Difference between revisions of "Multicast notes"

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==== Cisco Managed Switches  ====
==== 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
# conf t
# no ip igmp snooping
# no ip igmp snooping
==== Netgear Managed Switches  ====
==== Netgear Managed Switches  ====

Revision as of 20:29, 13 October 2014


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 and higher, which uses corosync and cman, and would apply to any other solution which utilizes those clustering tools.

If multicast does not work in your network infrastructure, you should fix it so that it does. If all else fails, use unicast instead, but beware of the node count limitations with unicast.

IGMP snooping

IGMP snooping prevents flooding multicast traffic to all ports in the broadcast domain by only allowing traffic destined for ports which have solicited such traffic. IGMP snooping is a feature offered by most major switch manufacturers and is often enabled by default on switches. In order for a switch to properly snoop the IGMP traffic, there must be an IGMP querier on the network. If no querier is present, IGMP snooping will actively prevent ALL IGMP/Multicast traffic from being delivered!

If IGMP snooping is disabled, all multicast traffic will be delivered to all ports which may add unnecessary load, potentially allowing a denial of service attack.

IGMP querier

An IGMP querier is a multicast router that generates IGMP queries. IGMP snooping relies on these queries which are unconditionally forwarded to all ports, as the replies from the destination ports is what builds the internal tables in the switch to allow it to know which traffic to forward.

IGMP querier can be enabled on your router, switch, or even linux bridges.

Configuring IGMP/Multicast

Ensuring IGMP Snooping and Querier are enabled on your network (recommended)

Juniper - JunOS

Juniper EX switches, by default, enable IGMP snooping on all vlans as can be seen by this config snippet:

[edit protocols]
user@switch# show igmp-snooping
vlan all;

However, IGMP querier is not enabled by default. If you are using RVIs (Routed Virtual Interfaces) on your switch already, you can enabled IGMP v2 on the interface which enables the querier. However, most administrators do not use RVIs in all vlans on their switches and should be configured instead on the router. The below config setting is the same on Juniper EX switches using RVIs as it is on Juniper SRX service gateways/routers, and effectively enables IGMP querier on the specified interface/vlan. Note you must set this on all vlans which require multicast!:

set protocols igmp $iface version 2



Linux: Enabling Multicast querier on bridges

If your router or switch does not support enabling a multicast querier, and you are using a classic linux bridge (not Open vSwitch), then you can enable the multicast querier on the Linux bridge by adding this statement to your /etc/network/interfaces bridge configuration:

  post-up ( echo 1 > /sys/devices/virtual/net/$IFACE/bridge/multicast_querier )

Disabling IGMP Snooping (not recommended)

Juniper - JunOS

set protocols igmp-snooping vlan all disable

Cisco Managed Switches

# conf t
# no ip igmp snooping

Netgear Managed Switches

the following are pics of setting to get multicast working on our netgear 7300 series switches. for more information see http://documentation.netgear.com/gs700at/enu/202-10360-01/GS700AT%20Series%20UG-06-18.html





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>


Testing multicast

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 (if all else fails)

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