Border Leaf - Prefer One of Two

[weylin.piegorsch]

On the ACI side, how do I configure things so that all leafs (border or non-border) choose one border leaf for all packets, with automatic failover to the other border leaf?

I have two locations, one with Catalyst 9500 routers and the other as ACI.  I'm using OSPFv2/v3, but MP-BGP is an option also (not EIGRP or RIP/RIPng for organizational reasons).  To connect them, I have a lambda on an agreement with a carrier to use it however I want.  The topology is a ring-route, where there is a long path and a short path:

The clients on my network have massive throughput demands (we part of the LHC's ATLAS project), who historcally are very sensitive to even 1ms delay.  I can't overcome the speed of light, but I can at least choose the shorter path with failover to the long path.

I know how to do this on the Catalyst routers.  But to achieve proper redundancy, each location needs a pair of routers.  On the ACI side, I infer that to mean a pair of Border Leafs that each take one of the paths, but please let me know if there's a better ACI approach.

Ideally I'd control this from the Catalyst 9500s since their traffic engineering capability is superior to anything in ACI (at least, "anything I've found so far"), but I'll use any suggestion that works.

(Edited to move the question to the top; original post had the question at the bottom.)

[M02@rt37]

Hello @weylin.piegorsch 

To achieve optimal path selection and redundancy between the catalyst 9500 routers and the ACI fabric, you can indeed use a pair of Border Leafs on the ACI side to handle the connections to the "short-path" and "long-path" routes. In ACI, you would configure two Border Leafs, with one connected to the short path and the other to the long path. The key is to ensure that routing policies favor the short-path Border Leaf as the primary gateway, while the long-path Border Leaf serves as the backup. You can accomplish this by using OSPF or MP-BGP metrics to manipulate the preferred path. For instance, on the ACI side, you can adjust the OSPF cost or BGP local preference to make the short-path Border Leaf more attractive for all routes, ensuring that traffic prefers the shorter route by default. In the event of a failure on the short path, the routing protocol will automatically converge to use the long-path Border Leaf. Although ACI does have some traffic engineering capabilities, controlling path selection from the Catalyst 9500s may provide finer control over routing metrics. You can leverage these capabilities by influencing OSPF or BGP route attributes on the Catalyst routers to ensure that the ACI fabric consistently chooses the optimal path based on the configured preferences.