PIM Dense mode using a shared tree?? 2

[norteldude78]

Hello,

I have R2,R3, and R4 connected to R1 in a star topology. All interfaces are sparse-dense mode.
R4 is my mapping agent.
R1 is announcing itself as an rp candidate to r4.

172.12.14.4 = R4 interface connected to R1
172.12.14.1 = R1 interface connected to R4

On R2 i have the following route table:

R2#show ip mroute

(*, 224.0.1.39), 00:01:13/00:02:59, RP 0.0.0.0, flags: D
Incoming interface: Null, RPF nbr 0.0.0.0
Outgoing interface list:
Serial1/0, Forward/Sparse-Dense, 00:01:13/00:00:00

(172.12.14.1, 224.0.1.39), 00:01:13/00:01:46, flags:
Incoming interface: Null, RPF nbr 0.0.0.0
Outgoing interface list:
Serial1/0, Forward/Sparse-Dense, 00:01:13/00:00:00

(*, 224.0.1.40), 00:01:44/00:00:00, RP 0.0.0.0, flags: DCL
Incoming interface: Null, RPF nbr 0.0.0.0
Outgoing interface list:
Serial1/0, Forward/Sparse-Dense, 00:01:44/00:00:00

(172.12.14.4, 224.0.1.40), 00:01:29/00:01:30, flags: CL
Incoming interface: Null, RPF nbr 0.0.0.0
Outgoing interface list:
Serial1/0, Forward/Sparse-Dense, 00:01:30/00:00:00

I have been reading that dense mode does not use shared trees, but isn't that what (*, 224.0.1.39) and (*,224.0.1.40) are? They both have the D flag set, but they are (*, G).

Can someone explain what I am misunderstanding? Isn't (*,G) a shared tree?

Thanks,
-B

 

[BuckWeet]

No, that's correct.. Since its a *, there is no defined source for the multicast session.. That means that the devices can never build a SPT back to the source because they don't know who is sending it, instead it just floods it and doesn't care about the trees..


I know this is a short answer, but hopefully it helps and makes sense..

BuckWeet

 

[norteldude78]

Thanks BuckWeet,

That explains it a little, I am starting to understand. I guess I was also wondering why I had two routes:

(*, 224.0.1.39), 00:01:13/00:02:59, RP 0.0.0.0, flags: D
Incoming interface: Null, RPF nbr 0.0.0.0
Outgoing interface list:
Serial1/0, Forward/Sparse-Dense, 00:01:13/00:00:00

(172.12.14.1, 224.0.1.39), 00:01:13/00:01:46, flags:
Incoming interface: Null, RPF nbr 0.0.0.0
Outgoing interface list:
Serial1/0, Forward/Sparse-Dense, 00:01:13/00:00:00

One has a * source, the other is 172.12.14.1 (the RP).

I will keep reading, hopefully it will 'click' soon...

Thanks,

B

 

[burtsbees]

Here is some more on it...

http://www.netcraftsmen.net/welcher/papers/multicast02.html

Quote...
"The entry starting with (*, 233.1.1.1) is a shared multicast tree entry, sometimes referred to as a (*, G) entry. (G here is just 223.1.1.1). PIM-DM doesn't use these for packet forwarding, but does list interfaces with a role in multicast (known IGMP receiver or PIM neighbor) as outgoing interfaces under such entries."
Of course, your multicast route is 224.0.1.39.

Burt

 

[norteldude78]

Thanks Burt. I have read many of those netcraftsman papers! They have some great articles. I think that quote explains it though.

My whole concern was that PIM-DM uses sourced based trees so there should never be a star-G route with a D flag...but obviously there is such a route.

-Bryan