--> Unicast Routing feature in ACI allows the Leaf Switches to learn the IP addresses associated with the Endpoint which are connected to ACI Fabric.
--> The Unicast Routing feature also enables IP Routing under the bridge domain ( Similar to IP Routing Command in Nexus-OS/ Catalyst OS).
--> If the Unicast Routing is disabled then the endpoints within the bridge domain cannot communicate to different subnets other than local subnet.
--> In the above case, The endpoint should have the default gateway pointing towards to Outside of ACI Fabric to communicate with other subnets. ( Used in L2 Out Scenario)
--> Basically once you disable Unicast Routing feature on the Bridge Domain, Whatever the subnets you have configured under the Bridge Domain does not get pushed/Installed into the leaf switches thats why no routing takes place.
--> Once you disable Unicast Routing feature on the Bridge Domain, Whatever IP address information related to Endpoint is Flushed.
--> Before APIC 3.1 Version, Both IP address and MAC address information are Flushed.
--> If the BD Subnet is deleted when Unicast Routing is enabled then also IP address information is Flushed.
--> If you can in the below image, There are two endpoints:
Endpoint A: 192.168.1.1 ( BD1) ( Default Gateway is on outside of ACI Fabric)
Endpoint B: 192.168.2.1 ( BD2) ( Default Gateway is on Inside of ACI Fabric)
--> When Endpoint A wants to communicate Endpoint B, The traffic should go via L3OUT to reach Endpoint B even if they are connected to the same Leaf Switch as the default gateway is outside of ACI Fabric.
--> In this case, You should disable " Unicast Routing" on the Bridge Domain.
--> If "Unicast Routing" is enabled under Bridge Domain then Endpoint A and Endpoint B will communicate directly without going via L3 Out.
Reference:
https://www.cisco.com/c/en/us/solutions/collateral/data-center-virtualization/application-centric-infrastructure/white-paper-c11-739989.html#UnicastRouting
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