strange behavier with mtu

[ggzengel]

I have have two NICs which are not configured.

# ifreload -a -s
# ifreload -a

#root@zfs1.hq1.zmt.info:~# ip a | grep -E "(xyz|upper)" 
3: upper-left: <BROADCAST,MULTICAST> mtu 1500 qdisc mq state DOWN group default qlen 1000
5: upper-right: <BROADCAST,MULTICAST> mtu 1500 qdisc mq state DOWN group default qlen 1000

I put some config for them:

# cp test /etc/network/interfaces.d/

# cat test
auto upper-left
iface upper-left
	rx 4096
	tx 4096
        mtu 9000

auto upper-right
iface upper-right
	rx 4096
	tx 4096
        mtu 9000

auto xyz-bond
iface xyz-bond
	bond_mode 802.3ad
	bond_xmit_hash_policy layer2+3
        bond_slaves upper-left upper-right

auto xyz-bridge
iface xyz-bridge 
	bridge_ports xyz-bond

auto xyz-bridge100
iface xyz-bridge100
	bridge_ports xyz-bond.100

# ifreload -a -s
warning: xyz-bond: interface not recognized - please check interface configuration
warning: xyz-bridge: interface not recognized - please check interface configuration
warning: xyz-bridge100: interface not recognized - please check interface configuration
warning: xyz-bond.100: interface not recognized - please check interface configuration

Now I bring them up.
The MTU is still 1500

# ifreload -a
warning: b_lan: attribute bond-min-links is set to '0'

# ip a | grep -E "(xyz|upper)" 
3: upper-left: <NO-CARRIER,BROADCAST,MULTICAST,SLAVE,UP> mtu 1500 qdisc mq master xyz-bond state DOWN group default qlen 1000
5: upper-right: <BROADCAST,MULTICAST,SLAVE,UP,LOWER_UP> mtu 1500 qdisc mq master xyz-bond state UP group default qlen 1000
52: xyz-bond: <BROADCAST,MULTICAST,MASTER,UP,LOWER_UP> mtu 1500 qdisc noqueue master xyz-bridge state UP group default qlen 1000
53: xyz-bridge: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
54: xyz-bond.100@xyz-bond: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master xyz-bridge100 state UP group default qlen 1000
55: xyz-bridge100: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000

I didn't change the config and run it again.
e voila: the MTU is now 9000 but no inheritance

# ifreload -a
warning: b_lan: attribute bond-min-links is set to '0'
warning: xyz-bond: attribute bond-min-links is set to '0'

# ip a | grep -E "(xyz|upper)" 
3: upper-left: <NO-CARRIER,BROADCAST,MULTICAST,SLAVE,UP> mtu 9000 qdisc mq master xyz-bond state DOWN group default qlen 1000
5: upper-right: <BROADCAST,MULTICAST,SLAVE,UP,LOWER_UP> mtu 9000 qdisc mq master xyz-bond state UP group default qlen 1000
52: xyz-bond: <BROADCAST,MULTICAST,MASTER,UP,LOWER_UP> mtu 1500 qdisc noqueue master xyz-bridge state UP group default qlen 1000
53: xyz-bridge: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
54: xyz-bond.100@xyz-bond: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master xyz-bridge100 state UP group default qlen 1000
55: xyz-bridge100: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000

now I add mtu 9000 to xyz_bond:
still no inheritance

# ifreload -a
warning: b_lan: attribute bond-min-links is set to '0'
warning: xyz-bond: attribute bond-min-links is set to '0'

# ip a | grep -E "(xyz|upper)" 
3: upper-left: <NO-CARRIER,BROADCAST,MULTICAST,SLAVE,UP> mtu 9000 qdisc mq master xyz-bond state DOWN group default qlen 1000
5: upper-right: <BROADCAST,MULTICAST,SLAVE,UP,LOWER_UP> mtu 9000 qdisc mq master xyz-bond state UP group default qlen 1000
56: xyz-bond: <BROADCAST,MULTICAST,MASTER,UP,LOWER_UP> mtu 9000 qdisc noqueue master xyz-bridge state UP group default qlen 1000
57: xyz-bridge: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
58: xyz-bond.100@xyz-bond: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9000 qdisc noqueue master xyz-bridge100 state UP group default qlen 1000
59: xyz-bridge100: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000

If I put mtu 9000 to xyz-bridge100:
it annoys with: warning: xyz-bridge100: bridge inherits mtu from its ports. There is no need to assign mtu on a bridge

# ifreload -a -s
warning: xyz-bridge100: bridge inherits mtu from its ports. There is no need to assign mtu on a bridge

# ifreload -a
warning: b_lan: attribute bond-min-links is set to '0'
warning: xyz-bond: attribute bond-min-links is set to '0'

# ip a | grep -E "(xyz|upper)" 
3: upper-left: <NO-CARRIER,BROADCAST,MULTICAST,SLAVE,UP> mtu 9000 qdisc mq master xyz-bond state DOWN group default qlen 1000
5: upper-right: <BROADCAST,MULTICAST,SLAVE,UP,LOWER_UP> mtu 9000 qdisc mq master xyz-bond state UP group default qlen 1000
56: xyz-bond: <BROADCAST,MULTICAST,MASTER,UP,LOWER_UP> mtu 9000 qdisc noqueue master xyz-bridge state UP group default qlen 1000
57: xyz-bridge: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
58: xyz-bond.100@xyz-bond: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9000 qdisc noqueue master xyz-bridge100 state UP group default qlen 1000
59: xyz-bridge100: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9000 qdisc noqueue state UP group default qlen 1000

#ifreload -V
ifupdown2:3.0.0-1.1

#lsb_release -a
No LSB modules are available.
Distributor ID:	Debian
Description:	Debian GNU/Linux 12 (bookworm)
Release:	12
Codename:	bookworm