Issue Report
Bug
Between Container Linux releases, the default interface MTU used on AWS instances supporting Jumbo packets has changed/regressed from 9001 to 1500. This causes connectivity problems between pods across node boundaries. CNI providers don't expect the MTU to change over time node-by-node. It also means Jumbo packets aren't being used. I received pages shortly after the OS auto-update occurred.
Container Linux Version
Stable 1745.4.0
Stable 1745.3.1
NAME="Container Linux by CoreOS"
ID=coreos
VERSION=1745.4.0
VERSION_ID=1745.4.0
BUILD_ID=2018-05-24-2146
PRETTY_NAME="Container Linux by CoreOS 1745.4.0 (Rhyolite)"
ANSI_COLOR="38;5;75"
HOME_URL="https://coreos.com/"
BUG_REPORT_URL="https://issues.coreos.com"
COREOS_BOARD="amd64-usr"
Environment
AWS
Reproduction Steps
Launch an EC2 instance that supports Jumbo packets. A t2.small will suffice. With stable 1688.5.3, the interface comes up with MTU 9001.
$ ip link show eth0
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9001 qdisc pfifo_fast state UP mode DEFAULT group default qlen 1000
link/ether 06:06:9f:dc:08:10 brd ff:ff:ff:ff:ff:ff
Now try with stable 1745.3.1 or 1745.4.0. The MTU has changed to 1500, as though Jumbo frame support isn't available.
$ ip link show eth0
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT group default qlen 1000
link/ether 02:6d:6b:43:05:ec brd ff:ff:ff:ff:ff:ff