ACI BD - Multicast IP Address

[Xdxb]

Hi,

I can see every Broadcast Domain has one multicast IP address in our ACI, I am curious to know that where do they get the multicast IP address from? Is it from APIC multicast POOL?

I dont see any multicast pool configured in Fabric Access Policies.

[Sergiu.Daniluk · Accepted solution]

Hi @Xdxb 

When you perform the initial setup of the APIC, you are asked for "IP address pool for bridge domain multicast address (GIPo)"

GIPo stands for Group IP outer address.

The multicast IP addresses for both BDs and VRFs are allocated from this pool, and will be used to forward BUM traffic within ACI fabric and also between Pods in a Multi-Pod Setup.

In case you are curious when VRF GIPo is used, the answer is simple - only for user multicast traffic when you have PIM enabled on the BD. Here is a table to visualize the options:

Traffic	Non-MC Routing-enabled BD	MC Routing-enabled BD
Broadcast	BD GIPo	BD GIPo
Unknown Unicast Flood	BD GIPo	BD GIPo
Multicast	BD GIPo	VRF GIPo

If you want to find more details of the GIPo specific to your fabric you can use these commands:

1. Find the GIPo range configured during initial APIC setup:

apic# cat /data/data_admin/sam_exported.config
Setup for Active and Standby APIC
<snip>
GIPo = 225.0.0.0/15
</snip>

2. Find multicast IP address for a specific BD:

apic# moquery -c fvBD -f 'fv.BD.name=="BD1"' 
<snip>
bcastP                   : 225.1.91.128
</snip>

3. Find the VRF GIPo (will return data only if you have PIM enabled on VRF level):

apic# moquery -c pimCtxP 
<snip>
vrfGipo      : 225.1.192.0/3
</snip>

Stay safe,

Sergiu