Not able to set persistent MTU on NIC

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Hi.

We're running RHEL 7.5 (3.10.0-693.21.1.el7.x86_64) on a HPE ProLiant DL360 Gen10.

The server has the following NICs:

"HPE StoreFabric CN1300R 10/25Gb CNA"
Firmware 08.35.43
Interface names in RHEL:
ens2f0
ens2f1

"Network Controller"
Firmware 20.12.41
Interface names in RHEL:
eno1
eno2
eno3
eno4

"HPE Eth 10/25Gb 2p 622FLR-SFP28 CNA"
Firmware 08.35.43
Interface names in RHEL:
eno5
eno6

eno5 and ens2f0 are teamed (using LACP) as team0. Works fine.

eno6 is going to receive ERSPAN traffic on a specific VLAN, and thus needs increased MTU size and a subinterface.

MTU is set like this:
/etc/sysconfig/network-scripts/ifcfg-eno6

MTU=1600

The subinterface is set like this:
/etc/sysconfig/network-scripts/ifcfg-eno6.1234

VLAN=yes
VLAN_ID=1234
MTU=1550

The subinterface works fine, and it is able to receive ERSPAN traffic, but it drops any packet larger than 1500. ERSPAN adds an overhead of 50 bytes, so we need to be able to receive packets of 1550 bytes.

ifconfig tells that both eno6 and eno6.1234 has an MTU of 1500.

If I use this command:
sudo ifconfig eno6 mtu 1600
it works.
ifconfig shows that eno6 is 1600, eno6.1234 is 1550, and I can see in my captures on eno6.1234 that it is able to receive all ERSPAN traffic that is sent.

But: If I reboot the system, MTU is back to 1500 again.

Any ideas?

GR

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