Seems like this commit introduced the regression: http://cgit.freedesktop.org/NetworkManager/NetworkManager/commit/?id=9b79e6c732ffb2fb105647c1465070d36a6cc180
I'm including a reversion patch for the relevant piece, though, it is likely not a clean patch for upstream.
diff --git a/src/nm-ip4-config.c b/src/nm-ip4-config.c index 5918f3d..3d67d54 100644 --- a/src/nm-ip4-config.c +++ b/src/nm-ip4-config.c @@ -267,6 +267,7 @@ gboolean nm_ip4_config_commit (const NMIP4Config *config, int ifindex, gboolean routes_full_sync, gint64 default_route_metric) { NMIP4ConfigPrivate *priv = NM_IP4_CONFIG_GET_PRIVATE (config); + guint32 mtu = nm_ip4_config_get_mtu (config); int i; gs_unref_ptrarray GPtrArray *added_addresses = NULL; @@ -339,6 +340,10 @@ nm_ip4_config_commit (const NMIP4Config *config, int ifindex, gboolean routes_fu return FALSE; } + /* MTU */ + if (mtu && mtu != nm_platform_link_get_mtu (NM_PLATFORM_GET, ifindex)) + nm_platform_link_set_mtu (NM_PLATFORM_GET, ifindex, mtu); + return TRUE; }