Users Guide

IPv4 and IPv6 Multicast 1539
• Limiting the number of packets sent to the RP by the first-hop router.
When a multicast data source (S) starts sending data destined for a
multicast group (G), the first-hop router receives these packets and traps
them to its local CPU. A Dell EMC Networking first-hop router
immediately blocks further data packets in the stream and prevents them
from reaching the CPU. The first-hop router then unicast-encapsulates
the first received data packet in the form of a PIM Register message and
software forwards it to the RP.
When a Dell EMC Networking first-hop router subsequently receives the
PIM Join from the RP, the block is replaced with a regular multicast
forwarding entry so that subsequent data packets are forwarded in the
hardware.
If the initial Register message(s) does not reach the RP, or the PIM Join
sent in response does not reach the first-hop router, then the data stream
would never get forwarded. To solve this, the negative entry is timed out
and removed after 3 seconds so that the process can be repeated until it
succeeds.
• In Phase 3—Shortest Path Tree, the last-hop router initiates a switchover
to the SPT tree by sending a PIM (S,G) Join message towards the source as
soon as it receives the first data packet via the (*,G) shared tree. Per the
standard, this function is used to detect suboptimal routing of multicast
traffic. Dell EMC Networking multicast eliminates the
SwitchToSptDesired(S,G) function and performs as if the
SwitchToSptDesired(S,G) function always returns “true” as soon as it
receives the first multicast packet instead of waiting for 30 seconds.
• Dell EMC Networking RPs do not wait to receive the native multicast
data but immediately respond to the PIM (S,G) Join by sending a 'Register
Stop' message to the source's first-hop router to inform it that it can stop
sending the encapsulated Register messages. This removes the load from
the CPU of the first-hop router and the RP, as they no longer need to
encapsulate and de-encapsulate register messages with multicast data.
These optimizations significantly reduce the load on first-hop routers and
RPs to encapsulate/de-encapsulate PIM register messages and their associated
multicast data. In addition, the switchover to the SPT is initiated
immediately upon the first multicast packet reaching the last-hop router. This