Hello
In summary you could run both old and new cores in parallel, having a trunk and l3 interconnect between the two cores
To minimize down time,create all the same layer 3 interfaces on the new cores that are active on the old cores but have them in a shutdown down state, As/when you are ready for migration shutdown the old layer interfaces and bring them up on the new core, This can be done piecemeal if need be, until eventually all L3 is on the new core, then shutdown old core.
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Kind Regards
Paul
