H3C Technologies H3C S12500 Series Switches - Manual (Page 394)

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DRs at the receiver side. The RP acts as a transfer station for all IPv6 multicast packets. The whole process
involves the following issues:
• The DR at the source side and the RP need to implement complicated encapsulation and
de-encapsulation of IPv6 multicast packets.
• IPv6 multicast packets are delivered along a path that might not be the shortest one.
• An increase in IPv6 multicast traffic heavily burdens the RP, increasing the risk of failure.
To solve the issues, IPv6 PIM-SM allows an RP or the DR at the receiver side to initiate a switchover to SPT
process when the traffic rate exceeds the threshold:
1. The RP initiates a switchover to SPT process.
After receiving an IPv6 multicast packet, the RP sends an (S, G) join message hop by hop toward
the IPv6 multicast source to establish an SPT between the DR at the source side and the RP.
Subsequent IPv6 multicast data travels along the established SPT to the RP.
For more information about the switchover to SPT initiated by the RP, see "
Multicast source
registration ."
2. The receiver-side DR initiates a switchover to SPT process
After receiving the first multicast packet, the receiver-side DR initiates a switchover to SPT process,
as follows:
{
The receiver-side DR sends an (S, G) join message hop by hop toward the IPv6 multicast source.
When the join message reaches the source-side DR, all the routers on the path have installed the
(S, G) entry in their forwarding table, and thus an SPT branch is established.
{
When the IPv6 multicast packets travel to the router where the RPT and the SPT deviate, the
router drops the multicast packets received from the RPT and sends an RP-bit prune message hop
by hop to the RP. After receiving this prune message, the RP sends a prune message toward the
IPv6 multicast source—suppose only one receiver exists—to implement switchover to SPT.
{
Finally, IPv6 multicast data is directly sent from the source to the receivers along the SPT.
IPv6 PIM-SM builds SPTs through switchover to SPT more economically than IPv6 PIM-DM does through
the flood-and-prune mechanism.
Assert
IPv6 PIM-SM uses the similar assert mechanism as IPv6 PIM-DM does. For more information, see
" Assert ."
IPv6 administrative scoping overview
Division of IPv6 PIM-SM domains
Typically, an IPv6 PIM-SM domain contains only one BSR, which is responsible for advertising RP-set
information within the entire IPv6 PIM-SM domain. The information for all multicast groups is forwarded
within the network scope administered by the BSR. We call this IPv6 non-scoped BSR mechanism.
To implement refined management, an IPv6 PIM-SM domain can be divided into one IPv6 global-scoped
zone and multiple IPv6 administratively scoped zones (IPv6 admin-scoped zones). We call this IPv6
administrative scoping mechanism.
The IPv6 administrative scoping mechanism effectively releases stress on the management in a
single-BSR domain and enables provision of zone-specific services using private group addresses.
IPv6 admin-scoped zones correspond to IPv6 multicast groups with different scope values in their group
addresses. The boundary of the IPv6 admin-scoped zone is formed by zone border routers (ZBRs). Each