我有一个 Debian Jessie 8.2 服务器作为 Xen VM 主机。服务器有两个使用 LACP 绑定在一起的物理网络接口。还有一个xenbr0接口。我想启用巨型帧，但我无法做到。这是网络配置：
auto eth0
iface eth0 inet manual
bondmaster bond0
mtu 9000
auto eth1
iface eth1 inet manual
bondmaster bond0
mtu 9000
auto bond0
iface bond0 inet manual
bond-mode 4
bond-miimon 100
bond-lacp-rate 1
bond-slaves eth0 eth1
mtu 9000
auto xenbr0
iface xenbr0 inet static
bridge_ports bond0
address 10.0.0.12
gateway 10.0.0.1
netmask 255.255.255.0
mtu 9000
将 MTU 添加到每个接口后，我运行了sudo ifdown bond0 && sudo ifup bond0. MTU 仍报告为 1,500。然后我在界面上尝试了相同的操作xenbr0，但得到以下输出：
user@host:~$ sudo ifdown xenbr0 && sudo ifup xenbr0
ifdown: interface xenbr0 not configured
Waiting for xenbr0 to get ready (MAXWAIT is 32 seconds).
RTNETLINK answers: File exists
Failed to bring up xenbr0.
以下是 的输出ip a，没有 VM 来宾的虚拟接口：
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default
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: eth0: <BROADCAST,MULTICAST,SLAVE,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master bond0 state UP group default qlen 1000
link/ether e8:94:f6:08:aa:30 brd ff:ff:ff:ff:ff:ff
3: eth1: <BROADCAST,MULTICAST,SLAVE,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master bond0 state UP group default qlen 1000
link/ether e8:94:f6:08:aa:30 brd ff:ff:ff:ff:ff:ff
4: bond0: <BROADCAST,MULTICAST,MASTER,UP,LOWER_UP> mtu 1500 qdisc noqueue master xenbr0 state UP group default
link/ether e8:94:f6:08:aa:30 brd ff:ff:ff:ff:ff:ff
17: xenbr0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default
link/ether e8:94:f6:08:aa:30 brd ff:ff:ff:ff:ff:ff
inet 10.0.0.12/24 brd 10.0.0.255 scope global xenbr0
valid_lft forever preferred_lft forever
inet6 2001:****:****:****:****:****:****:****/64 scope global mngtmpaddr dynamic
valid_lft 2591736sec preferred_lft 604536sec
inet6 fe80::ea94:f6ff:fe08:aa30/64 scope link
valid_lft forever preferred_lft forever
我做错了什么？