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[MX] Interface goes to admin down when MTU of the physical interface is changed

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Article ID: KB34717 KB Last Updated: 25 Jun 2019Version: 3.0
Summary:

This article provides an explanation as to why the "ifAdminStatus down" log message is reported when the operation was not performed by the administrator.

 

Symptoms:

The following log message is reported in the logs indicating that an interface has gone down:

Jun 24 11:30:47.381  rtr-re0 mib2d[17617]: SNMP_TRAP_LINK_DOWN: ifIndex 791, ifAdminStatus down(2), ifOperStatus down(2), ifName xe-1/1/0.0
Jun 24 11:30:47.481  rtr-re0 mib2d[17617]: SNMP_TRAP_LINK_UP: ifIndex 791, ifAdminStatus up(2), ifOperStatus up(1), ifName xe-1/1/0.0
 

The administrator did not shut down the interface. However, the administrator did change the physical interface MTU:

set interfaces xe-1/1/0 mtu 2000 ‚Äč

 

Solution:

When the protocol MTU of a physical interface is changed, all the content for that interface needs to be re-programmed again. Therefore, all the logical interfaces that are tied to the physical interface will bounce/flap. This is expected behavior.

It should be noted on most running systems that the flap will be much less than a second and that packet loss should be minimal. Only in environments that are configured for aggressive BFD liveliness settings, for example, could customer traffic be affected.

 

Modification History:

2019-06-25: Note added in Solution section about traffic loss and minor changes made in Symptom section.

 

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