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FTOS Configuration Guide for the C-Series - Force10 Networks

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3. If a host on <strong>the</strong> same subnet as ano<strong>the</strong>r multicast receiver sends an IGMP report <strong>for</strong> <strong>the</strong> same multicast<br />

group, <strong>the</strong> gateway takes no action. If a router between <strong>the</strong> host and <strong>the</strong> RP receives a PIM Join<br />

message <strong>for</strong> which it already has a (*,G) entry, <strong>the</strong> interface on which <strong>the</strong> message was received is<br />

added to <strong>the</strong> outgoing interface list associated with <strong>the</strong> (*,G) entry, and <strong>the</strong> message is not (and does<br />

not need to be) <strong>for</strong>warded towards <strong>the</strong> RP.<br />

Refusing Multicast Traffic<br />

A host requesting to leave a multicast group sends an IGMP Leave message to <strong>the</strong> last-hop DR. If <strong>the</strong> host<br />

is <strong>the</strong> only remaining receiver <strong>for</strong> that group on <strong>the</strong> subnet, <strong>the</strong> last-hop DR is responsible <strong>for</strong> sending a<br />

PIM Prune message up <strong>the</strong> RPT to prune its branch to <strong>the</strong> RP.<br />

1. Upon receiving an IGMP Leave message, <strong>the</strong> gateway removes <strong>the</strong> interface on which it is received<br />

from <strong>the</strong> outgoing interface list of <strong>the</strong> (*,G) entry. If <strong>the</strong> (*,G) entry has no remaining outgoing<br />

interfaces, multicast traffic <strong>for</strong> that group is no longer <strong>for</strong>warded to that subnet.<br />

2. If <strong>the</strong> (*,G) entry has no remaining outgoing interfaces, <strong>the</strong> last-hop DR sends a PIM Prune message to<br />

towards <strong>the</strong> RP. All routers along <strong>the</strong> way remove <strong>the</strong> interface on which <strong>the</strong> message was received<br />

from <strong>the</strong> outgoing interface list of <strong>the</strong> (*,G) entry. If on any router <strong>the</strong>re is at least one outgoing<br />

interface listed <strong>for</strong> that (*,G) entry, <strong>the</strong> Prune message is not <strong>for</strong>warded.<br />

Sending Multicast Traffic<br />

With PIM-SM, all multicast traffic must initially originate from <strong>the</strong> RP. A source must unicast traffic to <strong>the</strong><br />

RP so that <strong>the</strong> RP can learn about <strong>the</strong> source and create an SPT to it. Then <strong>the</strong> last-hop DR may create an<br />

SPT directly to <strong>the</strong> source.<br />

1. The source gateway router (first-hop DR) receives <strong>the</strong> multicast packets and creates an (S,G) entry in<br />

its multicast routing table. The first-hop DR encapulates <strong>the</strong> inital multicast packets in PIM Register<br />

packets and unicasts <strong>the</strong>m to <strong>the</strong> RP.<br />

2. The RP decapsulates <strong>the</strong> PIM Register packets and <strong>for</strong>wards <strong>the</strong>m if <strong>the</strong>re are any receivers <strong>for</strong> that<br />

group. The RP sends a PIM Join message towards <strong>the</strong> source. All routers between <strong>the</strong> RP and <strong>the</strong><br />

source, including <strong>the</strong> RP, create an (S,G) entry and list <strong>the</strong> interface on which <strong>the</strong> message was<br />

received as an outgoing interface, thus recreating a SPT to <strong>the</strong> source.<br />

3. Once <strong>the</strong> RP starts receiving multicast traffic via <strong>the</strong> (S,G) it unicasts a Register-Stop message to <strong>the</strong><br />

first-hop DR so that multicast packets are no longer encapsulated in PIM Register packets and unicast.<br />

Upon receiving <strong>the</strong> first multicast packet from a particular source, <strong>the</strong> last-hop DR sends a PIM Join<br />

message to <strong>the</strong> source to create an SPT to it.<br />

4. There are two paths, <strong>the</strong>n, between <strong>the</strong> receiver and <strong>the</strong> source, a direct SPT and an RPT. One router<br />

will receive a multicast packet on two interfaces from <strong>the</strong> same source in this case; this router prunes<br />

<strong>the</strong> shared tree by sending a PIM Prune message to <strong>the</strong> RP that tells all routers between <strong>the</strong> source and<br />

<strong>the</strong> RP to remove <strong>the</strong> outgoing interface from <strong>the</strong> (*,G) entry, and tells <strong>the</strong> RP to prune its SPT to <strong>the</strong><br />

source with a Prune message.<br />

628 PIM Sparse Mode

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