Hi Pat,
If your router have multiple RPs, you can use ISSU to upgrade.
From a software perspective, the Cisco ASR 1000 Series Router takes full advantage of Cisco IOS® Software High Availability features. The Cisco ASR 1000 IOS XE Software offers basic High Availability features such as Configuration Synchronization, Warm Standby, and Warm Reboot. It also supports advanced High Availability features such as Nonstop Forwarding with Stateful Switchover (NSF/SSO), Multiprotocol Label Switching (MPLS) High Availability, Bidirectional Forwarding Detection (BFD), High Availability for the Cisco Session Border Controller (SBC-HA), and In Service Software Upgrade (ISSU). A major differentiator of the Cisco ASR 1000 Series Router is that it provides integrated services with outstanding performance in a midrange platform - and it offers such services with High Availability.
Improved resilience against outages is even supported on the non hardware -redundant Cisco ASR 1002 Router and Cisco ASR 1004 Router. Both systems allow for two independent Cisco IOS XE Software processes to run in their own protective memory spaces on the route processor. Each Cisco IOS XE Software process maintains its own system state information for control-plane, Layer 2, and Layer 3 protocols. This state is continuously mirrored from the active to the standby Cisco IOS XE Software process. Upon encountering a failure or forced switchover, the standby Cisco IOS XE Software process immediately starts the execution of control-plane functions and any resulting updates in its system state tables.
The remainder of this paper discusses the Cisco ASR 1000 Series Router carrier-class advantages, from both chassis-design and software perspectives.
Link:
http://www.cisco.com/en/US/prod/collateral/routers/ps9343/solution_overview_c22-450809_ps9343_Product_Solution_Overview.html
HTH
