NETWORKING: Multicast Distribution Trees

Multicast Distribution Trees

In this section, We will discuss about Multicast distribution Trees / Design Models.

Multicast Distribution Tress is classified in two types. Source and Shared tree.

Source Tree – In this type of Multicast topology, Multicast Source will be on Top of Multicast Tree (Logically, source will be the first one and other receivers will be Tree branches) and other Multicast enabled routes will be acting like branches.

Deployment of Source Tree is very Easy. It is recommended for small Networks.

In Source Tree, forwarding of packets based on shortest path. It is based on both the Source Address that the packets originated from and the Group address G that the packets are addressed to.

For this reason we refer to the forwarding state on the SPT by the notation (S, G) (pronounced “S comma G”).

Where S and G stands for:

“S” is the Source IP address.

“G” is the multicast group address

Example 1:

The shortest path between Source 1 and Receiver 1 is via Routers A and C, and shortest path to Receiver 2 is A, C and E.

In this mode, the Tree is formed based on Source. That’s why it is known as Source Tree.

Shared Tree:

This Multicast Design is used in Large Enterprises and Service Provider Networks. It is widely acceptable in industry. In this topology all multicast source Register themselves to one Centre point known as RP (Rendezvous point) , this is very important in network. As the host request for any multicast stream, this time concern router will send that request to mapped RP. The RP will redirect it to source. No need to maintain any (S, G) entry every time. Only, we have to make sure that our unicast routing is proper and router is reachable to RP. Basically it is reducing the burden to searching the multicasting stream by own. In this way, Multicast is managed centrally. We can designate as many RP in network for redundancy.

Check the diagram for more understanding

In this topology, you can find we have one RP in network. Which is acting as centre point, As the request comes to Router C or Router D. It will send to RP, RP will redirect it to concern source.

Same time in case of, When Receiver 1 wants to reach Source 1. In that case first Router C will forward that request to Router D (RP). It will inform that router, that you can go to Router A, which is directly reachable to you. In this way, no need to go every time from RP. RP is only for redirecting us. But when Receiver 2 wants to access source 2, this time RP (Router D) will be every time responsible for this communication.

Hope this explanation will help you understand distribution trees. It’s just a design model..

PIM-Dense mode uses Source tree model and PIM – Sparse mode uses Shared Tree model.

We will discuss it this in next session, Plz let me know your views on this post... For any queries.. Plz put comments with mail id..