GCP cloud router is not accepting default route advertised by a on premise router

[Pseudonym10]

We have set up a VPN tunnel from our premise datacenter to a Google Cloud project. The BGP session on our on premise router, and on a Google Cloud router is successfully established.
The traffic flow is to route all egress traffic from Google Cloud instances to the on premise network. However, the default route is not accepted by Cloud router and not added to the routes table.

[azzi]

Hi Pseudonym10,

So you want 0.0.0.0/0 next hop is to On-Premises device? Based on my experiences, yes it is possible to do it. You just need to control the order of routes https://cloud.google.com/network-connectivity/docs/vpn/concepts/order-of-routes
https://cloud.google.com/vpc/docs/using-routes

You also need to check the possibility of suppressed routes. In case there is existing 0.0.0.0/0 with next-hop to Default Internet Gateway you should also delete it. Please be careful when deleting as if you don't have other 0.0.0.0/0 the Internet Connection in your VPC will be temporarily disconnected.

Regarding the possibility of the Cloud Router rejected the advertised routes from On-Premises, I don't think it is possible as the Cloud Router should always accept all the advertised routes. You just need to control the order of routes (priority) so that the routes is not suppressed https://cloud.google.com/network-connectivity/docs/router/how-to/updating-priority

Regards,
Iza

[Pseudonym10]

Hi, Iza.

Actually, the GCP routes does not show the default route advertise by my on premise router even deleting the  default route (GCP via default internet gateway).

[azzi]

What on-premises device are you using? Verify that the BGP correctly established (ASN, etc)