Multicast PIM Snooping

(Rene Molenaar) #1

This topic is to discuss the following lesson:

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(Alexander F) #2

Great content so far!
Please update the topology pic, there the IP adress in 192.168.123.0-network for R2 and R3 must be changed.
Is pim snooping ever used as it doesn’t seem to be well covered from Cisco’s documentation, too.

  • Why is R2 the DR as R3 has higher IP?
  • Why should one filter traffic to the DR as it break the ability to send join/register messages to the RP and to register sources or receivers? Or does this join/regeister forwarding by an DR only apply when the RP is in another broadcast domain?
    Best regards
    Alex
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(Lazaros Agapides) #3

Hello Alexander

Looking at the topology pic, I’m not sure where the error is. The 192.168.123.0/24 network is shared between the three routers on interface Ge0/1 with IP addresses of .1, .2 and .3. Please elaborate on what should be corrected.

As for your other questions:

Actually, R1 is the DR as is indicated in the PIM debug messages on both R1 and R2. Although you are correct that the higher IP should be chosen as the DR, in this case, the DR is statically assigned. If you look at the configuration files of R1, R2 and R3 you will see the following command:

ip pim rp-address 1.1.1.1

This statically assigns the R1 router as the DR.

Cisco answers this question quite elegantly:

By default, switches that have PIM snooping enabled will flood multicast traffic to the designated router (DR). This method of operation can send unnecessary multicast packets to the designated router. The network must carry the unnecessary traffic, and the designated router must process and drop the unnecessary traffic.

To reduce the traffic sent over the network to the designated router, disable designated-router flooding. With designated-router flooding disabled, PIM snooping only passes to the designated-router traffic that is in multicast groups for which PIM snooping receives an explicit join from the link towards the designated router.

This comes from this Cisco Documentation.

I hope this has been helpful!

Laz

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(Alexander F) #4

Hi. Thanks for your answers, didn’t see the hard-coded RP in the first moment.
Explanaition for PIM Snooping and DR traffic is sufficient.

Regarding the topology pic, the IP adresses for R2 and for R3 are wrong/ should be changed.
R2 has configured 192.168.123. 2 and topology pic shows 192.168.123. 3
R3 has configured 192.168.123. 3 and topology pic shows 192.168.123. 2

Best regards
Alex

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(Rene Molenaar) #5

Thanks Alex, just fixed it.

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(saif s) #6

dears
from cisco document:
By default, routers that have PIM snooping enabled will flood multicast traffic to the designated router
(DR). This method of operation can send unnecessary multicast packets to the designated router. The
network must carry the unnecessary traffic, and the designated router must process and drop the
unnecessary traffic.
To reduce the traffic sent over the network to the designated router, disable designated-router flooding.
With designated-router flooding disabled, PIM snooping only passes to the designated-router traffic that
is in multicast groups for which PIM snooping receives an explicit join from the link towards the
designated router.
my questions are :

  1. command no ip pim snooping dr-flood is for router or switch
  2. need senario more clear about dr-flooding bahavior
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(Lazaros Agapides) #7

Hello Saif

The no ip pim snooping dr-flood is a command available in both switches and routers. It is generally supported by higher end devices such as the 6500 series catalyst switches and the 7600 routers. In the example that Rene showed in the lesson, it is applied to the switches.

When PIM is enabled, any and all multicast traffic, when it arrives at a network device, will be flooded to the DR. The DR does not need to receive all of this traffic, since there may be no end devices being served that particular multicast traffic connected to the DR. This means that the DR will need to receive this traffic, determine that it is not responsible for routing it and will discard it. This takes time, CPU, and memory resources, which if avoided, will be more efficient for the network.

I hope this has been helpful!

Laz

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