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Aruba CX: ip ospf ignore-mtu

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  • 1.  Aruba CX: ip ospf ignore-mtu

    Posted Apr 15, 2024 12:13 AM

    Does Aruba CX have the the ability to do "ip ospf ignore-mtu" like cisco?  I have a cisco 6500 to connect to that has mtu 9216 set on the interface (i have no access to change this).  The 8360 i'm connecting only allows mtu 9198/ip mtu 9198.  The neighbour is stuck in exstart and show ip ospf statistics shows 

      MTU mismatch                36343

  • 2.  RE: Aruba CX: ip ospf ignore-mtu

    Posted Apr 15, 2024 12:33 AM

    An MTU of 9198 in an Aruba switch is the same as an MTU of 9216 in a Cisco switch. Aruba simply removes the 18 IP overhead bits from the number.  Why are you having adjacency issues is another question. Are you certain the Cisco switch has the MTU set in the L3 interface as well?

  • 3.  RE: Aruba CX: ip ospf ignore-mtu

    Posted Apr 15, 2024 01:21 AM

    Yes the Cisco interface is L3 routed, with mtu 9216 set.  The output of the Aruba ospf interface statistics is:

    OSPF Process ID 1 VRF default, interface 1/1/29 statistics  (cleared 32 days ago)

    Tx Hello Packets      : 24060        Rx Hello Packets      : 25079
    Tx Hello Bytes        : 1636076      Rx Hello Bytes        : 1979508
    Tx DD Packets         : 36378        Rx DD Packets         : 0
    Tx DD Bytes           : 1891656      Rx DD Bytes           : 0
    Tx LS Request Packets : 0            Rx LS Request Packets : 0
    Tx LS Request Bytes   : 0            Rx LS Request Bytes   : 0
    Tx LS Update Packets  : 0            Rx LS Update Packets  : 91676
    Tx LS Update Bytes    : 0            Rx LS Update Bytes    : 42567196
    Tx LS Ack Packets     : 0            Rx LS Ack Packets     : 0
    Tx LS Ack Bytes       : 0            Rx LS Ack Bytes       : 0

    Total Number of State Changes : 1
    Number of LSAs                : 0
    LSA Checksum Sum              : 0
    Total Transmit Failures       : 0
    Total OSPF Packets Discarded  : 0

    Reason                        Packets Dropped
    Invalid type                  0
    Invalid length                0
    Invalid checksum              0
    Invalid version               0
    Bad or unknown source         0
    Area mismatch                 0
    Self-originated               0
    Duplicate router ID           0
    Interface standby             0
    Total Hello packets dropped   0
      Network Mask mismatch       0
      Hello interval mismatch     0
      Dead interval mismatch      0
      Options mismatch            0
      MTU mismatch                36343
      Neighbor ignored            0
    Authentication errors         0
      Type mismatch               0
      Authentication failures     0
    Wrong protocol                0
    Resource failures             0
    Bad LSA length                0
    Bad DD packets                0
    Others                        0

  • 4.  RE: Aruba CX: ip ospf ignore-mtu

    Posted Apr 16, 2024 09:11 PM

    What is the connectivity between the two? Direct fiber in the same campus, dark fiber a crossed town, or an ISP service that runs through an ONT/ONU setup. IT sounds to me like the link between the two doesn't support jumbo frames, have you confirmed that? I have run into this a few times in the past.

  • 5.  RE: Aruba CX: ip ospf ignore-mtu

    Posted Apr 16, 2024 10:13 PM

    It's a simple dark fibre link with nothing in between.  It's the Aruba router showing dropped packets due to MTU in the logs below, so it must be interpreting the 9216 from Cisco as the frame size without CRC, whereas on Cisco it is counted.  We're getting the upstream lowered to 9198 by the support provider later, will see if that fixes it.