I've had an interesting situation with osmo-ggsn and kernel GTP module that I wonder if someone could shed some light on. I've set up osmo-ggsn (v1.7.1) on Debian 10. There are two ethernet interfaces on the system, ens224 (for the Gn Interface) and ens256 (for the Gi Interface). Using the following ggsn configuration seems to work fine: ggsn ggsn0 description Keith's Test APN gtp state-dir /tmp gtp bind-ip <ens224 IP> gtp control-ip <ens224 IP> apn keithtest gtpu-mode tun tun-device apn0 type-support v4 ip prefix dynamic 192.168.42.0/24 ip dns 0 <redacted> ip dns 1 <redacted> ip ifconfig 192.168.42.0/24 no shutdown default-apn keithtest no shutdown ggsn I get the following tun device: 5: apn0: <POINTOPOINT,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UNKNOWN group default qlen 500 link/none inet 192.168.42.0/24 scope global apn0 valid_lft forever preferred_lft forever inet6 fe80::fada:2e69:ad71:a61b/64 scope link stable-privacy valid_lft forever preferred_lft forever Traffic routes, all is good with the world. However, if I change to using Kernel GTP, by changing the gtpu-mode to "kernel-gtp", traffic will not route (and, yes, I have done modprobe gtp). After some exploration, it appears the tun device is being created with an MTU of zero which then, cannot pass traffic: 10: apn0: <POINTOPOINT,UP,LOWER_UP> mtu 0 qdisc noqueue state UNKNOWN group default qlen 1000 link/none inet 192.168.42.0/24 scope global apn0 valid_lft forever preferred_lft forever Here are the logs: Tue Apr 13 11:02:06 2021 DGGSN NOTICE <0002> ggsn.c:830 GGSN(ggsn0): Successfully started Tue Apr 13 11:02:06 2021 DGGSN INFO <0002> ggsn.c:186 APN(keithtest): Starting Tue Apr 13 11:02:06 2021 DGGSN INFO <0002> ggsn.c:204 APN(keithtest): Opening Kernel GTP device apn0 Tue Apr 13 11:02:06 2021 DGGSN NOTICE <0002> gtp-kernel.c:79 Initialized GTP kernel mode (genl ID is 24) Tue Apr 13 11:02:06 2021 DTUN NOTICE <0001> tun.c:217 GTP kernel configured Tue Apr 13 11:02:06 2021 DGGSN INFO <0002> ggsn.c:236 APN(keithtest): Setting tun IP address 192.168.42.0/24 Tue Apr 13 11:02:06 2021 DGGSN INFO <0002> ggsn.c:294 APN(keithtest): Creating IPv4 pool 192.168.42.0/24 Tue Apr 13 11:02:06 2021 DGGSN INFO <0002> ggsn.c:168 APN(keithtest): Blacklist tun IP 192.168.42.0/24 Tue Apr 13 11:02:06 2021 DGGSN NOTICE <0002> ggsn.c:325 APN(keithtest): Successfully started Note that if I manually alter the MTU, everything starts working again: ip link set dev apn0 mtu 1500 So I can put this into a script and run it via ipup-script, and this works fine. I just get the impression I shouldn't have to do this, and the system should be setting the MTU, just as it does when I use tun mode. Any thoughts on what I'm doing wrong here? Cheers, Keith