BD Subnet advertised over L3out

[AirBorn]

Hello

when a BD subnet is advertised over an OSPF L3out, i'm seeing tha subnet as E2 external route with metric 20 on the router connectd to BL (VPC link).

Why the subnet is shown as external and not internal OSPF route? Why the metric is 20 ?

Regards

[RedNectar]

Hi @AirBorn ,

The short answer is that a BD subnet is not part of your OSPF area. 

But to fully grasp this, you need to understand the dynamic nature of ACI - realise that a BD subnet may appear or disappear on any leaf dynamically as EPGs dynamically attach and detach.  True, this only happens when you statically map an EPG to a port on a new leaf where the EPG dis not previously exist, but the point is that it's dynamic.

So back to the BD subnet - no matter where it exists, it will be learned on the Border Leaf (or Border Leaves in your case, since you have a VPC) as a BGP route.  Check it out - issue the following command on your APIC substituting the green values appropriate to your system.

apic1# fabric <BORDER_LEAF1_ID>,<BORDER_LEAF2_ID> show ip route bgp vrf <YOUR_TENANT:VRF>

 You should see that the BD subnets appear as BGP routes on the Border Leaves, and therefore show up as E2 External on your external router. As for the metric, 20 is a pretty stock standard metric for external routes redistributed into OSPF.