Illustrated TCP/IP Illustrated TCP/IP
by Matthew G. Naugle
Wiley Computer Publishing, John Wiley & Sons, Inc.
ISBN: 0471196568   Pub Date: 11/01/98  

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Chapter 283
Adding Interfaces

A router can add interfaces for which it received a Graft message (rejoin a branch for a group) or an IGMP membership report. If the router already has state information about a group (it has built an entry for the group), it simply adds or refreshes the interface entry on which the IGMP message or Graft message was received. If the outgoing list entry is empty, the router will send a Graft message upstream towards the source. Any router that receives this message will use that received interface as the outgoing interface for the existing (source, group) pair.

If the router has no state information at all for the (source, group) pair, it will do nothing, for it knows that PIM-DM routers will deliver a multicast datagram to all interfaces when creating a state for the group.

PIM-Graft messages are positive acknowledged. A PIM-Graft message is unicast to the upstream router. The upstream router changes the Graft message into a Graft ACK and sends it back to the originating router.

Adding Interfaces

•  Routers add interfaces by using the Graft message.
•  When a router receives a Graft it will make one of the following decisions:
•  If the router already has information about the group, it simply adds or refreshes the interface for that group.
•  If the outgoing list is empty (but it knows about the group) that router will send a Graft message upstream.
•  If the router does not know about the group, it will do nothing with the received Graft message knowing that multicast datagrams for an unknown group will always be forwarded and it will wait for those frames.
•  Graft messages are acknowledged by each router to the source of the Graft message.
•  It is unicast to upstream routers.


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