VPP unexpected default MTU `9000` for bonding interface
```
vyos@r14# run show conf com | match vpp
set interfaces vpp bonding vppbond2 member interface 'eth1'
set interfaces vpp bonding vppbond2 mode 'active-backup'
set vpp settings interface eth1
set vpp settings poll-sleep-usec '2222'
[edit]
vyos@r14# run show interfaces vpp
Kernel Dataplane Type IP Address MAC MTU State
-------- ------------- ------ ------------ ----------------- ----- -------
BondEthernet2 bond 52:54:00:f4:d5:cd 9000 up
eth1 dpdk 52:54:00:f4:d5:cd 1500 up
local0 local 00:00:00:00:00:00 0 down
eth1 tap4096 virtio 02:fe:36:06:73:38 1500 up
vppbond2 tap4097 virtio 02:fe:c6:83:c4:96 9000 up
[edit]
vyos@r14#
```
Try to set the MTU
```
vyos@r14# set interfaces vpp bonding vppbond2 mtu 1500
[edit]
vyos@r14# commit
run s[edit]
vyos@r14# run show interfaces vpp
Kernel Dataplane Type IP Address MAC MTU State
-------- ------------- ------ ------------ ----------------- ----- -------
BondEthernet2 bond 02:fe:a9:c6:01:db 9000 up
eth1 dpdk 52:54:00:f4:d5:cd 1500 up
local0 local 00:00:00:00:00:00 0 down
eth1 tap4096 virtio 02:fe:36:06:73:38 1500 up
vppbond2 tap4097 virtio 02:fe:9d:1b:aa:7c 9000 up
[edit]
vyos@r14#
```
The same for IPIP/GRE/etc
````
set interfaces vpp gre vppgre1 remote '192.0.2.52'
set interfaces vpp gre vppgre1 source-address '192.0.2.1'
set interfaces vpp ipip vppipip2 remote '192.0.2.2'
set interfaces vpp ipip vppipip2 source-address '192.0.2.1'
set vpp settings interface eth1
set vpp settings poll-sleep-usec '2341'
```
All they have unexpected MTU `9000`
```
vyos@r14# run show interfaces vpp
Kernel Dataplane Type IP Address MAC MTU State
-------- ----------- ------ ------------ ----------------- ----- -------
eth1 dpdk 192.0.2.1/24 52:54:00:f4:d5:cd 1500 up
gre1 GRE 00:00:00:00:00:00 9000 up
ipip2 IPIP 00:00:00:00:00:00 9000 up
local0 local 00:00:00:00:00:00 0 down
eth1 tap4096 virtio 02:fe:05:e2:e8:73 1500 up
vppgre1 tun4097 virtio 00:00:00:00:00:00 9000 up
vppipip2 tun4098 virtio 00:00:00:00:00:00 9000 up
[edit]
vyos@r14#
```
---
There are several MTU option, I think we need `packet`
```
They have several MTU options
vpp# set interface mtu ?
set interface mtu set interface mtu [packet|ip4|ip6|mpls] <value> <interface>
vpp#
vpp# set interface mtu packet 1500 gre1
vpp#
vpp# show int
Name Idx State MTU (L3/IP4/IP6/MPLS) Counter Count
eth1 1 up 1500/0/0/0 rx packets 818
rx bytes 42591
tx packets 16
tx bytes 1308
drops 817
ip6 1
gre1 3 up 1500/0/0/0 tx packets 2
tx bytes 200
```