Bug 1920983

Summary: Custom MTU setting is not applied to the physical NIC
Product: OpenShift Container Platform Reporter: Eduardo Minguez <eminguez>
Component: NetworkingAssignee: Dan Winship <danw>
Networking sub component: ovn-kubernetes QA Contact: Anurag saxena <anusaxen>
Status: CLOSED DUPLICATE Docs Contact:
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Version: 4.5   
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Last Closed: 2021-02-01 17:46:50 UTC Type: Bug
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Description Eduardo Minguez 2021-01-27 10:56:00 UTC 
Description of problem:

When setting a custom MTU via CNO, the physical NIC is not configured to fit the MTU size.
Related to https://bugzilla.redhat.com/show_bug.cgi?id=1846485
Maybe the fix is just adding a quote in the documentation explaining that the dhcp server needs to specify the custom MTU setting as well

Version-Release number of selected component (if applicable):

4.5 IPI baremetal but I guess it would be applicable to 4.X in any environment

How reproducible:

Install an environment with custom MTU without setting any dhcp option related to MTU

Steps to Reproduce:
1.
2.
3.

Actual results:

```
...
6: eno4: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master ovs-system state UP group default qlen 1000
...
8: ovn-k8s-mp0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 8900 qdisc noqueue state UNKNOWN group default qlen 1000
```

Expected results:

```
...
6: eno4: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9000 qdisc mq master ovs-system state UP group default qlen 1000
...
8: ovn-k8s-mp0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 8900 qdisc noqueue state UNKNOWN group default qlen 1000
```

Additional info:

https://access.redhat.com/solutions/5736191
Comment 2 Dan Winship 2021-02-01 17:46:50 UTC 
> Maybe the fix is just adding a quote in the documentation explaining that the dhcp server needs to specify the custom MTU setting as well

It's not even "as well". You *must* configure the DHCP server so that the nodes will get the right MTU, and then once you do that, there is no need to configure ovn-kubernetes's MTU because it will use the correct MTU automatically.

*** This bug has been marked as a duplicate of bug 1877570 ***