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Multicasts in ACI

RedNectar
VIP
VIP

Hi Experts,

Most of my life I've avoided anything to do multicasts at all costs. So my knowledge is pretty scanty.  However, now I need to know and I'm simply too lazy to lab it all up and figure out the answer to these questions

Q1. Regarding the Optimized Flood option.   The missing ACI Pilot's Operating Handbook (BRKACI-2210) states that 
"In optimize flood case send only to router ports detected by PIM hellos" Slide 34 - see below

In BRKACI-2608 Multicasts in ACI, I see for the same option the statement
"Optimized flood forwards only based on IGMP snooping table across all leaf switches where BD is deployed" Slide 32 - see below, and this is also hinted at by the lead guru of ACI Forwarding (Takuya Kishida) in Mastering ACI Forwarding Behavior - BRKACI-3545 (Slide 70 - see below)

So experts, which is it? Does the optimize flood option work

  1. only on ports that receive PIM Hellos or,
  2. only on ports that receive IGMP Join requests, or
  3. both the above?

Q2. This one is even more vague than the last - because I just can not find the source of a flowchart that I once copied.  It says

RedNectar_4-1677210353361.png

Now the bit that worries me is the statement

Multicast is "unknown" when there are no IGMP snooping/MLD entries for the group (per leaf)

because in BRKACI-2608 Multicasts in ACI, the exact same statement is used to describe how unknown multicasts are flooded out all ports in the BD. (Slide 29)

RedNectar_5-1677210658674.png

 

So Q2 is - If an unknown multcast frame arrives at Leaf 1 (as shown above) and there are no IGMP snooping entries on Leaf1, does the frame get dropped (as per the flowchart) or flooded in the BD (as per slide29 from BRKACI-2608 Multicasts in ACI)?


Slides referenced above

RedNectar_0-1677206593597.png

RedNectar_1-1677206785941.png

RedNectar_6-1677211063643.png

 

RedNectar aka Chris Welsh.
Forum Tips: 1. Paste images inline - don't attach. 2. Always mark helpful and correct answers, it helps others find what they need.
1 Accepted Solution

Accepted Solutions

Robert Burns
Cisco Employee
Cisco Employee

Hey Chris,

1. Optimized Multicast flooding will restrict forwarding of traffic to mrouter ports (which are detected PIM hellos or IGMP query messages), or ports where the group is learned in the IGMP snooping table for the BD.

2. Not sure where you found that flowchart, but Its not accurate (careful with Interweb mis-information)- unknown multicast is simply flooded (not dropped) similar to how we handle Unknown Unicast traffic flooding. Optimized flooding simply controls the scope of the flooding.  This chart would make sense if you moved the "IGMP Group Known?" decision action after the "Optimized Flood" check -  as this is where traffic could be dropped.  With OMF is enabled, when and a Leaf recieves multicast, but doesn't know about the group in IGMP snooping and doesn't have any mrouter ports learned in the BD, then the action depends on the "L3 Unknown Multicast Flooding" setting of the BD. If it is set to 'Flood' then the traffic is flooded out all ports in the BD until the group is learned in igmp snooping or an mrouter port is learned. If it is set to "Optimized Flood" then the traffic is dropped.

[Props to Joseph (Mr. Multicast) Young for keeping me honest with this]

Robert

View solution in original post

2 Replies 2

Robert Burns
Cisco Employee
Cisco Employee

Hey Chris,

1. Optimized Multicast flooding will restrict forwarding of traffic to mrouter ports (which are detected PIM hellos or IGMP query messages), or ports where the group is learned in the IGMP snooping table for the BD.

2. Not sure where you found that flowchart, but Its not accurate (careful with Interweb mis-information)- unknown multicast is simply flooded (not dropped) similar to how we handle Unknown Unicast traffic flooding. Optimized flooding simply controls the scope of the flooding.  This chart would make sense if you moved the "IGMP Group Known?" decision action after the "Optimized Flood" check -  as this is where traffic could be dropped.  With OMF is enabled, when and a Leaf recieves multicast, but doesn't know about the group in IGMP snooping and doesn't have any mrouter ports learned in the BD, then the action depends on the "L3 Unknown Multicast Flooding" setting of the BD. If it is set to 'Flood' then the traffic is flooded out all ports in the BD until the group is learned in igmp snooping or an mrouter port is learned. If it is set to "Optimized Flood" then the traffic is dropped.

[Props to Joseph (Mr. Multicast) Young for keeping me honest with this]

Robert

RedNectar
VIP
VIP

Hi @Robert Burns ,

Thanks for clearing that up. I feel much better now. And your answer to 2 probably explains why I can't find the source of that flowchart - maybe it was moi that didn't copy it properly (because I remember "copying" as in "transcribe" rather than any digital copying).

And thanks too to Joseph for his contribution.

 

RedNectar aka Chris Welsh.
Forum Tips: 1. Paste images inline - don't attach. 2. Always mark helpful and correct answers, it helps others find what they need.

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