Difference Between subnet under EPG and BD.

[Khansa'a Nasr]

Hi 

Could anyone please let me know what is a difference between Subnets in EPG and Subnets in BD?

[Rick1776]

It really depends on what you are trying to achieve.

Here is a good link to read

https://www.cisco.com/c/en/us/td/docs/switches/datacenter/aci/apic/sw/1-x/aci-fundamentals/b_ACI-Fundamentals/b_ACI-Fundamentals_chapter_010001.html#d10240e710a1635

[RedNectar · Accepted solution]

Hi Khansa,

[Edited to clear up ambiguity in #2 below and add #3]

You may find out all you need to know by reading my reply to this question, but let me re-iterate here anyway.

Subnets can be assigned to both BDs and EPGs.  Which is very confusing, but there is a subtle difference that doesn't really come into play until you need to configure contracts between VRFs or between Tenants.

If you haven't got into sharing contracts between VRFs or between Tenants, then this is all you probably need to know:

1. If you want, you can merrily configure all your IPs on EPGs rather than BDs.  And in some ways, that more closely resembles a Network Centric Approach.
2. If you ever need to provide a service to another VRF or another Tenant (apart from the common Tenant), you will NEED to add an IP Address to the EPG to specify which part of the subnet you wish to advertise to the consumer EPG, and mark that subnet as being Shared Between VRFs and that subnet will then get leaked into the Consumer's VRF.
3. On the Consumer side, the subnet also has to be marked as being Shared Between VRFs so that the route can be leaked into the Provider's VRF.  This can be done on the Consumer's DB or on a Subnet EPG.

I hope this helps

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[Khansa'a Nasr]

Thank A lot. It helped me put things into perspective

[nvermand · Accepted solution]

Hi,
Subnet under EPG should solely used in the context of VRF leaking (which works with contract) to apply the right classification and corresponding policy enforcement under the provider EPG. A new knob had been introduced in 2.3 to allow you to:
- Define a Subnet under the BD, and use it as the default gateway
- Configure carvings of this subnet under the EPGs and not having to change the default gateway on the servers
For example, you can have 10.10.10.1/24 under the BD as a default gateway, whilst deploying more specific classification under EPG, such as 10.10.10.5/30.

You can find additional info there: https://www.cisco.com/c/en/us/solutions/collateral/data-center-virtualization/application-centric-infrastructure/white-paper-c11-737909.html#_Toc492605185

Hope this helps

Nicolas

Technical Marketing - Insieme BU