So to move from where we are, I'd guess we delete the virtual IPs, that should then allow us to assign the VDSL and MG IPs to the warm spare device's WAN ports.
If both fibre WANs went down (as is expected in the roadworks), then the SD-WAN would just rebuild over the VDSL and the MG, as they are load balanced for fibre and the MX doesn't know it has different circuits now...
How would the solution deal with the fact that the fibre and VDSL/MG bandwidths could be very different, is it just a 'hope' moment?
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