VPC and L3 Design Scenarios
Whenever any layer 3 is connected to vPC domain through vPC, each vPC peer device is seen as distinct L3 device by rest of the network.
Below figure shows the distinct views depending on layer 2 and Layer 3 version on network.
Connecting any L3 device to vPC domain using vPC is not supported because of vPC loop avoidance rule. If you want to connect any Layer 3 device to vPC domain than use L3 links from l3 device to each vPC peer device where these link can be one and to up to 16 and thus provides ECMP function and load balances the traffic across all L3 links.
L3 device will not initiate the L3 routing protocol adjacencies with both vPC peer devices. Traffic from L3 device to vPC domain (i.e. vPC peer device 1 and vPC peer device 2) can be load-balanced across all the L3 links interconnecting the 2 entities together.
There is then no penalty to use L3 links to connect L3 device to vPC domain compared to a vPC connection (in the sense that multiple links can also be leveraged with L3 connectivity)
vPC & L3 connectivity Designs
Following are some supported topology and shows how we should connect the Layer 3 device to vPC domain.