Broken PMTU Discovery - LTM not returning ICMP Type 3 Code 4
Hello,
I have an issue where I can't ping an upstream router interface through f5 LTM. I've created specific IP virtual forwarding servers for inbound and outbound ICMP (IP protocol 1) and applied a FastL4 profile with "Reassemble IP Fragments" checked. I've tried to change the server type to a Performance (Layer 4), but still no luck.
I am running LACP on the ingress and egress interfaces. I've monkeyed with the VLAN MTUs. It appears that LTM is black-holing PMTUD. If I ping the upstream interface with a payload size of 1454, the ping's successful. A size of 1455 breaks. TCPDUMP on the inside and outside interface show no attempt to forward an ICMP Destination Unreachable to me, and the packet is never forwarded upstream.
I've also monkeyed with the various TMM options:
tmsh modify sys db tm.pathmtudiscovery value enable/disable
tmsh modify sys db tm.enforcepathmtu value enable/disable
tmsh modify sys db route.metrics.mtu value enable/disable
As for the route.metrics.mtu, I've tried modifying the individual route MTUs...no luck.
Anyone ever see this kind of behavior? I'm wondering if LACP is causing me some sort of issue - why a payload size max of 1454? I should not fragment up to 1472 bytes for payload: 1472 + 8 byte ICMP + 20 byte IP headers. There's an 18 byte difference, as if LTM is taking into account the 14 byte Ethernet header and the 4 byte .1Q tag? We're utilizing Ethernet II/ARPA framing, so there's no 4 byte CRC.
Any thoughts/ideas/suggestions would be appreciated!
-Jason