PIM- SM and DM ? Forouzan and Larry Peterson books

Unanswered Question
May 4th, 2007

While I was reading through the below resources, I came across these :

? Computer Network A system Approach ? book 3ed by Larry L. Petersom page 336 Article 4.4.3 syas:

<quote>

PIM-SM allows two types of trees to be constructed: a shared tree, which may be used by all senders, and a source-specific tree, which may be used only by a specific sending host. The normal mode of operation creates the shared tree first, followed by one or more source-specific tree if there is enough traffic to warrant it.

</quote>

In another book ? Data Communications and Networking ? book 3ed by Forouzan page 570, I came a cross below :

http://highered.mcgraw-hill.com/sites/0072967757/student_view0/chapter22/powerpoint_slides.html

http://img514.imageshack.us/img514/4489/multicastprotocolsforouwk0.jpg

If you look at Figure 22.39 Taxonomy you will see that PIM-SM uses only Group-shared tree (i.e does not know what is Source ?based tree).

And PIM-DM uses only Source ?based tree (i.e does not know what is Group-shared tree).

1- Isn ' t there a contradiction between Taxonomy and the below phrase (Larry L. Petersom book ) ??

? PIM-SM allows two types of trees to be constructed ? .

In another resource I read this :

<quote>

There are two types of multicast trees that can be formed in. The default tree type is a source based tree.

A source based tree is rooted at the source of the multicast stream. The tree is built using the least cost path between the source and destination or destinations. This is sometimes referred to a shortest path tree.

The second type of tree, is called a shared tree. With a shared tree all multicast packets are sent to the RP and then down

to the receivers.

</quote>

2- Isn ' t there a contradiction between these two phrases (the resource and first book) :

? The default tree type is a source based tree ? ----compare with---- ? The normal mode of operation creates the shared tree first ? ?

3- Referring to this phrase : ? This is sometimes referred to a shortest path tree. ?.

As we know that that PIM-SM, and PIM-DM both protocols are unicast-protocol-independent ,,,,now when he said : ?This is sometimes referred to a shortest path tree.? , does that means PIM would have been using OSPF as its unicast routing protocol, therefore it has been referred to as : shortest path tree ?? (i.e if OSPF was not used as unicast routing protocol, we would not referred to ?Source Based Tree? as :shortest path tree ? )

Please throw some light ?

I have this problem too.
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Harold Ritter Sat, 05/05/2007 - 12:17

1- With PIM-SM, a shared tree is first built between the receiver(s) and the RP. Upon reception of the first data packet from a given source, the designated router connected to an active receiver sends a join toward the source thus creating the source tree. The shift from the shared tree to the source tree is done by default with IOS but can be turned off.

2- As stated in 1, PIM SM starts by creating a shared tree and then switches to the source tree as Multicast traffic starts flowing between the sender and the receiver(s).

3- The two terms (shortest path tree and source tree) are used interchangeably no matter what routing protocol is used in the network.

Hope this helps,

zillah2004 Sat, 05/05/2007 - 20:11

Thanks hritter

1- Since we have got only one default mode which : "default is shared trees"

>>There are two types of multicast trees that can be formed in. The default tree type is a source based tree.<<

What does the author mean by: "The default tree type is a source based tree",,,,Is this a typo ?

2- Does that mean we have got another typo in the link below :

http://img514.imageshack.us/img514/4489/multicastprotocolsforouwk0.jpg

When the author pointed that PIM-SM uses only Group-Shared Tree ?

Harold Ritter Sun, 05/06/2007 - 07:19

1- Not sure what the author meant by that. PIM-DM would only use a source based tree, whereas PIM-SM uses both a shared tree and a source tree (or shortest path tree).

2- Indeed, this diagram kind of indicates that PIM-SM doesn't make use of a source based tree. PIM-SM certainly does so between the RP and the multicast source.

Hope this helps,

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