Multicast mac address isn't learned, igmp-snooping

[egor zem]

I have PIM router which connects to the cat 2960 switch and also I have host which connects to another port on the same switch. Host was joined to the IGMP group 224.1.1.1. I see that the router generates igmp-query and the host respons. IGMP-snooping process sees that process and updates appropriate entries:

2960-5#sh ip igmp snooping mrouter
Vlan    ports
----    -----
  15    Gi2/0/32(dynamic)

2960-5#sh ip igmp snooping groups

15        224.1.1.1                igmp        v2          Gi2/0/32, Gi2/0/33

But when I command "sh mac address-table multicast" I see nothing:

2960#sh mac address-table multicast
Vlan    Mac Address       Type        Ports
----    -----------       ----        -----

What is reason of this problem?

[luckymike33]

Hi,

The command show mac address-table multicast will not display the mac address of hosts who have joined this particular group - remember that traffic is never sourced from a multicast mac address, and the source mac address of traffic is a requirement for the mac address learning process used by a switch. 

Remember also that IGMP is a layer 3 protocol that allows a host to communicate with the router that it wants to receive traffic for a particular group. It does not mean that a particular Multicast MAC address is found off a particular switch port.

HTH

Mike

[egor zem]

There is the following statement from the "CCNP Practical Studies: Switching:

the process of populating the bridge table with multicast MAC addresses is based upon inspection of the destination MAC address, unlike unicast MAC addresses where the source MAC address of unicast frames is examined to generate bridge table entries.

And this book describes other parts of the mac learning process and says that after exchanging IGMP-message MAC-table must be populated by multicast mac-addresses. But later I found some Cisco and Jupiner documentation which says there is two way to perform multicast forwarding - MAC and IP. Default metod is IP multicast forwarding. When this metod is used multicast MAC-addresses isn't learnt and process of packet forwarding uses special forwarding cache which includes list of mapping IP and appropriate interfaces. It all means that this book isn't actual. All modern switchs perform multicast forwarding by IP metod and MAC-addresses don't populate CAM. 

[luckymike33]

Hi,

I am not saying that the switch just broadcasts multicast traffic out all ports, that is the point of igmp snooping, whereby the switch listens in on the IGMP messages between the end hosts and the router to discover off which ports interested hosts reside.

The switch normally uses the IP multicast group address as the reference in the multicast forwarding table, and not the translated multicast MAC address. But that is not to say that you can't configure a switch with a static entry, that is you are configuring a switch to forward for a particular multicast MAC address out a particular port.

Have a look at this

http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3750e_3560e/software/release/12-2_46_se/configuration/guide/scg/swigmp.pdf

HTH

Mike