112
Figure 47 RPT building at the multicast source side
, the process for building a source-side RPT is relatively simple:
4.
When a multicast source sends multicast packets to the multicast group G, the DF in each
subnet unconditionally forwards the packets to the RP.
5.
The routers along the path from the source's directly connected router to the RP constitute an
RPT branch. Each router on this branch adds to its forwarding table a (*, G) entry.
After a bidirectional RPT is built, the multicast sources send multicast traffic to the RP along the
source-side RPT. Then, the RP forwards the traffic to the receivers along the receiver-side RPT.
IMPORTANT:
If a receiver and a multicast source are at the same side of the RP, the source-side RPT and the
receiver-side RPT might meet at a node before reaching the RP. In this case, the multicast packets
from the multicast source to the receiver are directly forwarded by the node, instead of by the RP.
Administrative scoping overview
Typically, a PIM-SM domain or a BIDIR-PIM domain contains only one BSR, which is responsible for
advertising RP-set information within the entire domain. The information about all multicast groups is
forwarded within the network that the BSR administers. This is called the "non-scoped BSR
mechanism."
Administrative scoping mechanism
To implement refined management, you can divide a PIM-SM domain or BIDIR-PIM domain into a
global-scoped zone and multiple administratively-scoped zones (admin-scoped zones). This is
called the "administrative scoping mechanism."
The administrative scoping mechanism effectively releases stress on the management in a
single-BSR domain and enables provision of zone-specific services through private group
addresses.
Admin-scoped zones are divided for multicast groups. Zone border routers (ZBRs) form the
boundary of an admin-scoped zone. Each admin-scoped zone maintains one BSR for multicast
groups within a specific range. Multicast protocol packets, such as assert messages and BSMs, for a
specific group range cannot cross the boundary of the admin-scoped zone for the group range.
Source
Server A
Server B
Host B
Host C
Receiver
Receiver
Multicast packets
Source-side RPT
RP
Source
Host A
Receiver
