[Linux-cluster] Multicast

Erwan Velu erwan at seanodes.com
Tue Aug 21 08:09:24 UTC 2007

Fajar A. Nugraha wrote:
> Patricio A. Bruna wrote:
>> Hi,
>> Does cluster suite use Multicast in Red Hat 5?
>> If is so, how can i test my Switchs allow multicast traffic?
> The switches that I tested allow multicast traffic by default.
> You can test it by running some programs that uses multicast address, like :
> - ping Note that by default RHEL ignores broadcast pings, so
Regarding the **IANA Guidelines for IPv4 Multicast Address Assignments 
using some 224.0.0/24 based addresses will bypass  some multicast mecanisms.

The 224.0.0/24 addresses are dedicated to the local network routing 
messages meaning all ports of your switch will receive this traffic.
But for the others multicast addresses, 3com/cisco and most of high end 
switches need to be configured to do the routing.

I mean you can find the "snooping" and the "querier/query" options in 
the multicast's switch configuration. Regarding the configuration, the 
mulitcast will works or not.

Snooping Off: all multicast traffic works
Snoop On: query off: Mcast is not working except for 224.0.0/24
Snoop On, query on: all multicast traffic works

This have been confirmed on a 3com 3870 switch. Its default 
configuration is set on "snoop on/query off".
In such configuration, you must enable the "query/querier" option at 
least on a switch to make the multicast working.

So pinging 224.0.0/24 could generates some false positive results 
because in some configuration 224.0.0/24 could works whereas all the 
other multicast adresses are not.

On some low & middle end switches (dell etc..), activating the snooping 
enable the "querier" at the same time so this situation doesn't exists.

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