Fabric Switchover; Active/Standby Status Interpretation; Non-Stop Forwarding; Nonstop Routing - Cisco ASR 9001-S Overview And Reference Manual

High Availability Router Operations

Fabric Switchover

In the Cisco ASR 9010 Router, Cisco ASR 9006 Router, and Cisco ASR 9904 Router, the RSP card
makes up most of the fabric. The fabric is configured in an "active/active" configuration model, which
allows the traffic load to be distributed across both RSP cards. In the case of a failure, the single "active"
switch fabric continues to forward traffic in the systems.
In the Cisco ASR 9922 Router and Cisco ASR 9912 Router, fabric switching across the RP and line
cards is provided by a separate set of seven OIR FC cards operating in 6+1 redundancy mode. Any FC
card can be removed from the chassis, power-cycled, or provisioned to remain unpowered without
impacting system traffic. All FC cards remain active unless disabled or faulty. Traffic from the line cards
is distributed across all FC cards.

Active/Standby Status Interpretation

Status signals from each RSP/RP card are monitored to determine active/standby status and if a failure
has occurred that requires a switchover from one RSP/RP card to the other.

Non-Stop Forwarding

Cisco IOS XR Software supports non-stop forwarding (NSF) to enable the forwarding of packets without
traffic loss during a brief outage of the control plane. NSF is implemented through signaling and routing
protocol implementations for graceful restart extensions as standardized by the Internet Engineering Task
Force (IETF).
For example, a soft reboot of certain software modules does not hinder network processors, the switch
fabric, or the physical interface operation of forwarding packets. Similarly, a soft reset of a non-data path
device (such as a Ethernet Out-of-Band Channel Gigabit Ethernet switch) does not impact the forwarding
of packets.

Nonstop Routing

Nonstop routing (NSR) allows forwarding of data packets to continue along known routes while the
routing protocol information is being refreshed following a processor switchover. NSR maintains
protocol sessions and state information across SSO functions for services such as MPLS VPN. TCP
connections and the routing protocol sessions are migrated from the active RSP/RP card to the standby
RSP/RP card after the RSP/RP switchover without letting peers know about the switchover. The sessions
terminate and the protocols running on the standby RSP/RP card reestablish the sessions after the
standby RSP/RP goes active. NSR can also be used with graceful restart to protect the routing control
plane during switchovers. The NSR functionality is available only for Open Shortest Path First Protocol
(OSPF) and Label Distribution Protocol (LDP) routing technologies.

Graceful Restart

Graceful restart (GR) provides a control plane mechanism to ensure high availability by allowing
detection and recovery from failure conditions while preserving Nonstop Forwarding (NSF) services.
Graceful restart is a way to recover from signaling and control plane failures without impacting the
Cisco ASR 9000 Series Aggregation Services Router Overview and Reference Guide
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Chapter 3 High Availability and Redundant Operation
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