doc/examples: remove unnecessary route for IPv6 on-link gateways

[sbraz]

Description

Hi,
When the on-link flag is enabled, the additional route has no effect. This extra route is only required when the on-link flag is not set.

Indeed, for netorkd, when "on-link" is true, the resulting systemd-networkd config file contains "GatewayOnLink=true", which translates to "flags: onlink" in its logs. The resulting route appears with this "onlink" flag in the output of "ip route".

Similarly, for NetworkManager, "route1_options=onlink=true" gets added by

netplan/src/nm.c

Lines 228 to 230 in e5ff9f6

	if (cur_route->onlink) {
	/* onlink for IPv6 addresses is only supported since nm-1.18.0. */
	g_string_append_printf(tmp_val, "onlink=true,");

Checklist

• Runs make check successfully → except for test_with_empty_config which also fails on the main branch on my Ubuntu 22.10 test server, so I'm assuming this is entirely unrelated.
• Retains 100% code coverage (make check-coverage).
• New/changed keys in YAML format are documented.
• (Optional) Adds example YAML for new feature.
• (Optional) Closes an open bug in Launchpad.

[daniloegea]

Hello and thank you for your PR.

That sounds correct.

We also should mention in the docs that scope only applies to IPv4 routes (I can check that later). We mention that in the code but we include it in the generated configuration anyway:

netplan/src/nm.c

Line 204 in e5ff9f6

/* For IPv6 addresses, kernel and NetworkManager don't support a scope.

[daniloegea]

Tagging @slyon for further thoughts.

[slyon]

Thanks for your contribution. Yes, I agree with the understanding that an additional scope: link route is not needed for IPv6. Checking the origin of this example doesn't give any special reason for it to be included, neither: canonical/netplan.io@ed72e3a

And checking the relevant systemd-networkd docs for Scope= states the following:

For IPv4 route, defaults to "host" if Type= is "local" or "nat", and "link" if Type= is "broadcast", "multicast", "anycast", or "unicast". In other cases, defaults to "global". The value is not used for IPv6.

LGTM!

Also, I agree with @daniloegea's assessment that we should add a comment about scope being effective only for IPv4. I'll add such comment before merging.