[IPV6]: ROUTE: Add router_probe_interval sysctl.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index 87bbd77..88efed0 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -789,6 +789,12 @@
 	Default Maximum Transfer Unit
 	Default: 1280 (IPv6 required minimum)
 
+router_probe_interval - INTEGER
+	Minimum interval (in seconds) between Router Probing described
+	in RFC4191.
+
+	Default: 60
+
 router_solicitation_delay - INTEGER
 	Number of seconds to wait after interface is brought up
 	before sending Router Solicitations.