Multicast notes

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Yellowpin.svg Note: Articles about Proxmox VE 2.0



Multicast allows a single transmission to be delivered to multiple servers at the same time.

This is the basis for cluster communications in Proxmox VE 2.0.

If multicast does not work in your network infrastructure, use unicast instead.


not all hosting companies allow multicast traffic.

Some switches have multicast disabled by default.

test if multicast is working between two nodes with omping

aptitude install omping

start omping on all nodes with the following command and check the output, e.g:

omping node1 node2 node3

test if multicast is working between two nodes with ssmping

Copied from a post by e100 on forum .

  • this uses ssmping

Install this on all nodes .

aptitude install ssmping

run this on Node A:


then on Node B:

asmping ip_for_NODE_A_here

example output

asmping joined (S,G) = (*,
pinging from
  unicast from, seq=1 dist=0 time=0.221 ms
  unicast from, seq=2 dist=0 time=0.229 ms
multicast from, seq=2 dist=0 time=0.261 ms
  unicast from, seq=3 dist=0 time=0.198 ms
multicast from, seq=3 dist=0 time=0.213 ms
  unicast from, seq=4 dist=0 time=0.234 ms
multicast from, seq=4 dist=0 time=0.248 ms
  unicast from, seq=5 dist=0 time=0.249 ms
multicast from, seq=5 dist=0 time=0.263 ms
  unicast from, seq=6 dist=0 time=0.250 ms
multicast from, seq=6 dist=0 time=0.264 ms
  unicast from, seq=7 dist=0 time=0.245 ms
multicast from, seq=7 dist=0 time=0.260 ms

for more information see

man ssmping


less /usr/share/doc/ssmping/README.gz

ssmping notes

  • there are a few other programs included in ssmping which may be of use. here is a list of the files in the package:
apt-file list ssmping
ssmping: /usr/bin/asmping
ssmping: /usr/bin/mcfirst
ssmping: /usr/bin/ssmping
ssmping: /usr/bin/ssmpingd
ssmping: /usr/share/doc/ssmping/README.gz
ssmping: /usr/share/doc/ssmping/changelog.Debian.gz
ssmping: /usr/share/doc/ssmping/copyright
ssmping: /usr/share/man/man1/asmping.1.gz
ssmping: /usr/share/man/man1/mcfirst.1.gz
ssmping: /usr/share/man/man1/ssmping.1.gz
ssmping: /usr/share/man/man1/ssmpingd.1.gz
  • If you want to use apt-file do this:
aptitude install apt-file
apt-file update

then set up a cronjob to do apt-file update weekly or monthly ..

cman & iptables

In case cman crashes with cpg_send_message failed: 9 add those to your rule set:

iptables -A INPUT -m addrtype --dst-type MULTICAST -j ACCEPT
iptables -A INPUT -p udp -m state --state NEW -m multiport –dports 5404,5405 -j ACCEPT

Use unicast instead of multicast

Unicast is a technology for sending messages to a single network destination. In corosync, unicast is implemented as UDP-unicast (UDPU). Due to increased network traffic (compared to multicast) the number of supported nodes is limited, do not use it with more that 4 cluster nodes.

<cman keyfile="/var/lib/pve-cluster/corosync.authkey" transport="udpu"/>
  • activate via GUI
  • add all nodes you want to join in /etc/hosts and reboot
  • before you add a node, make sure you add all other nodes in /etc/hosts

Multicast with Infiniband

IP over Infiniband (IPoIB) supports Multicast but Multicast traffic is limited to 2044 Bytes when using connected mode even if you set a larger MTU on the IPoIB interface.

Corosync has a setting, netmtu, that defaults to 1500 making it compatible with connected mode Infiniband.

Changing netmtu

Changing the netmtu can increase throughput The following information is untested.

Edit the /etc/pve/cluster.conf file Add the section:
<totem netmtu="2044" />

<?xml version="1.0"?>
<cluster name="clustername" config_version="2">
  <totem netmtu="2044" />
  <cman keyfile="/var/lib/pve-cluster/corosync.authkey">
  <clusternode name="node1" votes="1" nodeid="1"/>
  <clusternode name="node2" votes="1" nodeid="2"/>
  <clusternode name="node3" votes="1" nodeid="3"/></clusternodes>

Netgear Managed Switches

the following are pics of setting to get multicast working on our netgear 7300 series switches. for more information see





Cisco Managed Switches

Some cisco switchs are a feature enabled by default : igmp snooping.

These feature is used to filter multicast traffic, to avoid to forward it on each ports.

But this can sometimes do problems with corosync, so it's better to disable it.

For cisco 2960G, by example, you can disable it with:

# conf t
# no ip igmp snooping
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