Juniper ACX1000 Configuration Manual page 1682

ACX Series Universal Access Router Configuration Guide
1624
NOTE:
The MTU for an IRB interface is calculated by removing the Ethernet header
overhead [6(DMAC)+6(SMAC)+2(EtherType)]. Because, the MTU is the
lower value of the MTU configured on the IRB interface and the MTU
configured on the IRB's associated bridge domain IFDs or IFLs, the IRB MTU
is calculated as follows:
In case of Layer 2 IFL configured with the
the IRB MTU is calculated by including 8 bytes overhead (SVLAN+CVLAN).
In case of Layer 2 IFL configured with the
MTU is calculated by including a single VLAN 4 bytes overhead.
flexible-vlan-tagging statement,
statement, the IRB
vlan-tagging
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