vyos@cpe-1# show interfaces tunnel tun0
 address 192.168.254.1/26
 encapsulation gre
 multicast enable
 parameters {
     ip {
         key 1
     }
 }
+source-address 0.0.0.0
-source-interface pppoe1

vyos@cpe-1# commit

Key "1" for source-address "0.0.0.0" is already used for tunnel "tun0"!

[[interfaces tunnel tun0]] failed
Commit failed