[Linux-cluster] multicast howto

[Kovacs, Corey J.]

Wolfgang, you can't arbitrarily ping multicast addresses as they don't really
exist in 
the sense that an ethernet interface exists as a card. Multicast is a
"subscription" 
based concept. You have to be listening for multicast traffic to receive it
at all. To
"listen", one "joins" a multicast group (bind to a mcast ip address) and the
router, if
configured properly will route mcast traffic to your interface. There are
certain mcast
addresses you should never use explicitly. As mentioned, 224.0.0.1 is one of
them, there
are others but I can't recall what they are. Your other config didn't work
(with respet 
to multicast) because all of your nodes were listening to different multicast
"groups"
which is like trying to dial in on a party line, but everone using the wrong
phone number.

Important to reitterate, Multicast has to be configured by whoever runs your
router(s).
If Multicast is not enabled on a switch, even if it is on your network, it
turns in to 
broadcast on that switch, which defeats the purpose of multicast in the first
place.


Hope this clears some multicast stuff up for you.

-----Original Message-----
From: linux-cluster-bounces at redhat.com
[mailto:linux-cluster-bounces at redhat.com] On Behalf Of Wolfgang Pauli
Sent: Wednesday, April 26, 2006 8:46 PM
To: sdake at redhat.com; linux clustering
Subject: Re: [Linux-cluster] multicast howto

Thanks!

First, I had to figure out how multicast works. Still don't fully understand
it. I can ping 224.0.0.1 and I get responses from all hosts in the same
subnet. It tried to ping 225.0.0.8 but that does not really work. But I don't
know whether it has to.
I have changed the cluster.conf to have only two nodes. Just to get the basic
understanding. I have node dream on subnet 210 and neo on 223. They still
form their own clusters. Should I just try different addresses, or do the
switches/routers have to be programmed for that?

regards,

wolfgang

<?xml version="1.0" ?>
<cluster config_version="2" name="alpha_cluster">
        <fence_daemon post_fail_delay="0" post_join_delay="3"/>
        <clusternodes>
                <clusternode name="dream" votes="1">
                        <multicast addr="225.0.0.9" interface="eth0"/>
                        <fence>
                                <method name="1">
                                        <device name="human" 
nodename="dream"/>
                                </method>
                        </fence>
                </clusternode>
                <clusternode name="neo" votes="1">
                        <multicast addr="225.0.0.9" interface="eth0"/>
                        <fence>
                                <method name="1">
                                        <device name="human" nodename="neo"/>
                                </method>
                        </fence>
                </clusternode>
        </clusternodes>
        <cman expected_votes="1" two_node="1">
                <multicast addr="225.0.0.9"/>
        </cman>
        <fencedevices>
                <fencedevice agent="fence_manual" name="human"/>
        </fencedevices>
        <rm>
                <failoverdomains/>
                <resources/>
        </rm>
</cluster>

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