In ACX Series with EVO based platform such as ACX7100, ACX7024, and ACX7509, any change including either MTU change or addition/deletion will cause an interface to flap. 

This article reiterates that this behavior is expected as specified in product documentation.

In ACX Series with EVO based platform such as ACX7100, ACX7024, and ACX7509, any change including either MTU change or addition/deletion will cause the interface to flap. 

[edit]
labroot@router# set interfaces et-0/0/0 mtu ?
Possible completions:
  <mtu>                Maximum transmit packet size (256..9216)
[edit]
labroot@router# set interfaces et-0/0/0 mtu 9200 

[edit]
labroot@router# commit and-quit 
commit complete
Exiting configuration mode

labroot@router> show interfaces terse et-0/0/0    
Interface               Admin Link Proto    Local                 Remote
et-0/0/0                up    up
et-0/0/0.100            up    up   multiservice
et-0/0/0.32767          up    up   multiservice

labroot@router> show interfaces  et-0/0/0         
Physical interface: et-0/0/0, Enabled, Physical link is Up
  Interface index: 1065, SNMP ifIndex: 521
  Link-level type: Flexible-Ethernet, MTU: 9200, LAN-PHY mode, Speed: 100Gbps, BPDU Error: None, Loop Detect PDU Error: None, MAC-REWRITE Error: None, Loopback: Disabled, Source filtering: Disabled,
  Flow control: Enabled, Auto-negotiation: Disabled, Media type: Fiber
  Device flags   : Present Running
  Interface flags: SNMP-Traps
  CoS queues     : 8 supported, 8 maximum usable queues
  Current address: ac:78:d1:2d:65:34, Hardware address: ac:78:d1:2d:65:34
  Last flapped   : 2023-09-21 08:24:36 PDT (00:00:17 ago)   <<<<<<<<<<<<<<<<
  Input rate     : 0 bps (0 pps)
  Output rate    : 0 bps (0 pps)
  Active alarms  : None
  Active defects : None
  PCS statistics                      Seconds
    Bit errors                             0
    Errored blocks                         0
  Ethernet FEC Mode  :                   NONE
  Ethernet FEC statistics              Errors
    FEC Corrected Errors                    0
    FEC Uncorrected Errors                  0
    FEC Corrected Errors Rate               0
    FEC Uncorrected Errors Rate             0
  Interface transmit statistics: Disabled
  Link Degrade :                      
    Link Monitoring                   :  Disable

  Logical interface et-0/0/0.100 (Index 1063) (SNMP ifIndex 692)
    Flags: Up SNMP-Traps VLAN-Tag[ 0x8100.100 ]  Encapsulation: ENET2
    Input packets : 0
    Output packets: 0
    Protocol multiservice, MTU: Unlimited
      Flags: None

  Logical interface et-0/0/0.32767 (Index 1064) (SNMP ifIndex 691)
    Flags: Up SNMP-Traps VLAN-Tag[ 0x0000.0 ]  Encapsulation: ENET2
    Input packets : 0
    Output packets: 0
    Protocol multiservice, MTU: Unlimited
      Flags: None

labroot@router>  

labroot@router> configure 
Entering configuration mode

[edit]
labroot@router# rollback 1 
load complete

[edit]
labroot@router# show | compare 
[edit interfaces et-0/0/0]
-   mtu 9200;

[edit]
labroot@router# commit and-quit 
commit complete
Exiting configuration mode

labroot@router> show interfaces  et-0/0/0    
Physical interface: et-0/0/0, Enabled, Physical link is Up
  Interface index: 1067, SNMP ifIndex: 521
  Link-level type: Flexible-Ethernet, MTU: 1522, LAN-PHY mode, Speed: 100Gbps, BPDU Error: None, Loop Detect PDU Error: None, MAC-REWRITE Error: None, Loopback: Disabled, Source filtering: Disabled,
  Flow control: Enabled, Auto-negotiation: Disabled, Media type: Fiber
  Device flags   : Present Running
  Interface flags: SNMP-Traps
  CoS queues     : 8 supported, 8 maximum usable queues
  Current address: ac:78:d1:2d:65:34, Hardware address: ac:78:d1:2d:65:34
  Last flapped   : 2023-09-21 08:29:30 PDT (00:00:00 ago)   <<<<<<<<<
  Input rate     : 0 bps (0 pps)
  Output rate    : 0 bps (0 pps)
  Active alarms  : None
  Active defects : None
  PCS statistics                      Seconds
    Bit errors                             0
    Errored blocks                         0
  Ethernet FEC Mode  :                   NONE
  Ethernet FEC statistics              Errors
    FEC Corrected Errors                    0
    FEC Uncorrected Errors                  0
    FEC Corrected Errors Rate               0
    FEC Uncorrected Errors Rate             0
  Interface transmit statistics: Disabled
  Link Degrade :                      
    Link Monitoring                   :  Disable

  Logical interface et-0/0/0.100 (Index 1065) (SNMP ifIndex 692)
    Flags: Up SNMP-Traps VLAN-Tag[ 0x8100.100 ]  Encapsulation: ENET2
    Input packets : 0
    Output packets: 0
    Protocol multiservice, MTU: Unlimited
      Flags: None

  Logical interface et-0/0/0.32767 (Index 1066) (SNMP ifIndex 691)
    Flags: Up SNMP-Traps VLAN-Tag[ 0x0000.0 ]  Encapsulation: ENET2
    Input packets : 0
    Output packets: 0
    Protocol multiservice, MTU: Unlimited
      Flags: None

user@router> 

In log messages, the following logs will be observed for the respective interfaces.

Sep 21 08:24:34  router mib2d[9668]: SNMP_TRAP_LINK_DOWN: ifIndex 692, ifAdminStatus down(2), ifOperStatus down(2), ifName et-0/0/0.100
Sep 21 08:24:34  router mib2d[9668]: SNMP_TRAP_LINK_DOWN: ifIndex 691, ifAdminStatus down(2), ifOperStatus down(2), ifName et-0/0/0.32767
Sep 21 08:24:34  router mib2d[9668]: SNMP_TRAP_LINK_DOWN: ifIndex 521, ifAdminStatus down(2), ifOperStatus down(2), ifName et-0/0/0

Sep 21 08:24:34  router mib2d[9668]: SNMP_TRAP_LINK_UP: ifIndex 521, ifAdminStatus up(1), ifOperStatus up(1), ifName et-0/0/0
Sep 21 08:24:34  router mib2d[9668]: SNMP_TRAP_LINK_UP: ifIndex 692, ifAdminStatus up(1), ifOperStatus up(1), ifName et-0/0/0.100
Sep 21 08:24:34  router mib2d[9668]: SNMP_TRAP_LINK_UP: ifIndex 691, ifAdminStatus up(1), ifOperStatus up(1), ifName et-0/0/0.32767

Changing the MTU at IFD level is a catastrophic event and so the IFD hierarchy will be deleted and re-added. The event is expected to cause a flap of the interface. This behavior is documented in Media MTU and Protocol MTU.

2023-12-27: Article created

2024-10-30: add qfx device to Categories . This issue affects junos and evo