High Avail Overview


High availabilty is runing a network with 99.999% (5 min) or 99.9999% (30 sec) downtime per year.There a number of methods that provide the capability of HA, some of these include NSF and NSR.


  • HA is recoving a failure by continuing to flow traffic through a device experiencing a fault, instead of around it (RP failure).
  • HA does not change the topology during faulty recovery.
  • HA increases the redundancy of a single device.
  • There are two main components of HA:
    • Non-Stop Forwarding/Gracefull Restart (NSF)
    • Non-Stop Routing (NSR)
  • HA maintains the data plane forwarding while control plane is failingover to a standby.
  • HA requires redundant RP or a chassis (VSS).
  • Without HA, the following occurs:
    • RP fails, data plane still works
    • Control plane detects the failure and reset both control plane and data plane, which causes an outage.
    • Control plane has to reconverge to route around the failure.


  • Requires protocol extension to support NSF.
  • Neighboring routers need to be NSF aware to understand the NSF messages.
  • With NSF, the following occurs:
    • RP fails, data plane still works
    • Control plane detects the failure but keeps data plane marking the data as stale
    • NSF aware neighbor will count down hold timer until failed device's RP comes up and signals with a gracefull restart message
    • NSF aware neighbor updates the faulty device.


  • NSR is a improvement to a NSF, which works by syncing control plane information between two RPs. In the event of an active RP failure, the standby can pick up control plane sessions.
  • NSR does not require protocol extensions, but only synchronization within the chassis.
  • Input packets to the routing process are sent to both RPs, but only one responds.
  • Protocols that Cisco supports NSF include:


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