LACP fallback allows an interface that is part of a LAG to continue forwarding traffic even when LACP negotiation temporarily fails -- such as during a Mobility Gateway reboot or upgrade.
This enables the upgrade to proceed without removing interfaces from the LAG, since fallback mode keeps the link operational until LACP becomes available again.

When using UBT (User-Based Tunneling), packets are encapsulated, which increases their size.
HTTPS traffic is sensitive to MTU issues -- if packets exceed the path MTU and PMTUD is not functioning, sessions break or stall.
To prevent this, Path MTU Discovery must be enabled, ensuring devices automatically determine the correct MTU and avoid fragmentation-related issues.
Jumbo frames or manual fragmentation do not solve the underlying PMTU mismatch, and fragmentation is generally avoided for tunneled traffic.

In a VSX pair, each switch maintains its own configuration and can only aggregate its local interfaces into a LAG. To create a LAG that uses redundant links from both VSX peers toward an access switch, each chassis must configure a multi-chassis LAG with its local member port. The configuration that defines a LAG as multi-chassis and adds only the single local interface to that LAG is therefore the correct example for the required setup on both VSX switches.

The route-map includes set as-path prepend 65345 65345 65345
This increases the AS-path length of the exported routes, making them less preferred by external peers.
The advertised AS path is longer than the default, which is the correct interpretation of the configuration.