F5 BIG-IP Pool Member Down: How to Troubleshoot It
When a BIG-IP pool member shows red (down), the load balancer's health monitor is failing for it. The member may be fine — the monitor, port, or path is what's broken. Here's how to tell.
Step 1 — Confirm what's down
tmsh show ltm pool <pool> members
tmsh show ltm node <node>A red member with a failing monitor means the BIG-IP's health check isn't getting the expected response. If the node itself is down, every pool using it fails.
Step 2 — Check the monitor
- Wrong port: the monitor must target the port the service actually listens on.
- Receive string: an HTTP monitor expecting a specific response string fails if the server's reply changed (e.g., a redirect or a 401).
- Send string / interval: too-aggressive timeouts or a wrong request can mark a healthy member down.
Step 3 — Test the monitor path from the BIG-IP
The monitor sources from a self-IP, not your laptop. Test from the BIG-IP itself:
curl -sv http://<member-ip>:<port>/
tcpdump -ni 0.0 host <member-ip> and port <port>If this fails, the member is genuinely unreachable from the BIG-IP — a routing, VLAN, or firewall issue between the self-IP and the server (a common cause that looks like a "member down").
Step 4 — Fix
Correct the monitor (port, send/receive string, timers), fix reachability from the self-IP to the member, or address the real server outage. Check /var/log/ltm for the monitor's failure reason. Once the monitor passes, the member returns to green.
How Tech Matrix solves this in ~60 seconds
A down member could be the monitor, the path, or the server — and they look identical in the GUI. Tech Matrix reads the pool/monitor config and tests the path from the self-IP, tells you which of the three it is, and gives the fix for your TMOS version, with your approval.
Frequently asked questions
The health monitor assigned to the pool is failing for that member — usually the wrong port, a receive-string mismatch, or the BIG-IP self-IP can't reach the member, rather than the server being down.
From the BIG-IP CLI, curl the member on the monitored port (curl -sv http://member:port/) and tcpdump to confirm the self-IP can reach it — monitors source from the self-IP, not your client.
/var/log/ltm records monitor state changes and the reason a member was marked down.