Hour 99: PIM Dense Mode vs PIM Sparse Mode Part 1

Today I will be using an analogy to explain PIM and two of its variations. I wish I had this analogy a couple of years back when I was first introduced to multicast networks and PIM.

As you may know, multicast is a technique used to distribute information from one to many. PIM or Protocol Independent Multicast is a protocol that can be used to distribute information. Similarly to an IGP that shares routing information, PIM will share multicast information. It is important to know that PIM, as the name says, is an Independent Protocol and does not have a topology discovery mechanism; it implies an IGP is already running to do the routing. PIM has several “flavours”, the most common ones are PIM Dense mode (PIM-DM) and PIM Sparse mode (PIM-SM). In order to explain these different variations let me bring up an analogy.

You have just bought an apartment block in a nice neighborhood with five apartments. In order to make money you need to rent these apartments to people. There are different methods you could use to do this.

  1. You could send letters to everyone in the neighborhood announcing that you have apartments to rent. This method is inefficient because you will have to use a lot of resources to send these letters to everyone.
  2. You could put an ad in the newspaper saying that you have apartments to rent. This method is a lot more efficient but only good if people see your ad in the newspaper.

Similarly to the first method where you will send letters to everyone and get called back by those interested, PIM-DM will flood multicast traffic domain wide and then prune back where there are no receivers. In contrast, PIM-SM will use a Rendez-vous Point (RP), or in this analogy we use an ad in the newspaper, as an intermediary point to get your announcement across. This method assumes that everyone in the network knows about the RP. PIM-SM saves resources because it does not flood unused parts of the network unnecessarily.

This is just a crude analogy on how PIM-DM and PIM-SM work. You can remember these easily by thinking about Dense Mode as a PUSH technology and Sparse Mode as a PULL technology. I will get into more details about these in Part 2 but I think this analogy explains well to beginners the core concepts behind the two main variations of PIM.

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