Hi,
I've not tried this, but I don't see how you would be able to use the Anycast Gateway feature without having Layer-2 connectivity between your leaf switches. That might be an encapsulation mechanism such as FabricPath or VXLAN, but could perhaps also be 802.1Q trunks with vPC or Spanning Tree.
Let's consider a simple scenario where we have two VLANs, VLAN 10 and VLAN 11, with Host 1 (H1) in VLAN 10 and Host 2 (H2) in VLAN 11. Let's assume these two hosts are connected to a single switch, Switch 1 (S1), which has SVI and is the default gateway for both VLAN 10 and 11.
When H1 sends traffic to H2 that traffic is received by S1 in VLAN 10, routed to VLAN 11, and then forwarded on at Layer-2 to H2. It's important to note that once the traffic is routed in S1, the destination MAC address of the frame would be that of H2.
Now let's consider what happens when using two switches with the Anycast Gateway function. Here we have Host 1 connected to Leaf 1 (L1) in VLAN 10 and Host 2 connected to Leaf 2 (L2) in VLAN 11. In this case the traffic would be received by the Anycast Gateway function in L1, routed to VLAN 11 where we'd need to build the frame with the MAC address of H2 and send on at Layer-2.
We're now in a situation where we have a frame created by L1 that needs to be carried using a Layer-2 mechanism to L2 and then H2. This would be where FabricPath or VXLAN would come in.
You don't state the type of connectivity you're considering between the Nexus 7K and Nexus 9K, but today there's no compatible technology except IEEE 802.1Q with vPC or Spanning Tree. The Nexus 7K currently doesn't support VXLAN with MP BGP EVPN, and the Nexus 9K doesn't, and never will, support Cisco FabricPath.
Perhaps you might want to consider VXLAN with MP BGP EVPN on the Nexus 9K, and then you could build a Layer-3 leaf/spine with the Nexus 7K at the spine and the Nexus 9K at the leaf. As the Nexus 9K would be doing the Layer-2 "encap/decap" the Nexus 7K spine would have to do no more than simple Layer-3 routing.Take a read of the VXLAN Network with MP-BGP EVPN Control Plane white paper for more information.
Regards
