How Can I Check ELB Unavailability or Routing Interruption?

Check the health of the backend server. If the backend server is unhealthy, traffic will be routed to those healthy ones.
Check whether security policies of the backend server allow access from 100.125.0.0/16.
Check the timeout duration of TCP connections between the client and the load balancer. By default, the timeout duration is 300s, which cannot be modified. If the timeout duration exceeds 300s, the load balancer sends an RST message to the client and backend server and disconnects the connection.
Check the source IP address before the request reaches the load balancer if the Source IP hash algorithm is used.
For example, if ELB works with the Content Delivery Network (CDN) or Web Application Firewall (WAF) service, the IP address of the request is changed after the request passes through CDN or WAF. As a result, the IP address is changed, and the session stickiness fails. If you want to use CDN or WAF, it is recommended that you add an HTTP or HTTPS listener and configure cookie-based sticky sessions.
Check the cookie value if sticky sessions are enabled for an HTTP or HTTPS listener. If the cookie value changes, traffic is routed to other backend servers.
Check the stickiness duration set for the backend server group. The default stickiness duration is 1 minute for TCP or UDP listeners, and 1440 minutes (24 hours) for HTTP or HTTPS listeners. If the stickiness duration times out, ELB becomes unavailable.