CCNP Tutorial For The BSCI Exam:
An Illustrated Look At PIM Dense Mode
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 previous installment of this CCNP tutorial series for the BSCI exam, we examined the differences between the three versions of IGMP.
Now that the hosts are using IGMP to join the desired multicast group, we've got to get that traffic to them. For that, we'll use PIM - Protocol Independent Multicast.
There are three modes of PIM you must be fluent with to pass the BSCI exam, as well as two different PIM versions. You'll see all these modes and versions in production networks as well, so it's vital to understand the concepts of all of them.
You may be asking "If PIM is Protocol-Independent, is there protocol-dependent multicasting?" Yes - actually, there are two different protocol-dependent multicasting protocols.
The Distance Vector Multicast Routing Protocol (DVMRP) uses a type of RIP to discover the best paths for multicast traffic to take, while Multicast OSPF (MOSPF) is actually an extension of the OSPF protocol we've all come to know and love. The actual configuration of these protocols is out of the scope of the BSCI exam.
We'll take an illustrated look at PIM Dense and Sparse Mode 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!
 
PIM Dense Mode Operation
The first decision to make when implementing a multicasting protocol is which one to choose. PIM Dense is well suited for the following situations:
The multicast source and recipients are physically close
There will be few senders, but many recipients
All routers in the network have the capability to forward multicast traffic
There will be a great deal of multicast traffic
The multicast streams will be constant
When PIM Dense is first configured, a router will send a Hello message on every PIM-enabled interface to discover its neighbors. Once a multicast source begins transmitting, PIM Dense will use the prune-and-flood technique to build the multicasting tree.
Despite the name, the flooding actually comes first. The multicast packets will be flooded throughout the network until the packets reach the leaf routers.
The initial flooding ensures that every router has a chance to continue to receive multicast traffic for that specific group. If a leaf router has no hosts that need this multicast group's traffic. the leaf router will send a Prune message to the upstream router. The Prune message's IP destination address is 224.0.0.13.
The routers with hosts who belong to this multicast group are marked with "MG". Since none of the leaf routers know of hosts who need this multicast group's traffic, they will all send a Prune message to 224.0.0.13. 
If the router receiving the prune also has no hosts that need this multicast group's traffic, that router will then send a Prune to its upstream neighbor as well.
In this example, the router in the right-hand column that is receiving a Prune from its downstream neighbor knows of no hosts that need the traffic, so that router will send a Prune upstream. In the other two columns, the routers receiving the Prune do have a need for the multicast traffic, so the pruning in those branches stops there.
The router receiving that Prune also knows of no hosts that are members of this multicast group, so -- you guessed it -- that router will send a Prune to the upstream router.
Logically, the multicast tree now looks like this:
One branch of the tree has been completely pruned, while the leaf routers on the other two branches have been pruned. This group's multicast traffic will now only be seen by these five routers.
The other routers were pruned to prevent sending multicast traffic to routers that didn't need that traffic flow, but what if one of the pruned routers later learns of a host that needs to join that group? The pruned router will then send a Join message to its upstream neighbor.
Where PIM Dense builds the multicasting tree from the root down to the branches, PIM Sparse takes the opposite approach - and we'll take an illustrated look at PIM Sparse Mode in action in the next installment of this CCNP tutorial series.
Bookmark the BSCI Exam Resource Page and 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!
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