Unified In-Service Software Upgrades (ISSU)

D

Unified In-Service Software Upgrade (ISSU) is a technique that enables network devices to upgrade their software without affecting the network traffic. ISSU helps minimize disruption, prevent traffic loss, and improve network availability.

Among the given options, the correct statement that describes unified ISSU is option C. Unified ISSU permits both upgrades and downgrades with no control plane disruption and is based on the graceful Routing Engine switchover (GRES) feature.

Here is a more detailed explanation of the correct answer:

C. Unified ISSU permits both upgrades and downgrades with no control plane disruption and is based on the graceful Routing Engine switchover (GRES) feature.

Unified ISSU is a technique that allows a network device to upgrade its software without interrupting its forwarding of traffic. Unified ISSU works by allowing the device to upgrade its software while the device is still forwarding traffic. This is achieved by the device installing the new software on standby Routing Engines without disrupting the control plane or forwarding plane operations.

Unified ISSU builds upon the concept of Graceful Routing Engine Switchover (GRES). GRES is a feature that allows the backup Routing Engine to take over the duties of the primary Routing Engine in case of a failure, without disrupting the control plane operations.

Unified ISSU takes GRES a step further by allowing software upgrades to take place during the GRES process. During the upgrade process, the primary Routing Engine is upgraded first. Once the primary Routing Engine is upgraded, control plane operations are switched to the backup Routing Engine. The backup Routing Engine then becomes the primary Routing Engine and begins forwarding traffic. The process of switching control from the primary Routing Engine to the backup Routing Engine is a seamless process and results in no control plane disruption.

In summary, unified ISSU is a technique that allows network devices to upgrade their software without interrupting network traffic. Unified ISSU builds upon the GRES feature and permits software upgrades and downgrades with no control plane disruption.