Do not delete IPv6 link-local route in reconciler

Signed-off-by: wenyingd <wenyingd@vmware.com>

pkg/agent/route/route_linux.go

@@ -891,6 +891,12 @@ func (c *Client) Reconcile(podCIDRs []string) error {
 		if desiredPodCIDRs.Has(route.Dst.String()) {
 			continue
 		}
+		// A route destined to an IPv6 link-local CIDR is always system auto-generated along with a link-local
+		// address, which is not configured by antrea and is supposed to be ignored in the "deletion" list.
+		// Such routes are helpful in some case, e.g., IPv6 NDP.
+		if route.Dst.IP != nil && route.Dst.IP.IsLinkLocalUnicast() && route.Dst.IP.To4() == nil {
+			continue
+		}
 		// IPv6 doesn't support "on-link" route, routes to the peer IPv6 gateways need to
 		// be added separately. So don't delete such routes.
 		if desiredIPv6GWs.Has(route.Dst.IP.String()) {

pkg/agent/route/route_linux_test.go

@@ -676,6 +676,7 @@ func TestReconcile(t *testing.T) {
 		{Dst: ip.MustParseCIDR("2001:ab03:cd04:55ee:1001::1/128")}, // existing podCIDR, should not be deleted.
 		{Dst: ip.MustParseCIDR("fc01::aabb:ccdd:eeff/128")},        // service route, should not be deleted.
 		{Dst: ip.MustParseCIDR("2001:ab03:cd04:55ee:100b::/80")},   // non-existing podCIDR, should be deleted.
+		{Dst: ip.MustParseCIDR("fe80::/80")},                       // link-local route, should not be deleted.
 	}, nil)
 	mockNetlink.EXPECT().RouteDel(&netlink.Route{Dst: ip.MustParseCIDR("192.168.11.0/24")})
 	mockNetlink.EXPECT().RouteDel(&netlink.Route{Dst: ip.MustParseCIDR("2001:ab03:cd04:55ee:100b::/80")})