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    Understanding the Unified ISSU Process on a Router

    This topic describes the processes that take place on a router with dual Routing Engines when you initiate a unified in-service software upgrade (ISSU).

    Unified ISSU Process on a Router

    After you use the request system software in-service-upgrade command, the following process occurs.

    In Figure 1 through Figure 6 that follow:

    • A solid line indicates the high-speed internal link between a Routing Engine and a Packet Forwarding Engine.
    • A dotted line indicates the messages exchanged between the Packet Forwarding Engine and the chassis process (chassisd) on the Routing Engine.
    • RE0m and RE1b indicate master and backup Routing Engines, respectively.
    • The check mark indicates that the device is running the new version of software.

    Note: Unified ISSU can only upgrade up to three major releases ahead of the current release on a device. To upgrade to a release more than three releases ahead of the current release on a device, use the unified ISSU process to upgrade the device to one or more intermediate releases until the device is within three major releases of the target release.

    Note: The following process pertains to all supported routing platforms except the TX Matrix router and TX Matrix Plus router. On most routers, the Packet Forwarding Engine resides on a Flexible PIC Concentrator (FPC). However, on an M120 router, the Forwarding Engine Board (FEB) replaces the functions of a Packet Forwarding Engine. In the illustrations and steps, when considering an M120 router, you can regard the Packet Forwarding Engine as an FPC. As an additional step on an M120 router, after the FPCs and PICs have been upgraded, the FEBs are upgraded.

    1. The master Routing Engine validates the router configuration to ensure that it can be committed when you use the new software version.

      Checks are made for the following:

      • Disk space is available for the /var file system on both Routing Engines.
      • The configuration is supported by a unified ISSU.
      • The PICs are supported by a unified ISSU.
      • Graceful Routing Engine switchover is enabled.
      • Nonstop active routing is enabled.

      These checks are the same as the checks made when you enter the request system software validate in-service-upgrade command. If there is insufficient disk space available on either of the Routing Engines, the unified ISSU process fails and returns an error message. However, unsupported PICs do not prevent a unified ISSU. If there are unsupported PICs, the system issues a warning to indicate that these PICs will restart during the upgrade. Similarly, if there is an unsupported protocol configured, the system issues a warning that packet loss might occur for the unsupported protocol during the upgrade.

      Figure 1: Device Status Before Starting a Unified ISSU

      Device Status Before Starting
a Unified ISSU
    2. After the validation succeeds, the management process installs (copies) the new software image to the backup Routing Engine.
    3. The backup Routing Engine is rebooted.
    4. After the backup Routing Engine is rebooted and is running the new software, the kernel state synchronization process (ksyncd) synchronizes (copies) the configuration file and the kernel state from the master Routing Engine.

      Figure 2: Device Status After the Backup Routing Engine Is Upgraded

      Device Status After
the Backup Routing Engine Is Upgraded
    5. After the configuration file and the kernel state are synchronized to the backup Routing Engine, the chassis process (chassisd) on the master Routing Engine prepares other software processes for the unified ISSU. The chassis process informs the various software processes (such as rpd, apsd, bfdd, and so on) about the unified ISSU and waits for responses from them. When all the processes are ready, the chassis process sends an ISSU_PREPARE message to the FPCs installed in the router. You can display the unified ISSU process messages by using the show log messages command.
    6. The Packet Forwarding Engine on each FPC saves its state and downloads the new software image from the backup Routing Engine. Next, each Packet Forwarding Engine sends an ISSU_READY message to the chassis process.

      Figure 3: Device Status After One Packet Forwarding Engine Downloads the New Software

      Device Status After
One Packet Forwarding Engine Downloads the New Software
    7. After receiving an ISSU_READY message from a Packet Forwarding Engine, the chassis process sends an ISSU_REBOOT message to the FPC on which the Packet Forwarding Engine resides. The FPC reboots with the new software image. After the FPC is rebooted, the Packet Forwarding Engine restores the FPC state, and a high-speed internal link is established with the backup Routing Engine running the new software. The chassis process link is also reestablished with the master Routing Engine.

      Note: The Packet Forwarding Engine reboots that occur during an unified ISSU are designed to have a very short window of down time.

    8. After all Packet Forwarding Engines have sent a READY message using the chassis process on the master Routing Engine, other software processes are prepared for a Routing Engine switchover. The system is ready for a switchover at this point.

      Figure 4: Device Status Before the Routing Engine Switchover

      Device Status Before
the Routing Engine Switchover

      Note: For M120 routers, the FEBs are upgraded at this point. When all FEBs have been upgraded, the system is ready for a switchover.

    9. The Routing Engine switchover occurs, and the Routing Engine (re1) that was the backup now becomes the master Routing Engine.

      Figure 5: Device Status After the Routing Engine Switchover

      Device
Status After the Routing Engine Switchover
    10. The new backup Routing Engine is now upgraded to the new software image. (This step is skipped if you have specified the no-old-master-upgrade option in the request system software in-service-upgrade command.)

      Figure 6: Device Status After the Unified ISSU Is Complete

      Device Status
After the Unified ISSU Is Complete
    11. When the backup Routing Engine has been successfully upgraded, the unified ISSU is complete.

    Modified: 2016-02-03