Unanswered Question
Giuseppe Larosa Wed, 02/18/2009 - 08:52
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Hello Ali,

Dense Mode uses a logic of the following type:

every network segment needs to receive multicast so

a multicast stream is sent out all possible interfaces

if some of my neighbors say they don't need it (PRUNE message) the sending over that interface is stopped for some time.

This is broadcast and prune

PIM Sparse mode has a different approach:

an explicit Join message is needed to add an interface to the list of outgoing interface or the reception of some IGMP message from an interested receiver.

This changes the behaviour and saves resources.

PIM SM is also more scalable because it stores state information for groups instead for the pairs (source, groups) like PIM DM does.

PIM SM uses the rendez-vous point : a central node that is used as a reference to build the distribution tree

(shared tree)

PIM DM builds a tree rooted at the source.

So again if four sources send traffic to PIM DM builds 4 trees Source based.

PIM SM only one with traffic from sources tunneled to RP

So most real implementations use PIM SM.

The real Cisco implementation takes advantage of both:

the RP is used to build a shared tree.

As soon as a source starts to send traffic in volume a source based tree rooted at the pim router connected to the source is built and can reuse the work of the shared tree.

to be noted interfaces can be configured for mixed mode with

ip pim sparse-dense-mode

groups that are mapped to an RP are treated as sparse the others as dense

Hope to help


Joseph W. Doherty Wed, 02/18/2009 - 10:04
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Assuming you're asking about multicast modes (Giuseppe's post provides additional details), dense mode multicast initially sends the source multicast stream everywhere (known as flooding) although it will terminate forwarding the multicast stream to those parts of the network which don't appear to desire it (known as pruning). The flood/prune cycle usually repeats while the multicast stream is active.

Spare mode multicast only forwards multicast streams to those parts of the network that a client is known to want it.

The "dense" vs. "sparse" connote density of receiving clients as the two methods are oriented toward either many clients (dense) or few clients (sparse) (BTW: clients in the sense of network segments, not clients per network segment). I.e. if most network segments will want to receive the multicast stream, dense mode may serve well; if most networks segments do NOT want to receive the multicast stream, sparse mode may serve better.


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