querier, and <strong>the</strong> remaining hosts suppress <strong>the</strong>ir responses (see Adjusting Query and<br />

Response Timers on page 590 <strong>for</strong> how <strong>the</strong> delay timer mechanism works).<br />

3. The querier receives <strong>the</strong> report <strong>for</strong> a group and adds <strong>the</strong> group to <strong>the</strong> list of multicast groups associated<br />

with its outgoing port to <strong>the</strong> subnet. Multicast traffic <strong>for</strong> <strong>the</strong> group is <strong>the</strong>n <strong>for</strong>warded to that subnet.<br />

Sending an Unsolicited IGMP Report<br />

TTL<br />

(1)<br />

Protocol<br />

(2)<br />

Header<br />

Checksum<br />

A host does not have to wait <strong>for</strong> a general query to join a group. It may send an unsolicited IGMP<br />

Membership Report, also called an IGMP Join message, to <strong>the</strong> querier.<br />

584 Internet Group Management Protocol<br />

Src IP Addr<br />

Dest IP Addr Options<br />

(Router Alert)<br />

FCS<br />

Padding<br />

Type Max. Response<br />

Time<br />

Code: 0x11: Membership Query<br />

0x12: IGMP version 1 Membership Report<br />

0x16: IGMP version 2 Membership Report<br />

0x17: IGMP Leave Group<br />

Checksum<br />

8 bits 16 bits<br />

IGMP Packet<br />

Group Address<br />

May be zero and ignored by hosts <strong>for</strong><br />

general queries or contain a group<br />

address <strong>for</strong> group-specific queries<br />

fnC0069mp

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