Improve source routing example

[rbricheno]

Summary

On at least Ubuntu 18.04, the example given at https://netplan.io/examples#configuring-source-routing does not seem to fully describe how to configure source based routing.

Process

Copy the example, substituting my IP addresses and networks etc.

Current and expected result

Should configure source based routing. Doesn't. The only interface that ever "works" is the one that has a gateway4 configured.

It is however possible to create a working configuration using netplan following the example here:

https://askubuntu.com/a/1030528

[pmahnke]

@rbricheno thanks for filing this.

@sil2100 could you help with this issue?