This has been tested to work on Mikrotik 7.1.1<br>
<br>
/ip route<br>
add dst-address=8.8.8.8/32 gateway=10.0.1.1 scope=11 target-scope=10 comment="Validate Primary"<Br>
add gateway=8.8.8.8  distance=1 scope=30 target-scope=11 check-gateway=ping   comment=Primary<Br>
add gateway=10.0.2.1 distance=2                      comment=Secondary<Br>
<br>
<Br>
"Destination unreachable" is ICMP type 3.<Br>
<br>
/ip firewall filter add protocol=icmp icmp-options=3 chain=forward action=drop<br>
/ip firewall filter add action=drop chain=input icmp-options=3 protocol=icmp<br>
/ip firewall filter add action=drop chain=output icmp-options=3 protocol=icmp<br>
<Br>
should prevent any such ICMP packets from being forwarded through the router. <br>
If you want to prevent RouterOS itself from generating such messages dropping them in the output chain should do the trick.<br>

<br>
-Mimbach 20220131/  2020102501 / 2021092501