[Linux-cluster] people having trouble with the default multicast address 239.192.x.x

Nikolas Lam nlam87346 at library.usyd.edu.au
Sun Aug 26 04:01:33 UTC 2007

On Fri, 2007-08-24 at 09:17 -0700, Steven Dake wrote:
> If you are having problems with the default multicast address
> 239.192.x.x it is probably because your switch is not sending those
> packets because of a configuration issue.
> If you have this problem please contact me.  I would like to make a
> cookbook of switch manufacturer/models and possible solutions.
> Using 224.0.0.x is not recommended as these multicast packets are
> broadcoast over the network.
> Regards
> -steve

Hi Steve,

I'm not sure if the problems I'm having are caused by switch
configuration or compatibility - would the following be a good way to
determine if the multicast communications are working?

1) Check what multicast group (IP) the nodes have chosen using netstat
-gn ?

e.g. for host0, I'm guessing that cman is using (as I
think is for the fence_xvmd Xen virtualised host group and I
can't see any activity on

[root at host0 ~]# netstat -g
IPv6/IPv4 Group Memberships
Interface       RefCnt Group
--------------- ------ ---------------------
lo              1      all-systems.mcast.net
eth0            1
eth0            1
eth0            1
eth0            1      all-systems.mcast.net
[root at host0 ~]# 

2) Use tcpdump to check that this node is receiving transmissions from
*other* nodes to the multicast group.


[root at host0 ~]# tcpdump -i any -nn host and ip multicast
tcpdump: WARNING: Promiscuous mode not supported on the "any" device
tcpdump: verbose output suppressed, use -v or -vv for full protocol
listening on any, link-type LINUX_SLL (Linux cooked), capture size 96
13:42:03.674676 IP > UDP, length
13:42:03.767820 IP > UDP, length
13:42:03.962382 IP > UDP, length
13:42:04.162652 IP > UDP, length
13:42:04.267768 IP > UDP, length
13:42:04.450195 IP > UDP, length
13:42:04.650646 IP > UDP, length
13:42:04.766404 IP > UDP, length
13:42:04.940367 IP > UDP, length

9 packets captured
19 packets received by filter
0 packets dropped by kernel
[root at host0 ~]# 

In the above example I can only see transmissions to from (hosts0's IP address). If things were working properly, I'd
be able to see transmissions from other source addresses wouldn't I?

E.g. I should have at least some entries like this

13:42:04.940367 IP > UDP, length

where the source address is a different node, right?


Nik Lam

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