I have a requirement to allow access between VRFs
The scenario is a follows:
We are handed customers cellular LTE connections on a RFC1918 IP address via a GRE Tunnel from our carrier (the use case is fail over for their Ethernet service)
There are several clients being terminated into this handoff VRF.
Each client has their own VRF for their multi site WAN and their Ethernet leased line services terminate in here and in turn break out to the internet.
My question is, hi can I allow specific IP addresses from the LTE VRF access into the customer specific VRF. I have read this may be achievable using mangle rules but I can’t get my head around the logic 
Does anyone have a similar configuration or any suggestions?