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 11.0.0.0 test default:test si-2/0/0.3221225479 11.0.0.0 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 Bundle: Multilink: Input : 0 0 0 0 Output: 0 0 0 0 Network: Input : 0 0 0 0 Output: 0 0 0 0 IPV6 Transit Statistics Packets Bytes Network: Input : 0 0 Output: 0 0 Link: si-2/2/0.3221225478 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 Local: 10.0.0.1
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>