At this stage all the preconditions have been met. The unified ISSU process shuts down all management interfaces to the router in order to prevent changes in the configuration.
Final preparation for the upgrade phase includes the following actions:
The reason for raising the link cost is that once the upgrade of the line module control plane begins, routing protocol updates cannot be installed in the line modules until that upgrade completes. That period can be in the range 2–15 minutes. During the control plane upgrade, the routing protocols can still accept new routes and communicate with their neighbors but cannot install the routes.
Once the upgrade of the line module control plane begins, routing protocol updates cannot be installed in the line modules until that upgrade completes. That period can be in the range 2–15 minutes. During the control plane upgrade, the routing protocols can still accept new routes and communicate with their neighbors but cannot install the routes.
Raising the link cost for routes through the upgrading router enables neighbors to recompute better routes to those destinations that do not pass through the upgrading router. If you choose to raise the link cost, the higher costs can take some time to propagate through the network. Because the router is unable to determine when this has completed, it waits for 2 minutes before proceeding to the next step in the upgrade.