DELL N3000 User Manual

Managing IPv4 and IPv6 Multicast

creates a performance problem in that it limits the number of packets that 

can be processed and places a high load on the CPUs in the first hop and RP 

routers, which can then adversely affect other router functions.

Dell Networking switches perform the following optimizations to reduce the 

impact of multicast encapsulation/de-encapsulation and provide a higher 

level of multicast performance in the network.

• 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 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 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 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 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