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Highlighted
Participant

Multicast Dense Mode

Hi All,

Please advise on the below. I am just learning Multicast Dense Mode. Say R6 is the DR for the PIM, and currently there are no receivers. There is only Sender R9. When I checked on the R4 and R1, for #sh ip mroute, it is showing as forwarding for R1. Can please advise why is this so?

R6#sh ip pim neighbor
PIM Neighbor Table
Mode: B - Bidir Capable, DR - Designated Router, N - Default DR Priority,
      P - Proxy Capable, S - State Refresh Capable, G - GenID Capable
Neighbor          Interface                Uptime/Expires    Ver   DR
Address                                                            Prio/Mode
192.168.16.1      Ethernet1/1              00:12:48/00:01:30 v2    1 / S P G
192.168.16.4      Ethernet1/1              00:12:31/00:01:31 v2    1 / S P G
192.168.67.7      Ethernet1/4              03:18:26/00:01:39 v2    1 / DR S P G

 


R1#sh ip mroute

(192.168.79.9, 224.5.5.5), 00:20:03/00:01:58, flags: T
  Incoming interface: Ethernet1/2, RPF nbr 192.168.16.6
  Outgoing interface list:
    Ethernet1/1, Forward/Dense, 00:10:29/stopped

R4#sh ip mroute

 

(192.168.79.9, 224.5.5.5), 00:20:42/00:02:02, flags: PT
  Incoming interface: Ethernet1/3, RPF nbr 192.168.16.6
  Outgoing interface list:
    Ethernet1/1, Prune/Dense, 00:01:46/00:01:13

 

R8#sh run int eth 1/2
Building configuration...

Current configuration : 81 bytes
!
interface Ethernet1/2
 ip address 192.168.58.8 255.255.255.0
 duplex full
end

 

5 REPLIES 5
Highlighted
Enthusiast

Hi,It's been a while since

Hi,

It's been a while since dense mode but I think your provided output is correct.

R6 floods the multicast group(s) every 3 minutes (in dense mode) out all multicast enabled interfaces.

R1 receives the best flood message in on e1/2 and announces out e1/1

R4 receives the best flood message in on e1/3 and announces out e1/1

Since you don't have any active receivers, multicast is pruned back

Frank

Highlighted
Participant

Hi Frank, Thanks for your

Hi Frank,

 

Thanks for your reply.

Can you please if my understanding is correct?

flags: PT-->For router 4 means it is pruned and not sent out the interface e1/1?

 

Thanks

Highlighted
Enthusiast

HI,I think you are asking if

HI,

I think you are asking if R4 does not receive any IGMP multicast client request, will R4 prune the multicast back and the answer is yes.

Please reply if I did not understand and answer your question correctly.

Also if I did, mark it as complete!!!

Thank you

Frank

Highlighted
Participant

Hi Frank, May I know why

Hi Frank,

 

May I know why Router 1 is forwarding even when there are no clients?

flags: T

  Outgoing interface list:
    Ethernet1/1, Forward/Dense, 00:10:29/stopped

Highlighted
Enthusiast

Hey,

Hey,

R1 has stopped as shown below - see same output in your original message.

In dense mode -the original multicast mode- each layer-3 device will always announce (forward) downstream [away from the DR] known multicast groups to all multicast enabled routers and those further downstream routers will in-turn announce (forward) the multicast messages downstream until the multicast messages reach the edge of the network where no layer-3 multicast enabled devices exist.

The multicast messages being announced (flooded) contain the multicast sources available in your network. Sort of like a TV station always announcing (flooding) each of their stations. When you set your TV to the correct channel, you start receiving that channel on your screen.

At the edge of your network, if no clients are asking (IGMP messages) for the announced groups, the edge routers start PRUNEing back the multicast announcements. Then the next in-line upstream routers follow this pruning process. This pruning process continues until no other upstream routers exist. This process happens over and over and over forever, in Cisco world every 3 minutes Flood and Prune.

Does this make sense?

Frank

R1#sh ip mroute

(192.168.79.9, 224.5.5.5), 00:20:03/00:01:58, flags: T
  Incoming interface: Ethernet1/2, RPF nbr 192.168.16.6
  Outgoing interface list:
    Ethernet1/1, Forward/Dense, 00:10:29/stopped

R4#sh ip mroute

(192.168.79.9, 224.5.5.5), 00:20:42/00:02:02, flags: PT
  Incoming interface: Ethernet1/3, RPF nbr 192.168.16.6
  Outgoing interface list:
    Ethernet1/1, Prune/Dense, 00:01:46/00:01:13

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