Recent Articles

Google And The Need For Speed
If the scientists who claim news content has a half-life of 36 hours online before it falls off the radar of Internet users are correct, Google's successor online might match up...

EIGRP Stuck-In-Active Routes
Passing the BSCI exam and earning your CCNP is all about knowing the details, and when it comes to EIGRP SIA routes, there are plenty of details to know.

Frame Relay DLCIs And Mappings
Passing the CCNA is tough, and one of the toughest parts is keeping all the acronyms straight! Frame Relay has plenty of those, and today we´re going to examine what DLCIs do and how they´re mapped on a Cisco router.

Switches, QoS, And Cisco's Networking Model
QoS is a big topic on your BCMSN and CCNP exams...

AT&T Building 40Gb Net Backbone
The telecom giant plans to spend between $8 billion and $8.5 billion to upgrade its Multi Protocol Label Switching (MPLS) backbone.

Troubleshooting Directly Connected Serial Interfaces
CCNA exam success depends largely on noticing the details, and this is especially true of configurations involving directly connected serial interfaces.

Route Summarization And The OSPF Null Interface
CCNP exam success, particularly on the BSCI exam, demands you understand the details of route summarization.

Server Load Balancing (SLB)
When you´re working on your BCMSN exam on your way to CCNP certification, you´ll read at length about how Cisco routers and multilayer switches can work to provide...

07.24.06


Configuring CGMP On Routers & Switches

By Chris Bryant


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. CGMP allows the multicast 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


When CGMP is first enabled on both the multicast router and switch, the router will send a CGMP Join message, informing the switch that a multicast router is now connected to it. This particular CGMP Join will contain a Group Destination Address (GDA) of 0000.0000.0000 and the MAC address of the sending 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, letting the switch know that the multicast router is online.


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

A workstation connected to the switch on 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. (Don´t forget that the MAC address table is also referred to as the CAM table or the bridging table.)

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 fast 0/5. 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. Instead, the switch will forward a copy of the frame to each port that it knows leads to a member of the multicast group.

Two major benefits of CGMP are the explicit Join and Leave Group messages. In the next part of this BCMSN exam tutorial, we'll take a look at the Leave Group messages.


About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage, home of FREE CCNA and CCNP tutorials and daily exam questions, as well as The Ultimate CCNA and CCNP Study Packages.

About NetworkNewz
NetworkNewz editors, writers and contributors focus on both the big picture and the details of networking. At NetworkNewz our goal is to deliver to you The Key To Network Management.

NetworkNewz is brought to you by:

SecurityConfig.com NetworkingFiles.com
ITmanagementNews.com WebProASP.com
DatabaseProNews.com SQLProNews.com
ITcertificationNews.com SysAdminNews.com
LinuxProNews.com WirelessProNews.com
CProgrammingTrends.com ITmanagementNews.com


-- NetworkNewzis an iEntry, Inc. publication --
iEntry, Inc. 2549 Richmond Rd. Lexington KY, 40509
2006 iEntry, Inc.  All Rights Reserved  Privacy Policy  Legal

archives | advertising info | news headlines | free newsletters | comments/feedback | submit article


The Keys To Network Management Ask Questions in the Networking Forum NetworkNewz News Archives About Us Feedback NetworkNewz Home Page About Article Archive News Downloads WebProWorld Forums Jayde iEntry Advertise Contact