CCNP Tutorial For The BSCI Exam:
An Illustrated Look At PIM Sparse 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 took an illustrated look at the operation of PIM Dense Mode. We'll now compare Dense to PIM Sparse Mode.
Where PIM Dense builds the multicasting tree from the root down to the branches, PIM Sparse takes the opposite approach. PIM Sparse builds the multicast tree from the leaf nodes up.
PIM Dense creates a source-based multicast tree , since the tree is based around the location of the multicast traffic's source. PIM Sparse creates a shared multicast tree, referring to the fact that multiple sources can share the same tree - "one tree, many groups".
PIM Sparse Mode is best suited for the following situations:
- The multicast routers are widely dispersed over the network
- There are multiple, simultaneous multicast streams
- There are few receivers in each group
- The multicast traffic will be intermittent
The root of a PIM Sparse tree is not even necessarily the source of the multicasting traffic. A PIM Sparse tree has a Rendezvous Point (RP) for its root. The RP serves as a kind of "central distribution center", meaning that a shared tree will create a single delivery tree for a multicast group.
The routers discover the location of the RP in one of three different fashions, and we'll look at those 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 Sparse Mode Operation
The routers discover the location of the RP in one of three different fashions:
Statically configuring the RP's location on each router
Using an industry-standard bootstrap protocol to designate an RP and advertise its location
Using the Cisco-proprietary protocol Auto-RP to designate an RP and advertise its location
The shared tree creation begins in the same fashion that a source-based tree does - with a host sending a Membership Report to a router.
Hang on - lots of "ifs" ahead!
If there is already an entry in the router's multicast table for 224.1.1.1, the ethernet interface shown here will be added as an outgoing interface for that group, and that's it.
If there is no entry for 224.1.1.1, the router will send a Join message toward the RP.
In turn..
If the upstream router is the RP, it will add the interface that received the Join to the list of outgoing interfaces for that group.
If the upstream router is not the RP, it will send a Join of its own toward the RP.
In the following illustration, the three routers marked MG have hosts that want to join this particular multicast group, and we're assuming that the multicast group is new, with no prior neighbors. Note that one router in the left column has no hosts that want to join the group, but it's still sending a Join message. 
R2 has hosts that want to join the multicast group 224.1.1.4. R2 has no entry in its multicasting table for this group, so it sends a Join toward the RP. R1 receives the Join, checks its multicast table, and sees it has no entries for 224.1.1.4.
Even though R1 has no hosts that need to join this group, R1 will send a Join of its own toward the RP. The RP receives the Join message and adds the interface upon which the Join was received to the outgoing multicast list for 224.1.1.4. Sparse Mode uses Join messages as keepalives as well. They are sent every 60 seconds, and the membership will be dropped if three hellos are missed. To avoid unnecessary transmission of multicast traffic, the multicast routers can send Prune messages to end their membership in a given multicast group.
Using the same network setup we used for the PIM Dense example, we see that while the operation of PIM Sparse is much different - there is no "flood-and-prune" operation - the resulting multicast tree is exactly the same. 
PIM Sparse-Dense Mode
Many multicasting networks use a combination of these two methods, Sparse-Dense mode. A more accurate name would be "Sparse-Or-Dense" mode, since each multicast group will be using one or the other. If an RP has been designated for a group, that group will use Sparse Mode. If there's no RP for a group, obviously Sparse Mode is out, so that group will default to Dense Mode.
RP Discovery Methods
It's one thing to decide which router should be the RP, but it's another to make sure all the other routers know where the RP is! The available methods depend on the PIM version in use, and we'll take an illustrated look at those options on Friday, March 26, on our BSCI Exam Resource Page.
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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