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[MX] MTU calculation for multilink PPP interface on MX L2TP LNS with Inline Service Interfaces



Article ID: KB32587 KB Last Updated: 02 May 2018Version: 1.0

The MLPPP subscriber interface, when terminated by an MX device configured as an LNS, may report an unexpected MTU value.  This behavior is seen in versions 14.1X51, 15.1 and later. 


Below is an example of an MLPPP client logged into an MX LNS.  The negotiation will complete successfully.

user@router> show subscribers 
Interface           IP Address/VLAN ID                      User Name                      LS:RI
si-2/2/0.3221225478                                test                           default:test
si-2/0/0.3221225479                                test                           default:test

Even though the client negotiated a different MRRU, the MTU selected by the LNS was inherited from the the anchor interface.  This result could lead to unexpected behaviors, such as discarded packets or tunnel fragmentation. 

user@router> show interfaces si-2/0/0.3221225479 
  Logical interface si-2/0/0.3221225479 (Index 536874941) (SNMP ifIndex 200004029)
    Flags: Up Encapsulation: Multilink-PPP
    Last flapped: Never
    Bandwidth: 0
    Bundle links information:
      Active bundle links       1
      Removed bundle links      0
      Disabled bundle links     0
    Statistics         Frames        fps         Bytes          bps
        Input :             0          0             0            0
        Output:             0          0             0            0
        Input :             0          0             0            0
        Output:             0          0             0            0
    IPV6 Transit Statistics         Packets        Bytes
        Input :                      0               0
        Output:                      0               0
        Input :             0          0             0            0
        Output:             0          0             0            0
    Protocol inet, MTU: 9192
    Max nh cache: 0, New hold nh limit: 0, Curr nh cnt: 0, Curr new hold cnt: 0, NH drop cnt: 0
      Flags: Unnumbered
      Donor interface: lo0.1 (Index 4474)
      Addresses, Flags: Is-Primary


By default, the MTU for an MLPPP subscriber's interface is inherited from the inline service interface (si-x/y/z). This interface had default MTU of 9192 bytes, so the behavior is expected. The default behavior can be changed by adding one additional command in the dynamic-profile configuration:

set dynamic-profiles <dynamic profile name> interfaces "$junos-interface-ifd-name" unit "$junos-interface-unit" ppp-options mtu <desired value>
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