Load balancing on a Junos device involves distributing packets to achieve efficient network traffic management. It primarily balances packets on a per-flow basis to maintain session integrity by ensuring that all packets of the same flow take the same path. Additionally, it is enabled through a policy, requiring explicit configuration to define how the traffic should be balanced. This approach is crucial for both performance and reliability, ensuring balanced resource utilization and consistent delivery of network services.

Unified ISSU is based on nonstop-routing (NSR) and graceful Routing Engine switchover (GRES), and permits software upgrades only. This statement is accurate as it clearly outlines the basis of the Unified ISSU and correctly states its limitation to software upgrades, ensuring minimal disruption.

Graceful Routing Engine switchover provides redundancy by preserving interface and kernel information, preserving the data plane information, and not restarting the PFE. Preserving interface and kernel information ensures that essential configurations and states are maintained. Preserving the data plane information ensures that packet forwarding continues without interruption. Not restarting the PFE maintains the operation of the packet forwarding engine, which is crucial for continuous network performance.

Unified In-Service Software Upgrade (ISSU) offers significant benefits such as reducing operating costs while maintaining high service levels and it necessitates the use of graceful Routing Engine switchover (GRES) and nonstop active routing (NSR) to ensure minimal disruption during upgrades.

When an OSPF adjacency is stuck in the ExStart state, it is often due to a mismatch in the MTU settings between the two OSPF peers. Ensuring that the MTU settings match on both sides typically resolves this issue. This is because OSPF requires the MTU to be the same to successfully create the adjacency and proceed past the ExStart state.