40 Chapter 2: IP Forwarding Plane: Achieving Nonstop Forwarding
Synchronization of router startup, running, and dynamic configurations from active
to standby RP. Preservation of synchronized configuration information across the
switchover.
External View of the IP SSO and NSF
This section describes the IP control-plane SSO and IP NSF operation from the perspective of
neighboring nodes. The following description should be useful to better understand interactions
between NSF-capable and NSF-incapable neighbors in the ensuing chapters:
The IP control-plane SSO capability of a router is not visible to neighbors because it does
not involve any protocol message exchange before or after the switchover. In that sense,
the IP control-plane SSO capability is internal to the router, and therefore does not depend
on support from neighbors (see Figure 2-9). The only thing a neighbor detects is the loss
of a protocol session (or protocol adjacency). It has no way to determine the cause of the
loss; the loss could be due to any of a number of reasons including a transient, very high
traffic load, a change in configuration, SSO, and so forth.
In contrast, the IP forwarding-plane NSF capability of a router is visible to its neighbors.
This is because NSF involves interaction with neighbors both before and after the
switchover. These interactions include the exchange of capabilities and the recovery of a
partial or complete control-plane state to update the existing forwarding state. As a result,
IP forwarding-plane NSF capability is known to its neighbors, and in that sense needs
support from neighbors.
Figure 2-9 Neighbor’s View of IP SSO and NSF
Line Card
Forwarding
Control Plane
(Active)
Control Plane
(Standby)
Switchover
Intercommunication
Pre-Switchover
Intercommunication
Interfaces
Pre-Switchover Control
Message Exchange
with Peers
Post-Switchover Control
Message Exchange
with Peers
SSO Is Transparent
to the Neighbors
NSF Capability Is Not
Transparent to the Neighbors
Restarting Router
Nonrestarting NeighborNonrestarting Neighbor
Post-Switchover
Intercommunication
Line Card
Forwarding
Line Card
Control Plane
(Active)
Control Plane
(Standby)
Switchover
Intercommunication
Pre-Switchover
Intercommunication
Interfaces
Post-Switchover
Intercommunication
Line Card
Forwarding Forwarding

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