Some confusion of Subnet and BD...

[SIMMN]

Hi,

Generally speaking within ACI, One EPG = One Subnet = One VLAN, right? If so, then

1. Can multiple EPGs be associated with a single BD? If so, then how does one EPG map to a particular subnet within the BD? There could be multiple subnets created within one BD. OR It should always be 1:1 mapping between EPG and BD in reality?
2. On APIC UI, there is also "Subnets" under EPG (Tenant->Application Profiles->Profile Name->EPG Name->Subnets). Will the subnet configured here override the subnet configured under the associated BD?
3. Is there really a use case to have multiple subnets under single BD in real world?
4. I know one VLAN ID cannot be associated with two EPGs. But is there a use case that endpoints (for example physical nodes) are using IP addresses from the same subnet but connecting to two ports belonging to different EPGs, hence different VLAN IDs associated? Even there is a corner use case, I would say it has to be L2 traffic OR use gateway outside of ACI for L3 traffic. Am I right?

Thanks,

/S

[gbrait85]

see inline

1. Can multiple EPGs be associated with a single BD? If so, then how does one EPG map to a particular subnet within the BD? There could be multiple subnets created within one BD. OR It should always be 1:1 mapping between EPG and BD in reality? Yes, you can have multiple EPG using the same BD. You could have multiple subnet associated to the same BD as well. EPG are not mapped to subnets. Let's imagine you have Endpoint A that belongs to EPG A that is using BD A with subnet 10.0.0.1/24 and you have Endpoint B that belong to EPG B that is using BD A with subnet 10.0.1.1/24 (Two subnets on the same BD A) and both of these endpoint are connected to Leaf 101. Leaf 101 will have both subnet IPs created as SVIs. When Endpoint A send an ARP request for it's GW, the Leaf will reply with the MAC of the SVI that has 10.0.0.1 ip.
2. On APIC UI, there is also "Subnets" under EPG (Tenant->Application Profiles->Profile Name->EPG Name->Subnets). Will the subnet configured here override the subnet configured under the associated BD?No, this won't "override" that. If you have one subnet here, and one subnet under the BD, both SVI's will exist on the leaf (As long as there is an endpoint that is consuming that BD). The subnet under the EPG, comes into play when you do inter-vrf leaking inside ACI.
3. Is there really a use case to have multiple subnets under single BD in real world?Yes, this would be an application centric deployment. Basically you could make some of the config simpler. If I have a one to one mapping, if I have 100 subnets, I will need to have 100 BD. Most of the times those BD would have the same config. It is a simpler config, if I only created 1BD with multiple subnets inside (there are some reason were you would need to have only one subnet per BD, for example when doing DHCP relay)
4. I know one VLAN ID cannot be associated with two EPGs. But is there a use case that endpoints (for example physical nodes) are using IP addresses from the same subnet but connecting to two ports belonging to different EPGs, hence different VLAN IDs associated? Even there is a corner use case, I would say it has to be L2 traffic OR use gateway outside of ACI for L3 traffic. Am I right? You can have one vlan id associated with two EPGs.