To close out our deep dive on PIM dense mode,
let's spend a few minutes discussing a PIM message that
only appears in this implementation, the PIM Graft and Graft‑Ack packet.
A PIM Graft is very similar to a PIM join with a few important differences.
The graft is used to clear a previous PIM prune for an
interface to get multicast traffic flowing.
In this example,
the receiver has not signaled an interest in multicast group, and the
upstream traffic has been flooded and then pruned back. When the receiver
starts to signal its interest in multicast, the LHR, Last Hop Router, must
start the process of clearing the prune upstream to get each router to add
its downstream interface to an outgoing interface list again. Unlike the
PIM join, a PIM Graft is sent to the unicast address of the upstream
designated router.
The PIM Graft is message Type 6.
Under the PIM options, we see that the upstream neighbor is the DR for the
segment and that there's one group in this PIM Graft.
The group record shows the group address and has one
join listed for the source 10.99.1.100.
This is functionally identical to the PIM join we already looked at.
Next is the PIM Graft‑Ack packet.
The Graft‑Ack is sent to the graft originator as a response
notifying the downstream router that it's in the process of
adding the interface to its OIL.
The Graft‑Ack serves as a reliable mechanism to ensure the
downstream router won't need to send more graft requests. From
the group details, it's clear that the same multicast group and
source are being acknowledged.
The only real difference between a Graft and Graft‑Ack message is
the message type. GraftAck is PIM message Type 7.
In this module, we discussed the basic operation of PIM
dense mode. There were command line demonstrations of the
dense mode flood and prune method, as well as grafting new multicast receivers.
From a packet analysis perspective,
we dissected the PIM join and prune packet, the PIM Graft packet and finished with the PIM Graft‑Ack.