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Created attachment 1791524 [details]
Reproducer

Description of problem:
When applying route rules to bridge with default MTU 1500, the MTU is lowered by 1 to 1499. 

Version-Release number of selected component (if applicable):
nmstate-2.0.0 latest git base

How reproducible:
100%

Steps to Reproduce:
1. Run reproducer script


Actual results:
-  mtu: 1500
+  mtu: 1499


Expected results:
The MTU should stay the same.

Additional info:
I was able to track down start of this failure to PR#1595
https://github.com/nmstate/nmstate/pull/1595

However I am not really sure if it\s the root cause.

RHV confirmed the scratch/copr build fixed the issue.

Verified with versions:
nmstate-1.1.0-1.el8.noarch
nispor-1.1.1-1.el8.x86_64
NetworkManager-1.32.3-0.z.2.20e3975fd2.el8.x86_64

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory (nmstate bug fix and enhancement update), and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2021:4157