CCNP Multicasting Tutorial - BSCI Exam:

The Cisco Group Membership Protocol And Its Join & Leave Messages

Chris Bryant, CCIE #12933

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CCNP BSCI Exam Tutorials

In the previous installment of this CCNP BSCI exam tutorial series, we took a look at IGMP Snooping and how it helps Layer 2 switches handle multicasting traffic in an efficient manner.

If a Layer Two switch doesn't have the capabilities to run IGMP Snooping, it will be able to run CGMP - Cisco Group Membership Protocol.  (As long as it's a Cisco switch, that is - CGMP is CIsco-proprietary!)

CGMP allows the multicasting router to work with the Layer Two switch to eliminate unnecessary multicast forwarding. 

CGMP will be enabled on both the multicast router and the switch, but the router's going to do all the work.

The router will be sending Join and Leave messages to the switch as needed.   PIM must be running on the router interface facing the switch before enabling CGMP, as you can see:

R1(config)#int e0
R1(config-if)#ip cgmp
WARNING: CGMP requires PIM enabled on interface
R1(config-if)#ip pim sparse
R1(config-if)#ip cgmp

Let's look at two examples of when and where CGMP Join and Leave messages will be sent.

When CGMP is first enabled on both the multicast router and switch, the router will send a CGMP Join message, informing the switch of the multicasting router's presence.

This particular CGMP Join will contain a Group Destination Address (GDA) of 0000.0000.0000 and the MAC address of the sending router interface.

The GDA is used to identify the multicast group, so when this is set to all zeroes, the switch knows this is an introductory CGMP Join.  This GDA lets the switch know that the multicast router is online.

The switch makes an entry in its MAC table that a multicast router can be found off the port that the CGMP Join came in on.  The router will send this CGMP Join to the switch every minute to serve as a keepalive.

We'll see how a host PC joins a multicast group right after this brief and important message - and I look forward to continuing to help you earn your CCNP!

Earn Your CCNP With The Personal Guarantee Of Chris Bryant, CCIE #12933:

“I GUARANTEE You'll Pass The Current CCNP Exams - BSCI, ONT, ISCW, and BCMSN - Before The July 31, 2010 Cutoff Date With My CCNP Study Packages ...

... And If You Don't Pass FOR ANY REASON Before That Time, I'll Give You a 100% Free CCNP Study Package Download For The New Exam Track!"

You Also Get A FREE CCNA Security Study Package With The Purchase Of Any CCNP Study Package!

A workstation connected to port 0/5 now wishes to join multicast group 225.1.1.1.  The Join message is sent to the multicast router, but first it will pass through the switch.

The switch will do what you'd expect it to do - read the source MAC address and make an entry for it in the MAC address table as being off port fast 0/5 if there's not an entry already there.

The router will then receive the Join request, and send a CGMP Join back to the switch. 

This CGMP Join will contain both the multicast group's MAC address and the requesting host's MAC address.  Now the switch knows about the multicast group 225.1.1.1 and that a member of that group is found off port fast0/5. 

In the future, when the switch receives frames destined for that multicast group, the switch will not flood the frame as it would an unknown multicast - the switch will forward a copy of the frame to each port that it knows leads to a member of the multicast group.

CGMP Leave Messages

CGMP Leaves work much the same way as Join messages, but the router and switch have to allow for the possibility that there are other members on the switch that still need that multicast group's traffic. 

In the following example, two hosts that are receiving traffic from the multicast group 225.1.1.1 are connected to the same switch.

One of the hosts is sending an CGMP Leave.  The multicast router receives this request, and in return sends a group-specific CGMP query back to the switch.  The switch will then flood this frame so hosts on every other port receives a copy.  Any host that wishes to continue to receive this group's traffic must respond to this query. 

As shown below, the remaining host will send such a response, and the router in turn will send a CGMP Leave to the switch, telling the switch to delete only the host that originally sent the CGMP Leave from the group.

If no other host responds to the Group-Specific Query, the router will still send a CGMP Leave to the switch.  However, the CGMP Leave will tell the switch to remove the entire group listing from the MAC table. 

You may be wondering how the switch differentiates CGMP Joins and Leaves from all the other frames it processes.  The switch recognizes both of those by their destination address of 01-00-0c-dd-dd-dd, a reserved Layer 2 address used only for this purpose.

I hope you've enjoyed our multi-part Multicasting Tutorial! Whether you're preparing for the BSCI or CCNP ROUTE exams, be sure to BSCI Exam Resource Page for plenty of additional tutorials!

Visit every day in March and April for new CCNP study material - and watch the Bulldog Blog, too!

Earn Your CCNP With The Personal Guarantee Of Chris Bryant, CCIE #12933:

“I GUARANTEE You'll Pass The Current CCNP Exams - BSCI, ONT, ISCW, and BCMSN - Before The July 31, 2010 Cutoff Date With My CCNP Study Packages ...

... And If You Don't Pass FOR ANY REASON Before That Time, I'll Give You a 100% Free CCNP Study Package Download For The New Exam Track!"

You Also Get A FREE CCNA Security Study Package With The Purchase Of Any CCNP Study Package!