GTP-C Echo Timeout: The Silent Killer of User Plane Sessions

Spent the morning chasing a phantom “no internet” complaint on a UGW9811 node. Users could attach, get an IP, but data stalled after ~30s.

The culprit? GTP-C Echo (Path Failure Detection).

Here’s what was happening:

• SGW → PGW Echo Request every 60s
• PGW answered fine for 3 cycles, then went silent
• After 3 missed echoes (N3 retry), PGW declared the path DOWN
• All GTP-U tunnels on that path got torn down → session reset

Fix: the PGW’s loopback interface had a flapping route. One display gtp path + a static host route later, echoes are clean.

Lesson: when users “connect but can’t browse,” don’t jump to APN or AAA. Check display gtp echo-statistics first — 80% of the time it’s path-level, not session-level.

Quick filter if you’re in Wireshark:

gtpv2c.message_type == 1 || gtpv2c.message_type == 2

(1 = Echo Request, 2 = Echo Response)

Zero responses = your path is dead. Simple as that.