Did you add vlan 20 on the trunk between mx and core switch? -The trunk link between the Core switch (Catalyst-M) C9300X-12Y is tagged on Management Vlan 5 with allowed VLANs 1-1000   Did you remove the static route for vlan20 from the mx to the coreswitch? Yes, this was a pre-requisite otherwise I was not able to add the Subnet onto the MX   Did you remove the vlan 20 svi from the switch? Yes, I removed this.   Core switch has one route on it 0.0.0.0/0 next hop of the MXs transit IP address.     ... View more

Thank you very much for this. I see that when you do this, you'll see the static routes appear in the Configure->S2S VPN page. So the process is actually super straightforward. A slight lack of experience there on my behalf.  ... View more

Thank you all for your advice thus far. ... View more

Do you use meshing in the network? If not, just disable it entirely; Network-Wide -> Configure -> General. ... View more