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Sending PIM Join Messages to the PE Routers
To receive a multicast broadcast from a multicast
network, a CE router must send a PIM Join message to the C-RP. The
process described in this section refers to Figure 18.
The CE5 router needs to receive a multicast broadcast
from multicast source 224.1.1.1. To receive the broadcast, it sends
a PIM Join message to the C-RP (the PE3 router):
- The PIM Join message is sent through the multicast interface,
and a GRE header is prepended to the message. The GRE header contains
the VPN group ID and the loopback address of the PE3 router.
- The PIM Join message is then sent through the PIM encapsulation
interface and a register header is prepended to the packet. The register
header contains the IP address of the SP-RP and the loopback address
of the PE3 router.
- The PIM Join message is sent to the SP-RP by means of
unicast routing.
- On the SP-RP, the register header is stripped off (the
GRE header remains) and the packet is sent to all the PE routers.
- The PE2 router receives the packet, and because the link
to the C-RP is through the PE2 router, it sends the packet through
the multicast interface to remove the GRE header.
- Finally, the PIM Join message is sent to the C-RP.
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