Dear Community,
I'm currently deploying ACI multisite for a client and I have to do it in a phase deployment where we are moving from the old environment to the new local ACI environment. This is not a migration from a existing ACI fabric into Multisite. My question is what is the best way to do this? They have different subnets at each site and they want to keep the same networking structure but want the ability to stretch "vlans" in the future. I was looking at some of the documentation (which doesn't really explain the BD behavior well) and one of the options I see for giving them their current environment is "L3 only across sites".
Since I'm doing this in phases their DR site would be ACI and the production site would still be in their old environment (non ACI) and will be routing in between the old environment and ACI environment. My concern is that since I've already create the BD and EPG with the subnets on each site (tenant and VRF is stretched) that when I connect the old environment to the new ACI environment ACI will try and use MPBGP-EVPN via my IPN to try and get to the subnet that is technically in the old environment (because we haven't moved that environment over yet). This would cause a blackhole in traffic because its trying to get to the subnet via MPBGP-EVPN via the Spines and not the L3Out.
Now according to the documentation using "l3 only across site" for those BDs and EPG would force traffic to use it local L3 out to get to the other side. If that's the case that would work because routing would show that a subnet that is in Site1 is located via the old environment and not via Site 1 ACI fabric. This would give us the ability to do a Phase deployment were we can still have Site 1's old environment connected to the ACI fabric at Site 2.
Obviously once both sites are in ACI we will start stretching BDs and EPGs but until then we have to make sure we can add resources to Site 2's ACI fabric and it uses it local L3Out to get to resources in Site 1's old environment.
Will ACI L3 only would for a Phased deployment or not? I attached a picture of the BD deployment I want to deploy at first just to get the client into ACI. The one confusing part is where it says "Independent ACI Fabric (no ACI Multisite)". I'm assuming if I deploy it using L3 only across site that it will use it L3out to get to the other subnets.
Or would it be better to create the template but don't deploy it to my soon to be production ACI fabric until it is time to cut that over? This way the only configured ACI fabric is my DR site until it is time to migrate the Production site.
