Cisco ACI - Bridge Domain Design

[Joshua Glenn]

Hello All,

I find myself hitting a wall when trying to design a tenant with a 3-tier web/app/db Application Profile.  I am struggling to understand what are the best practices for the number of Bridge Domains really needed/necessary per VRF.

Example: VRF-WEB has BD-WEB-1, VRF-APP has BD-APP-1, etc. and a unique /24 subnet for each Bridge Domain.  Then the Web EPG uses BD-WEB-1, the App EPG uses BD-APP-1, etc.

But in some labs, I build just ONE Bridge Domain and tie all the Web/App/DB EPGs to it.  So... is there a best practice for choosing the number of bridge domains used?  I'm trying to design a new Data Center and trying to shun the legacy mindset if there's a better way.  Happy to clarify if i'm not making sense.  

Many thanks,

Josh

[lpember · Accepted solution]

Hi Josh,

The number of Bridge Domains to use is really a result of what your network needs are. A Bridge Domain is an L2 Domain. So you will want to make sure any applications that need L2 adjacency are in the same bridge domain. Because in ACI we don't do a 1:1 mapping of 1 subnet = 1 L2 Domain, you are free to design it however best fits your needs.

One thing to consider is that if you have your BD set to ARP and L2 Unknown Unicast flood, then you will have increased "unnecessary" traffic the more endpoints you cover under a single BD. Again, there is no right answer. The configuration should keep in mind whatever your network needs are.

Does that answer your question?