Lab #17

Open vSwitch: Multicast transmission in a LAN with IGMP snooping

The purpose of this lab is to show how IGMP snooping is able to filter multicast IP traffic in a LAN context.

Preparation steps

This lab requires the GNS3 VM to instantiate Docker containers of Linux-based routers.
In particular, it requires that you have already executed once the preparation steps described for Lab #12 and Lab #13.

Experiment steps

Recreate the following topology in GNS3.

Right-click on mcjoin-1 and select "Edit config" to modify its network configuration to assign the 192.168.0.2 IP address to the eth0 interface with netmask 255.255.255.0.
Do the same for mcjoin-2 and mcjoin-3 to assign to their eth0 interfaces the IP addresses 192.168.0.3 and 192.168.0.4 (with netmask 255.255.255.0), respectively.
Start all devices.
Start capture on link connecting OpenvSwitch-1 to mcjoin-2.
Start capture on link connecting OpenvSwitch-1 to mcjoin-3.
Right-click on OpenvSwitch-1, mcjoin-1, mcjoin-2 and mcjoin-3 to open their Auxiliary Console terminal
In the OpenvSwitch-1 terminal and execute the following:

ovs-vsctl set Bridge br0 mcast_snooping_enable=true

to enable IGMP snooping

In the mcjoin-1 terminal execute the following:

```
ping 192.168.0.3
```
to verify that mcjoin-1 can exchange packets with mcjoin-2
```
ping 192.168.0.4
```
to verify that mcjoin-1 can exchange packets with mcjoin-3

In the mcjoin-2 terminal and execute the following:

```
mcjoin 225.1.2.4
```
to let mcjoin-2 join the 225.1.2.4 multicast group.

In the mcjoin-1 terminal and execute the following:

```
mcjoin -s 225.1.2.4
```
to transmit packets to the 225.1.2.4 group; stop packet transmission after a few seconds by pressing ctrl-c at the command line.

Expected results

An IGMP snooping switch does not generate or respond to any multicast messages; instead it passively snoops on IGMP query, report, and leave (IGMP version 2) messages transferred between IP multicast routers/switches and IP multicast hosts to determine the multicast group membership.
Basically, this experiment should show that multicast packets transmitted by mcjoin-1 to the 225.1.2.4 multicast group are transmitted by OpenvSwitch-1 only to ports connected to end-systems that joined the group, i.e. mcjoin-2 in our experiment.
The instance of Wireshark activated onto the link connecting OpenvSwitch-1 to mcjoin-3 should not show such packets.

Traffic analysis

Wireshark shows a sequence of UDP packets sent by 192.168.0.2 (mcjoin-1) to the multicast IP group 225.1.2.4.

Notice that the MAC destination address for the Ethernet frames carrying the multicast IP packets sent to 225.1.2.4 is 01:00:5e:01:02:04.
This address is computed according to what is described in RFC-1112 (Section 6.4):.

An IP host group address is mapped to an Ethernet multicast address
by placing the low-order 23-bits of the IP address into the low-order
23 bits of the Ethernet multicast address 01-00-5E-00-00-00 (hex).
Because there are 28 significant bits in an IP host group address,
more than one host group address may map to the same Ethernet
multicast address.

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Last updated: October 4, 2024 by Roberto Canonico