I'm confused with fusion router's concept. in our design, we put two C9500-12Q-A switches as the Fabric Borders with all 40G ports, So now it seems that all traffic that need to go out of the fabric will go through the Fusion Router, it mean we must purchase routers with at least 40g throughput. it doesn't make sense.
Does i get it wrong?
Whether you use dedicated Fusion routers or not, the Borders have to be connected to something based on your throughput needs. A Fusion router does not have to be a dedicated device, though it can sometimes make things simpler and cleaner.
The function of aggregating VRFs (VNs) and/or leaking routes between them can be done on any device that supports the features required (VRF, BGP, etc.) A lot of customers connect their Border devices to their Core routers, for example.
As mentioned you dont need a dedicated device just for Fusion but rather a hop in the path that can support the various functions needed.
Keep in mind that it is mainly used to provide access to services outside the Fabric as well as leaking in between VRFs, you don't necessarily need such overkill
It all comes down to this question: what is outside of your fabric? If there is only a "core switch", firewall, router and 1Gbps connection to the Internet then 40G is certainly overkill. If you have a large "Legacy" network, DC, and other networks outside of your fabric that have 40G of throughput then your Fusion Routers make sense.
I hope this helps,
Some services like dhcp, ntp, dns will also be needed to be leaked from global routing table to respective VN, so in my understanding fusion is always needed, I have used an old 3800 as fusion as well as cat9500 as fusion.
The selection I guess will depend on how many routes you end up with all VNs combined and if the platform of selection can handle those many ipv4 routes.