Both of these addresses are multicast addresses, however each one is used for a different purpose. The 126.96.36.199 IP address is a well known multicast IP and it addresses all the hosts on a network segment. This is used when the router sends out the membership query because all hosts are asked if they are (still) interested in receiving the multicast traffic.
The 188.8.131.52 mutlicast IP address refers to a multicast group only. This is the IP address that corresponds ONLY to those hosts that have expressed interest in taking part in the specific group. This is the actual IP address which is used to send the multicast traffic to the hosts participating in the group.
With regards to the Administratively-scoped (local) multicast addresses, am i right to say that we could select from the ranges below when assigning the multicast group address?
184.108.40.206 - 220.127.116.11
What is the exact rule for assigning the IP address for the multicast group?
Can we also select from the below addresses?
18.104.22.168 - 22.214.171.124 - Reserved for special “well-known” multicast addresses
224.01.0 - 126.96.36.199 - Globally-scoped
The 188.8.131.52 to 184.108.40.206 range of multicast IP addresses known as “Administratively scoped” addresses. These are defined by RFC 2365 and are reserved for private use within an organization. So to answer your question, yes, when configuring multicast groups it is best practice to use these addresses.
Having said that, as you probably know, there are various special-use multicast addresses that have various purposes as defined by the IANA and by various RFCs. Although I don’t like linking to Wikipedia, I find that their summary of the IPv4 multicast address groups is quite comprehensive and complete, and you can find it at the following link:
So can you use these other multicast addresses to define a multicast group? Well, yes, nothing stops you from doing that. However, if you overlap with various well known multicast addresses such as those used by OSPF or EIGRP, then this will result in the malfunctioning of any such routing protocols you may have running on your network.
Even though technically you can indeed configure them, practically, it can present a problem on your network.
I enabled debug ip igmp on cisco routers via GNS3 but I can’t see the debug report.
what is your lab environment? is it real router or emulator?
I tried with IOSv image and c7200 router both but same result.
Both real equipment as well as the GNS3 should have the same debug output. When implementing debugging, make sure that you also employ the terminal monitor command in order to tell the device that all debug output should appear on the terminal screen. Also, verify that IGMP is indeed enabled and that you have achieved the necessary connectivity between the hosts and R1 before implementing the commands on the hosts.
Take a look at these and let us know your results!
Hi Laz, Can you explain it please ?Please correct me if am wrong. 220.127.116.11 addresses all the host on a network segment. This IP is used by the router to send membership query to all host to ask whoever interested in multicast traffic. And then followed by this you say that 18.104.22.168 multicast IP is the actual IP address that is used to send the multicast traffic to the host participating in the group ? I don’t understand here.
Do you mean 22.214.171.124 is the multicast IP address that host uses when they are sending the membership report ?
Unlike the 126.96.36.199 address, which is a well known and reserved address, the 188.8.131.52 multicast group address is configurable. This means that some hosts choose to join it, while others may choose not to. It is the destination address used by multicast sources to send their content to. A video server, for example, would use this as the destination address to reach all hosts interested in its multicast stream.
Yes, this address is also used by hosts to send their membership reports. The membership reports actually tell the multicast router that the host sending the report wants to receive multicast traffic destined for this particular multicast IP address.
Speaking of enabling multicast routing, in your topology, H1 and H2 are Cisco routers, but in your configuration example, I did not see the command ip multicast-routing on H1/H2. For routers to participate in multicast, do you not have to enable on all routers in global config mode even when they are behaving as hosts??
As you correctly stated, H1 and H2 are actually routers, but are functioning as hosts. Within the framework of multicast, hosts (even if they are Cisco routers) do not perform multicast routing. So to answer your question, no, multicast routing does not (and actually should not) be enabled on routers that play the role of a host.
Think about it this way. If you have a Windows PC connected to the network, and it wants to participate in a multicast group, it can do so even through it does not have the capability of multicast routing itself. For hosts of any type to participate in multicast, they simply have to support IGMP and the functionality of joining of multicast groups.