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[Dimiter Naydenov <dimiter.naydenov@xxxxxxxxxxxxx>]

juju-br0 was reintroduced in 1.23.3 as the default behavior in MAAS,
unless the "address-allocation" feature flag is enabled.

AIUI the juju-br0 "swallows" the "interface-mtu" DHCP setting coming
from MAAS. It doesn't happen when eth0 is not bound to juju-br0. So it
looks like this is a pre-existing issue since for ever.

I'm open to suggestions how Juju can work around the issue (e.g. by
generating a slightly different /etc/network/interfaces file with the
juju-br0 definition and/or brctl commands).

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https://bugs.launchpad.net/bugs/1403955

Title:
  DHCP's "Option interface-mtu 9000" is being ignored on bridge
  interface br0

Status in juju-core:
  Triaged
Status in isc-dhcp package in Ubuntu:
  Confirmed

Bug description:
  In an env with jumbo frames enabled, and using MAAS as DHCP server,
  the client receives the following IPv4 lease:

  $ cat /var/lib/dhcp/dhclient.br0.leases
  lease {
    interface "br0";
    fixed-address 10.230.20.26;
    filename "pxelinux.0";
    option subnet-mask 255.255.248.0;
    option dhcp-lease-time 43200;
    option routers 10.230.16.1;
    option dhcp-message-type 5;
    option dhcp-server-identifier 10.230.20.1;
    option domain-name-servers 10.230.20.1;
    option interface-mtu 9000;
    option broadcast-address 10.230.23.255;
    option domain-name "ctsstack.qa.1ss";
    renew 3 2014/12/17 16:48:15;
    rebind 3 2014/12/17 21:52:09;
    expire 3 2014/12/17 23:22:09;
  }

  The interfaces show the following config after boot:

  $ ifconfig br0
  br0       Link encap:Ethernet  HWaddr a0:d3:c1:01:9d:58
            inet addr:10.230.20.26  Bcast:10.230.23.255  Mask:255.255.248.0
            inet6 addr: fe80::a2d3:c1ff:fe01:9d58/64 Scope:Link
            UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
            RX packets:530530 errors:0 dropped:0 overruns:0 frame:0
            TX packets:1591569 errors:0 dropped:0 overruns:0 carrier:0
            collisions:0 txqueuelen:0
            RX bytes:68713489 (68.7 MB)  TX bytes:213710979 (213.7 MB)

  $ ifconfig eth0
  eth0      Link encap:Ethernet  HWaddr a0:d3:c1:01:9d:58
            inet6 addr: fe80::a2d3:c1ff:fe01:9d58/64 Scope:Link
            UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
            RX packets:10539274 errors:0 dropped:3394 overruns:0 frame:454
            TX packets:2627412 errors:0 dropped:0 overruns:0 carrier:0
            collisions:0 txqueuelen:1000
            RX bytes:2320560616 (2.3 GB)  TX bytes:3562885157 (3.5 GB)
            Interrupt:32

  
  "option interface-mtu 9000;" from the lease file is being ignored by br0. Could it be related to eth0 MTU size? If that's the case, shouldn't both interfaces be updated?

  Other info:

  $ brctl show br0
  bridge name	bridge id		STP enabled	interfaces
  br0		8000.a0d3c1019d58	no		eth0

  $ cat /etc/network/eth0.config
  iface eth0 inet manual

  auto br0
  iface br0 inet dhcp
    bridge_ports eth0

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