On an MX Series device with dual Routing Engines, which two conditions cause a unified ISSU to fail? (Choose two.)
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A. B. C. D.CD
On an MX Series device with dual Routing Engines, unified In-Service Software Upgrade (ISSU) allows you to upgrade the software without any traffic interruption, which means that you can avoid any traffic loss that would occur during a normal reboot of the device.
However, there are certain conditions under which unified ISSU can fail. Two of those conditions are:
B. BFD is enabled: Bidirectional Forwarding Detection (BFD) is a protocol used to detect link failures between two routers. During a unified ISSU, the control plane is upgraded first, and BFD might detect the control plane upgrade as a link failure, leading to a failover to the other Routing Engine. If BFD is enabled, it can cause a failover and disrupt the ISSU.
C. GRES is disabled: Graceful Routing Engine Switchover (GRES) is a feature that allows a router to perform a non-disruptive switchover from the primary Routing Engine to the secondary Routing Engine. During a unified ISSU, GRES ensures that the switch is performed smoothly without any traffic loss. If GRES is disabled, the switch might not be smooth, and there may be traffic loss, leading to a failed unified ISSU.
Therefore, B and C are the correct answers to this question.
A. Graceful restart is disabled: Graceful restart is a feature that allows a router to recover quickly from a routing protocol process crash or a Routing Engine switchover. Although graceful restart is a useful feature, it is not related to unified ISSU, and disabling it will not cause a unified ISSU to fail.
D. NSR is disabled: Nonstop active routing (NSR) is a feature that enables the router to maintain its routing table during a Routing Engine switchover. Although NSR is a useful feature, it is not related to unified ISSU, and disabling it will not cause a unified ISSU to fail.