On 09/18/2013 12:47 AM, Andy Litzinger wrote:
This does not appear to always be true. I obviously haven't tested
every multicast address, but it seems that pretty much all multicast
traffic directed to 224.0.0.0-239.0.0.255 will cause the switch to
flood traffic to all ports in the vlan.
But addresses from 239.0.1.0 and up seem to work as I expect.
Sounds like you're running into the fact that >1 multicast group range
maps to 1 multicast MAC range because there's not enough room to fit the
28 bits into the multcast mac lower-half.
239.0.0.0/24 maps to the same MAC address range as 224.0.0.0/24 and the
latter is defined as "no snooping" because it's the link-local/control
range, hence both flood.
It's a cisco document, but see here:
http://www.cisco.com/en/US/tech/tk828/technologies_white_paper09186a00802d4643.shtml#wp1002391
Some newer equipment (e.g. Cisco Sup2T) does IP-based multcast snooping
- it reads into the IP header, rather than relying on the multicast mac
- which solves the problem. No idea if the EX4xxx can do this, but I
doubt it somehow.
Basically, don't use (224-239).{0,128).0.0/24 for multicast.
also- what I'm trying to do is relatively simple but maybe I'm going
about it the wrong way. I have groups of servers in the vlan that
use multicast packets as a periodic "heartbeat" to keep track of each
other. I'd like to make sure the multicast heartbeat only goes to
other servers that subscribe to the same multicast address- not send
it to every server in the vlan. does my config seem like a valid way
to do this? I don't need to route the multicast across subnets.
FWIW we've had big problems with apps that do that. IMHO a
broadcast-discovery / unicast-keepalive is superior to those kinds of
multicast solutions. App developers often fail to test in realistic
environments, and don't account for the myriad of ways that multicast
can go wrong (e.g. the above issue!). We discourage multicast heartbeats
for that reason.
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