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When Same router is FHR/LHR and effect of IP PIM DENSE/SPARSE-DENSE/SPARSE commands

Hi everyone,

Please consider the following set up:

 

Capture-3.PNG

ESW4 is both LHR/FHR.   This is what I see:

 

IP PIM DENSE /IP PIM SPARSE-DENSE/ IP PIM sparse commands on the interfaces facing SOURCE/LISTENERS have no impact on multicast forwarding when FHR/LHR is same router.

Above Multicast forwarding refers to when source sends multicast 235.1.1.1, FHR/LHR simply forwards it to LISTENERS regardless of  IP PIM  mode  configured on the interfaces facing SOURCE/LISTENERS.

 

NOTE:  FHR/LHR will send REGISTER for 235.1.1.1 to RP but our focus is specifically on  multicast forwarding between INTERFACE facing SOURCE and  INTERFACE facing LISTENERS on FHR/LHR.

 

Configuring of IP PIM regardless of mode on interfaces connected to SOURCE/LISTENERS have no bearing how the PIM router will forward multicast stream to its PIM neighbors.

However, the PIM mode configured on the interface connected to DOWN STREAM neighbor, will determine whether PIM router will use SPARSE logic or dense logic when forwarding stream to DOWN STREAM PIM neighbor.

 

 

CASE 1: WHEN INTERFACES FACING SOURCE/LISTENERS are SPARSE MODE:

 

1)  FHR/LHR all interfaces are is SPARSE MODE, FHR/LHR does send PIM JOIN towards RP upon receiving IGMP Report

   about 235.1.1.1 from ESW3:

 

ESW4#show running-config interface s0/0

Building configuration...

 

Current configuration : 84 bytes

!

interface Serial0/0

 ip address 24.24.24.4 255.255.255.0

 ip pim sparse-mode

end

 

ESW4#show running-config interface s0/1

Building configuration...

 

Current configuration : 84 bytes

!

interface Serial0/1

 ip address 43.43.43.4 255.255.255.0

 ip pim sparse-mode

 

 

Wireshark capture shows PIM JOIN BEING SENT TO RP:

 

 

 

Capture-45.PNG

 

 

Source generates pings at 235.1.1.1, capture between ESW4 and ESW3 shows , PINGS being forwarded:

 

Capture-67.PNG

 

 

2)When INTERFACES FACING SOURCE/LISTENRS are SPARSE -DENSE MODE

 

ESW4(config)#int s0/0

 

ESW4(config-if)#ip pim sparse-dense-mode

ESW4(config-if)#int s0/3

ESW4(config-if)#ip pim sparse-dense-mode

 

 

No change in behavior, same results:

 

 

FHR/LHR still sends PIM JOIN 235.1.1.1 towards RP:

 Capture-89.PNG

 

CASE 3: When  INTERFACES FACING SOURCE/LISTENERS are configured as DENSE

NO change, same behavior,

PIM JOIN ( *,235.1.1.1) is sent by ESW4 to RP:

 

Capture-87.PNG

 

 

Kindly share your thoughts.

 

 

Thanks and have a nice evening!!

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