ospfd, ospf6d: introduce the "graceful-restart hello-delay" command

This command makes unplanned GR more reliable by manipulating the
sending of Grace-LSAs and Hello packets for a certain amount of time,
increasing the chance that the neighboring routers are aware of
the ongoing graceful restart before resuming normal OSPF operation.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>

1 files changed, 13 insertions, 0 deletions

diff --git a/doc/user/ospfd.rst b/doc/user/ospfd.rst
index e076fae6c..effad0fd0 100644
--- a/doc/user/ospfd.rst
+++ b/doc/user/ospfd.rst
@@ -635,6 +635,19 @@ Interfaces
    :clicmd:`ip ospf dead-interval minimal hello-multiplier (2-20)` is also
    specified for the interface.
 
+.. clicmd:: ip ospf graceful-restart hello-delay (1-1800)
+
+   Set the length of time during which Grace-LSAs are sent at 1-second intervals
+   while coming back up after an unplanned outage. During this time, no hello
+   packets are sent.
+
+   A higher hello delay will increase the chance that all neighbors are notified
+   about the ongoing graceful restart before receiving a hello packet (which is
+   crucial for the graceful restart to succeed). The hello delay shouldn't be set
+   too high, however, otherwise the adjacencies might time out. As a best practice,
+   it's recommended to set the hello delay and hello interval with the same values.
+   The default value is 10 seconds.
+
 .. clicmd:: ip ospf network (broadcast|non-broadcast|point-to-multipoint|point-to-point [dmvpn])
 
    When configuring a point-to-point network on an interface and the interface