Nonstop Forwarding - Cisco ASR 9000 Series Configuration Manual

Implementing IS-IS
The keychain feature allows IS-IS to reference configured keychains. IS-IS key chains enable hello and LSP
keychain authentication. Keychains can be configured at the router level (in the case of the lsp-password
command) and at the interface level (in the case of the hello-password command) within IS-IS. These
commands reference the global keychain configuration and instruct the IS-IS protocol to obtain security
parameters from the global set of configured keychains.
IS-IS is able to use the keychain to implement hitless key rollover for authentication. ey rollover specification
is time based, and in the event of clock skew between the peers, the rollover process is impacted. The
configurable tolerance specification allows for the accept window to be extended (before and after) by that
margin. This accept window facilitates a hitless key rollover for applications (for example, routing and
management protocols).
See Cisco ASR 9000 Series Aggregation Services Router System Security Guide for information on keychain
management.

Nonstop Forwarding

On Cisco IOS XR software, NSF minimizes the amount of time a network is unavailable to its users following
a route processor (RP) failover. The main objective of NSF is to continue forwarding IP packets and perform
a graceful restart following an RP failover.
When a router restarts, all routing peers of that device usually detect that the device went down and then came
back up. This transition results in what is called a routing flap, which could spread across multiple routing
domains. Routing flaps caused by routing restarts create routing instabilities, which are detrimental to the
overall network performance. NSF helps to suppress routing flaps in NSF-aware devices, thus reducing
network instability.
NSF allows for the forwarding of data packets to continue along known routes while the routing protocol
information is being restored following an RP failover. When the NSF feature is configured, peer networking
devices do not experience routing flaps. Data traffic is forwarded through intelligent line cards while the
standby RP assumes control from the failed active RP during a failover. The ability of line cards to remain
up through a failover and to be kept current with the Forwarding Information Base (FIB) on the active RP is
key to NSF operation.
When the Cisco IOS XR router running IS-IS routing performs an RP failover, the router must perform two
tasks to resynchronize its link-state database with its IS-IS neighbors. First, it must relearn the available IS-IS
neighbors on the network without causing a reset of the neighbor relationship. Second, it must reacquire the
contents of the link-state database for the network.
The IS-IS NSF feature offers two options when configuring NSF:
• IETF NSF
• Cisco NSF
If neighbor routers on a network segment are NSF aware, meaning that neighbor routers are running a software
version that supports the IETF Internet draft for router restartability, they assist an IETF NSF router that is
restarting. With IETF NSF, neighbor routers provide adjacency and link-state information to help rebuild the
routing information following a failover.
In Cisco IOS XR software, Cisco NSF checkpoints (stores persistently) all the state necessary to recover from
a restart without requiring any special cooperation from neighboring routers. The state is recovered from the
neighboring routers, but only using the standard features of the IS-IS routing protocol. This capability makes
Cisco NSF suitable for use in networks in which other routers have not used the IETF standard implementation
of NSF.
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