High Availability

High Availability provides hardware redundancy to the LAN clients. A High Availability group consists of 1 primary router and 1 (or several) secondary router for redundancy.

There are two redundancy methods available – Hot-Standby and Active-Standby, users may choose either mode for redundancy, and generally Hot-Standby is more often to see. Following is the comparison table and network topology:

	WAN Source	Online Status	Configuration Sync
Hot-Standby	Primary and Secondary share the same WAN source. To share the same WAN source, a modem with several Ethernet ports or a hub/switch would be required.	Only Primary is online. When Primary goes down, Secondary comes up and use the same WAN line to dial up	Configuration Sync from Primary to Secondary
Active-Standby	Primary and Secondary have different WAN sources	Both Primary and Secondary are online via their own different WAN lines	No Configuration Sync

Hot-Standby

　 　

Active-Standby

Graceful Restart

The extensions signal neighboring routers about a router undergoing a restart and prevent the neighbors from propagating the change in state to the network during a graceful restart wait interval. 
The main benefits of graceful restart are uninterrupted packet forwarding and temporary suppression of all routing protocol updates. Graceful restart enables a router to pass through intermediate convergence states that are hidden from the rest of the network.

When a router is running graceful restart and the router stops sending and replying to protocol liveness messages (hellos), the adjacencies assume a graceful restart and begin running a timer to monitor the restarting router.
During this interval, helper routers do not process an adjacency change for the router that they assume is restarting, but continue active routing with the rest of the network. The helper routers assume that the router can continue stateful forwarding based on the last preserved routing state during the restart.