Multicast PIM-DM help

jovan88

Can anyone explain to me why a network running PIM Dense mode, will still have a (*, G) entry in its multicast routing table? What is the purpose of this? Wouldn't all traffic be using the (S,G) entries?

Multicast is really one of those areas which has me lost at times.

suffah

In PIM Dense mode, each source has it's own tree.

Basically, in Dense mode each source pushes (floods) the data out to the receivers and prunes the receivers that aren't requesting the information.

On the other hand, in Sparse mode, a request has to be made first for the multicast to be sent.

kryolla

jovan88 wrote: »

Can anyone explain to me why a network running PIM Dense mode, will still have a (*, G) entry in its multicast routing table? What is the purpose of this? Wouldn't all traffic be using the (S,G) entries?

Multicast is really one of those areas which has me lost at times.

Yes you are correct in that all traffic will use S,G entr. The *,G doesnt really serve any purpose in my opinion besides tell you what interfaces have pim enabled. The DR on the segment will create *,G entry wihen it receives an IGMP join and I also read somewhere that the *,G entry is a parent entry to S,G but S,G entry will tell you everything you need to know. *,G entry or shared tree is only used by the router for sparse mode.

(*, 233.3.3.3), 00:01:15/stopped, RP 0.0.0.0, flags: D
Incoming interface: Null, RPF nbr 0.0.0.0
Outgoing interface list:
FastEthernet0/0, Forward/Dense, 00:01:15/00:00:00
Serial0/1, Forward/Dense, 00:01:15/00:00:00

(155.1.45.4, 233.3.3.3), 00:01:15/00:01:53, flags: T
Incoming interface: Serial0/1, RPF nbr 0.0.0.0
Outgoing interface list:
FastEthernet0/0, Forward/Dense, 00:01:16/00:00:00

The S,G entry says the source is coming in s0/1 and you have a receiver downstream on fa0/0 no RPF neighbor due to the source is off this router

here is the downstream neighbor

(*, 233.3.3.3), 00:08:08/stopped, RP 0.0.0.0, flags: DCL
Incoming interface: Null, RPF nbr 0.0.0.0
Outgoing interface list:
Vlan10, Forward/Dense, 00:08:08/00:00:00
Vlan58, Forward/Dense, 00:08:08/00:00:00

(155.1.45.4, 233.3.3.3), 00:00:19/00:02:49, flags: LT
Incoming interface: Vlan58, RPF nbr 155.1.58.5
Outgoing interface list:
Vlan10, Forward/Dense, 00:00:20/00:00:00, H

The only difference is the RPF neighbor. So I wouldn't pay attention to *,G entry for dense mode

accely

kryolla wrote: »

Yes you are correct in that all traffic will use S,G entr. The *,G doesnt really serve any purpose in my opinion besides tell you what interfaces have pim enabled. The DR on the segment will create *,G entry wihen it receives an IGMP join and I also read somewhere that the *,G entry is a parent entry to S,G but S,G entry will tell you everything you need to know. *,G entry or shared tree is only used by the router for sparse mode.

(*, 233.3.3.3), 00:01:15/stopped, RP 0.0.0.0, flags: D
Incoming interface: Null, RPF nbr 0.0.0.0
Outgoing interface list:
FastEthernet0/0, Forward/Dense, 00:01:15/00:00:00
Serial0/1, Forward/Dense, 00:01:15/00:00:00

(155.1.45.4, 233.3.3.3), 00:01:15/00:01:53, flags: T
Incoming interface: Serial0/1, RPF nbr 0.0.0.0
Outgoing interface list:
FastEthernet0/0, Forward/Dense, 00:01:16/00:00:00

The S,G entry says the source is coming in s0/1 and you have a receiver downstream on fa0/0 no RPF neighbor due to the source is off this router

here is the downstream neighbor

(*, 233.3.3.3), 00:08:08/stopped, RP 0.0.0.0, flags: DCL
Incoming interface: Null, RPF nbr 0.0.0.0
Outgoing interface list:
Vlan10, Forward/Dense, 00:08:08/00:00:00
Vlan58, Forward/Dense, 00:08:08/00:00:00

(155.1.45.4, 233.3.3.3), 00:00:19/00:02:49, flags: LT
Incoming interface: Vlan58, RPF nbr 155.1.58.5
Outgoing interface list:
Vlan10, Forward/Dense, 00:00:20/00:00:00, H

The only difference is the RPF neighbor.

nice explanation

kryolla

thanks, multicasting is like any other technology it looks scary at first but once you get down the fundamentals its not that bad.