CCNP Tutorial For The BSCI Exam:

The Multicast Tree And IGMP

Chris Bryant, CCIE #12933

Vital Reading And Watching For All Cisco Certification Candidates:

The July 2010 CCNP Changes (And How To Beat Them!)

In the first part of this CCNP tutorial series on multicasting, we looked at the basics of multicasting and how it differs from the unicasts and broadcasts we used in our CCNA studies.

Today, we'll take a look at how a multicast tree is built - and this being Cisco networking, we know there will be multiple ways to join such a tree! We'll take a look at each of those options as well.

Just as there are different routing protocols, there are different multicasting protocols.  The BSCI test focuses on Protocol Independent Multicast (PIM), so we'll focus on the different variations of PIM.

Regardless of the PIM version you run, a multicast tree will be created after PIM is configured.

The source of the multicast traffic sits at the top of the tree, sending the multicast stream out to the network.  The recipients are on branches, and the router at the "top" of each branch needs to know if another router on that branch is part of the destination multicast group.

If there are no downstream routers that need the multicast stream, that router will not forward the traffic.  This prevents the network from being overcome with multicast traffic.

Naturally, if there are downstream routers that are members of that particular multicast group, the router at the top of the branch will forward the multicast traffic.

This is one of those concepts that sounds a bit strange until you can see it in action.

In the above illustration, there are three multicast group members, each labeled "MG".  Just as we don't want unnecessary broadcasts on our network - they take up valuable bandwidth and the routers still have to process them - we don't want multicast traffic being forwarded down a branch of this tree if there are no members of that particular multicast group on that branch. 

The middle branch of this multicast tree has no member of the multicast group, so multicast traffic shouldn't be sent down that branch. 

The left branch does have a member at the edge, as does the right branch, so traffic for that multicast group will flow all the way down those two branches.  The routers on the multicast tree branches that receive this traffic are referred to as leaf nodes.

That's all fine - but how does a device join a multicast group in the first place?  That job is performed by IGMP, the Internet Group Management Protocol.  There are three versions of IGMP in today's networks, and these three versions have dramatic differences in the way they work.

Hmm... one protocol with three versions and large differences in their operation... sounds like fertile ground for BSCI exam questions to me! We'll take an illustrated look at IGMP version 1 right after this brief yet important note - 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!

IGMP Version 1

A host running IGMP v1 will send a Membership Report message to its local router, indicating what multicast group the host wishes to join.

This Membership Report's destination IP address will reflect the multicasting group the host wishes to join. (This message is occasionally called a Host Membership Report as well.)

A router on every network segment will be elected IGMPv1 Querier, and that router will send a General Query onto the segment every 60 seconds. 

If there are multiple routers on the segment, only one router will fill this role, as there's no need for two routers to forward the same multicast traffic onto a segment.  (Different protocols elect an IGMPv1 querier in different ways, so there's no one way to make sure a given router becomes the Querier with v1.)

This query is basically asking every host on the segment if they'd like to join a multicast group. These queries are sent to the reserved multicast address 224.0.0.1, the "all hosts on this subnet"  address. A host must respond to this query with a Membership Request under one of two conditions:

That second bullet point means that a host, in effect, must renew a multicast group membership every minute.  That's a lot of renewing, and a lot of Membership Requests taking up valuable bandwidth on that segment. 

In effect, IGMPv1 gives a host two ways to join a multicast group:

In the above scenario, the host was a member of a multicast group, but stopped sending Membership Reports two minutes ago. 

The problem is that the router will not age that membership out for a total of three minutes, so not only has the router been unnecessarily sending multicast traffic onto this segment for the last two minutes, but will continue doing so for another minute before finally aging out the multicast group membership.

If it occurs to you that IGMPv1 could be a great deal more efficient, you're right.  That's why IGMPv2 and then v3 were developed - and we'll take a look at both of those in the next installment of this CCNP BSCI tutorial series, coming Thursday, March 18!

Bookmark the BSCI Exam Resource Page and visit every day in March and April for new CCNP study material!

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!