[Bug 1979874] Re: Unable to set bridge to lower MTU that interface its attached to

[Lukas Märdian]

Thanks for the feedback. I need to further investigate this.

** Changed in: netplan
       Status: Incomplete => New

** Also affects: netplan.io (Ubuntu)
   Importance: Undecided
       Status: New

** Tags added: rls-jj-incoming

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Title:
  Unable to set bridge to lower MTU that interface its attached to

Status in netplan:
  New
Status in netplan.io package in Ubuntu:
  New

Bug description:
  Affected Ubuntu 20.04 and 22.04

  I have two interfaces in an LACP bond that have been set to MTU 9000
  as it's required for a VLAN interface that then passes through to
  bridge114 which needs MTU 9000. The issue I'm having is I also require
  bridge-lan bridge on bond0 not to go through a VLAN and this needs to
  be MTU to be 1500 but is set to 9000. Below is my netplan and output
  of "ip a"

  network:
    version: 2
    renderer: networkd
    ethernets:
      enp2s0:
        optional: true
        dhcp4: false
        dhcp6: false
      enp3s0:
        optional: true
        dhcp4: false
        dhcp6: false
    bonds:
      bond0:
        interfaces: [enp2s0,enp3s0]
        dhcp4: false
        dhcp6: false
        mtu: 9000
        parameters:
          mode: 802.3ad
          lacp-rate: fast
          transmit-hash-policy: layer2
          mii-monitor-interval: 100
    vlans:
      vlan102:
        link-local: []
        accept-ra: no
        id: 102
        link: bond0
        mtu: 1500
      vlan114:
        link-local: []
        accept-ra: no
        id: 114
        link: bond0
        mtu: 9000
    bridges:
      bridge-lan:
        macaddress: 62:21:10:24:86:e7
        interfaces: [bond0]
        dhcp4: true
        dhcp6: true
        mtu: 1500
        parameters:
          stp: false
          forward-delay: 0
      bridge102:
        interfaces: [vlan102]
        mtu: 1500
        dhcp4: false
        dhcp6: false
        link-local: []
        accept-ra: no
        parameters:
          stp: false
      bridge114:
        interfaces: [vlan114]
        mtu: 9000
        dhcp4: true
        dhcp6: false
        link-local: []
        accept-ra: no
        parameters:
          stp: false

  1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
      link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
      inet 127.0.0.1/8 scope host lo
         valid_lft forever preferred_lft forever
      inet6 ::1/128 scope host
         valid_lft forever preferred_lft forever
  2: enp2s0: <BROADCAST,MULTICAST,SLAVE,UP,LOWER_UP> mtu 9000 qdisc fq_codel master bond0 state UP group default qlen 1000
      link/ether 62:21:10:24:86:e7 brd ff:ff:ff:ff:ff:ff permaddr 00:01:2e:67:c8:39
  3: enp3s0: <BROADCAST,MULTICAST,SLAVE,UP,LOWER_UP> mtu 9000 qdisc fq_codel master bond0 state UP group default qlen 1000
      link/ether 62:21:10:24:86:e7 brd ff:ff:ff:ff:ff:ff permaddr 00:01:2e:67:c8:3a
  4: wlp4s0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000
      link/ether e0:94:67:76:a0:e3 brd ff:ff:ff:ff:ff:ff
  5: bond0: <BROADCAST,MULTICAST,MASTER,UP,LOWER_UP> mtu 9000 qdisc noqueue master bridge-lan state UP group default qlen 1000
      link/ether 62:21:10:24:86:e7 brd ff:ff:ff:ff:ff:ff
  6: bridge-lan: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9000 qdisc noqueue state UP group default qlen 1000
      link/ether 62:21:10:24:86:e7 brd ff:ff:ff:ff:ff:ff
      inet 172.28.185.201/24 metric 100 brd 172.28.185.255 scope global dynamic bridge-lan
         valid_lft 6295sec preferred_lft 6295sec
      inet6 2a02:8010:6538:10:6021:10ff:fe24:86e7/64 scope global dynamic mngtmpaddr noprefixroute
         valid_lft 86245sec preferred_lft 14245sec
      inet6 fe80::6021:10ff:fe24:86e7/64 scope link
         valid_lft forever preferred_lft forever
  7: bridge102: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
      link/ether 72:f5:bc:58:9a:cb brd ff:ff:ff:ff:ff:ff
  8: bridge114: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9000 qdisc noqueue state UP group default qlen 1000
      link/ether ae:56:9d:92:a6:4e brd ff:ff:ff:ff:ff:ff
      inet 172.28.187.123/24 metric 100 brd 172.28.187.255 scope global dynamic bridge114
         valid_lft 6295sec preferred_lft 6295sec
  9: vlan102 at bond0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master bridge102 state UP group default qlen 1000
      link/ether 62:21:10:24:86:e7 brd ff:ff:ff:ff:ff:ff
  10: vlan114 at bond0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9000 qdisc noqueue master bridge114 state UP group default qlen 1000
      link/ether 62:21:10:24:86:e7 brd ff:ff:ff:ff:ff:ff
  11: lxdbr0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN group default qlen 1000
      link/ether 00:16:3e:4f:03:63 brd ff:ff:ff:ff:ff:ff
      inet 10.55.236.1/24 scope global lxdbr0
         valid_lft forever preferred_lft forever
      inet6 fd42:167e:91e4:b07a::1/64 scope global
         valid_lft forever preferred_lft forever
  13: veth5e7b7985 at if12: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master bridge102 state UP group default qlen 1000
      link/ether 86:79:cd:04:f8:a4 brd ff:ff:ff:ff:ff:ff link-netnsid 0
  15: veth5237a3b8 at if14: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9000 qdisc noqueue master bridge114 state UP group default qlen 1000
      link/ether aa:6a:74:5b:6d:26 brd ff:ff:ff:ff:ff:ff link-netnsid 0

  I can manually work around this by running "sudo ip link set mtu 1500
  bridge-lan" after reboot. Still, I have to do this after every
  automatic reboot due to unattended upgrades or I will have networking
  issues with this node as bridge-lan is on an MTU 1500 VLAN.

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