[j-nsp] EX cpu performance under multicast replication load?

[Clarke Morledge]

I am seeing some bothersome CPU performance issues on EX switches, mostly 
on the less powerful units like the 2200s, when it comes to handling 
multicast.

In practical situations, I do not see much multicast traffic in general, 
except on our campus we do get a lot of Apple Bonjour traffic related to 
Multicast DNS.  Sometimes, a single host will go a little bonkers with 
repeated MDNS packets.   In one case, I have seen where a flood of about 
100 multicast packets per second, related to Bonjour, will cause the CPU 
on the lower end EX switches to spike up dramatically, resulting in loss 
of management of the switch during peak loads.   For example, the switch 
will stop handling ICMP echo requests to its management IP, or it will 
miss RADIUS packets.

Can someone walk me through the EX architecture a bit to tell me if this 
is expected behavior?  I am assuming the EX CPU is actually handling the 
multicast replication of Ethernet frames received to be sent out other 
ports, but it seems like 100 packets per second should not be a big deal 
to worry over.  So something looks awry.

Oddly enough, I do not see any performance issues when straight-up 
broadcast traffic hits these kind of packet rates.

To mitigate against this, I guess I could use QoS to prioritize management 
frames over user multicast data, but if the issue is about packet 
replication and not forwarding, I am entirely convinced that the standard
marking and handling QoS parameters would be effective.

Any ideas?

Clarke Morledge
College of William and Mary
Information Technology - Network Engineering
Jones Hall (Room 18)
Williamsburg VA 23187