leaf-spine ECMP in overlay

[wintermute000]

Trying to firm up some detailed level understanding re: a reference leaf-spine architecture and the interplay between underlay ECMP, overlay ECMP and multi-path vs add-path, then again for specific address families.


Assume a vendor-C reference design using OSPF underlay and IBGP overlay with RRs (no discussions pls on protocol X vs Y for underlay... lets just assume it is what it is).
Lets assume we are using border gateways, not spine borders (again no religious debates lol).


Let's assume a vanilla leaf-spine underlay where equal metric OSPF routes result in normal ECMP behaviour at the underlay.


Now for the overlay


- Do I even need multi-path on non-border ToRs for purposes of endpoint VXLAN traffic? All destinations are VTEPs, all VTEPs are loopbacks which recurse to equal cost underlay routes and get ECMP via underlay.
- So following on from this, is my assessment correct that the main use of multi-path would be to get multiple paths via BGP to routes that come in from the border i.e. external routes?
- Do I even need PIC Edge i.e. add-path for these border routes? If they are coming in from identical border gateways, translated into iBGP with identical attributes, then multi-path takes care of it? Or just add it on the RRs?