[j-nsp] Multicast/Broadcast Packets going to EX CPU

[Phil Mayers]

On 05/03/14 16:23, Andy Litzinger wrote:
> Chris, can you elaborate on why low TTL on multicast frames will
> cause high CPU?
>
> Sebastien, as Chris pointed out anything in the 224.0.0.0/24 will hit
> the CPU, but so will a few other ranges that fall into the Link-Local

There's no inherent reason these packets need to hit the CPU on a purely 
layer2 vlan, any more than broadcast or unknown-unicast packets have to.

It might be that the architecture of the device - either by necessity, 
choice or neglect, and either hardware or software - means these packets 
hit the CPU, but just being link-local or multicast doesn't imply it.

TBH, most vendors have similar behaviour in similar product ranges e.g. 
the recent discussion on cisco-nsp re: layer2 multicast and Cisco 3xxx 
range, and I've seen Foundry/Brocade and Extreme switches do it as well.

I've always chalked it up to laziness of implementation; the forwarding 
hardware just isn't setup and/or capable of being set to forward these 
in hardware for VLANs where the CPU doesn't need to see them.

Even IGMP snooping should only be punting IGMP packets to CPU, not all 
multicast!