IPv6 AutoConfiguration: Disable Listen ke RA

The route you are trying to delete looks like a route that was automatically added by the kernel in response to a router advertisement (RA). Be aware that if you delete it then it will probably come back on its own within 5 minutes when the router sends out another advertisement, as it will normally do periodically. If you don't want your machine to listen to router advertisements then what you probably want is to do this:

echo 0 >/proc/sys/net/ipv6/conf/eth1/accept_ra

If you do that, you don't even have to bother removing it manually: it will disappear within a few minutes when it expires naturally.

Nevertheless, if you want to, you can remove it manually. It's just that it looks like the route command isn't able to do it. Use the ip command instead:

ip route del ::/0 via fe80::20c:29ff:fe87:f9e7 dev eth1

In fact, if you are working only under Linux then I recommend that you always use the ip command as a modern replacement for all of the following commands: route, ifconfig, and netstat. Its syntax is much easier to handle that any of those other commands, it unifies them all together, and there are functions (like ip rule and ip tunnel) that are only available through it.