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netplan: setting MTU for bridge devices

netplan is the new default network configuration system for Ubuntu 17.10 (Artful) and onwards. It replaces /etc/network/interfaces.

The YAML configuration files are quite a lot nicer but there are a couple of bugs being worked out.

One quirk I discovered is that some settings - e.g. MTU - don't work if you're working with renamed interfaces when you match by name. For example, in an Artful VM with some added network interfaces, this doesn't work:

network:
    version: 2
    ethernets:
        switchdevs:
            match:
                name: ens[78]
            mtu: 1280
    bridges:
        br0:
            interfaces: [switchdevs]
            addresses:
                - 10.10.10.2/24

This creates the bridge, but the MTUs are unchanged:

$ ip l
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: ens3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT group default qlen 1000
    link/ether 52:54:00:f2:4b:48 brd ff:ff:ff:ff:ff:ff
3: ens7: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master br0 state UP mode DEFAULT group default qlen 1000
    link/ether 52:54:00:e2:29:24 brd ff:ff:ff:ff:ff:ff
4: ens8: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master br0 state UP mode DEFAULT group default qlen 1000
    link/ether 52:54:00:63:33:e3 brd ff:ff:ff:ff:ff:ff
5: br0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DEFAULT group default qlen 1000
    link/ether 8a:43:6e:24:d7:17 brd ff:ff:ff:ff:ff:ff

This is tracked in LP#1724895.

The workaround at the moment is to specify the devices with something other than name, for example MAC address:

network:
    version: 2
    ethernets:
        ens7:
            match:
                macaddress: 52:54:00:e2:29:24
            mtu: 1280
        ens8:
            match:
                macaddress: 52:54:00:63:33:e3
            mtu: 1280
    bridges:
        br0:
            interfaces: [ens7, ens8]
            addresses:
                - 10.10.10.2/24

If you apply this with a reboot, everything works:

$ ip l
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: ens3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT group default qlen 1000
    link/ether 52:54:00:f2:4b:48 brd ff:ff:ff:ff:ff:ff
3: ens7: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1280 qdisc pfifo_fast master br0 state UP mode DEFAULT group default qlen 1000
    link/ether 52:54:00:e2:29:24 brd ff:ff:ff:ff:ff:ff
4: ens8: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1280 qdisc pfifo_fast master br0 state UP mode DEFAULT group default qlen 1000
    link/ether 52:54:00:63:33:e3 brd ff:ff:ff:ff:ff:ff
5: br0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1280 qdisc noqueue state UP mode DEFAULT group default qlen 1000
    link/ether 8a:43:6e:24:d7:17 brd ff:ff:ff:ff:ff:ff

However, if you take a system without a bridge, and apply this with netplan generate and netplan apply, rather than rebooting, this renames the devices! ens7 and ens8 have now become eth0 and eth1!

$ ip l
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: ens3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT group default qlen 1000
    link/ether 52:54:00:f2:4b:48 brd ff:ff:ff:ff:ff:ff
5: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1280 qdisc pfifo_fast master br0 state UP mode DEFAULT group default qlen 1000
    link/ether 52:54:00:e2:29:24 brd ff:ff:ff:ff:ff:ff
6: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1280 qdisc pfifo_fast master br0 state UP mode DEFAULT group default qlen 1000
    link/ether 52:54:00:63:33:e3 brd ff:ff:ff:ff:ff:ff
7: br0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1280 qdisc noqueue state UP mode DEFAULT group default qlen 1000
    link/ether 8a:43:6e:24:d7:17 brd ff:ff:ff:ff:ff:ff


This is tracked by LP: #1753868 . So for the moment, check your yaml file with netplan generate and reboot for best results.

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