Helper mode is a part of the OSPF Graceful restart feature. Helper mode uses the OSPF routers to help other OSPF routers on the network stay on the forwarding path while the software is restarting. The OSPF router sends a type of LSA called a GRACE-LSA to inform the other OSPF routers that it is restarting the software. When an OSPF router receives a GRACE-LSA from a neighbor OSPF Router, it enters the Helper mode for that neighbor on that network. An OSPF router supports Helper mode by default.
The following section describes the operations in the Helper mode:
Entering Helper mode — An OSPF router enters the Helper mode provided the following conditions are true:
The router is fully adjacent with the neighbor already.
No changes have been made in the LSDB since the neighbor router started.
The grace period has not expired.
Local policy configured parameters allow it to help the neighbor.
The router is not in the process of restarting itself.
The OSPF router does not help the neighbor if the preceding conditions are not met.
If the OSPF router is already helping a neighbor, and receives another GRACE-LSA from the neighbor, it accepts the latest GRACE-LSA, and updates the grace period accordingly. The OSPF router in Helper mode continues to advertise its LSAs like the neighbor it is helping is still full, until any changes are made on the network during the grace period.
Exiting Helper mode — An OSPF router exits the Helper mode, under the following conditions:
The GRACE-LSA is flushed. It means graceful restart has successfully terminated.
The GRACE-LSA‘s grace period expires.
There is a network topology change.
When an OSPF router exits Helper mode, the following actions occur:
It recalculates the DR for the network.
It re-originates its router LSA.
If it is the DR, it re-originates the network LSA for the network.
If it is a virtual link, it re-originates the router LSA for the virtual link transit area.