Multicast IGMP Join Group

Hi Jskfan,

Could you clarify what you are asking?

Multicast addresses function as an identifier to a group of hosts.

The group address should be one in the multicast range:
https://www.iana.org/assignments/multicast-addresses/multicast-addresses.xhtml

The 239.0.0.0/8 range (239.0.0.0 - 239.255.255.255) are described in RFC 2365 (Administratively Scoped IP Multicast):
https://tools.ietf.org/html/rfc2365

A significant point is:
"Administratively scoped IPv4 multicast addresses do not cross administratively defined organizational boundaries, and administratively scoped IPv4 multicast addresses are locally assigned and do not have to be globally unique." (Wiki, https://en.wikipedia.org/wiki/Multicast_address)

Does this help?

Kind regards,
Chris

for instance on this link: https://networklessons.com/cisco/ccie-routing-switching/igmp-version-2/
H2(config)#interface GigabitEthernet 0/1
H2(config-if)#ip igmp join-group 239.1.1.1

I think I understand you to mean "Why would you chose 239.1.1.1 as a group address, as opposed to another range of multicast addressing?".

The 239.1.1.1 address is a multicast address chosen by a network admin (manually) to identify a number of hosts to which the multicast traffic shall be sent. This range is "locally significant" to the network, and is not a global address.

The ip igmp join-group <address> statement tells H2 to join the multicast group (as defined here: https://www.iana.org/assignments/multicast-addresses/multicast-addresses.xhtml)

I notice the link you provide is from a CCIE study guide, if you are studying towards your CCIE, have you already completed the CCNA/CCNP first, if not, it is well worth looking through them as they do go into this stuff in a little more detail.

The 239.1.1.1 address is a multicast address chosen by a network admin (manually) to identify a number of hosts to which the multicast traffic shall be sent. This range is "locally significant" to the network, and is not a global address.

So the Network Admin, can enter any IP address from 224.0.0.0 - 239.255.255.255  ??

Not exactly no, the 224.0.0.0 - 239.255.255.255 range also has sub-networks such as 244.0.0.0 - 224.0.0.255, the hosts in this network are designated to local subnetworks only.

For example the multicast address 224.0.0.10 is used in EIGRP the Cisco interior gateway routing protocol for creating neighbourships and so on (sometimes these are done using unicast, just to add to complication)

If you look at the following two links you may find a little more information about the reserved ranges in multicast addressing.
https://en.wikipedia.org/wiki/Multicast_address
https://www.iana.org/assignments/multicast-addresses/multicast-addresses.xhtml

And this link for the cisco manual regarding join-group:
http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipmulti/command/imc-cr-book/imc_i1.html#wp2941931051

IGMP multicast addresses:
http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipmulti_igmp/configuration/15-sy/imc-igmp-15-sy-book/imc_customizing_igmp.html

The join-group command is not limited to the 239.x.x.x range but you should understand the ranges to understand which values can be used.

For example if you tried to use 224.0.0.9 you would have the following returned: Illegal multicast group address, this is because it is reserved for use with RIPv2.

I see on this link : http://www.tcpipguide.com/free/t_IPMulticastAddressing.htm

224.0.0.0 to 224.0.0.255: Reserved for special “well-known” multicast addresses.
224.0.1.0to 238.255.255.255 Globally-scoped (Internet-wide) multicast addresses.
239.0.0.0 to 239.255.255.255  Administratively-scoped (local) multicast addresses.

Thank you