This article provides information on how to calculate the maximum allowed frame size on an interface. It also explains the default behavior change in EX 3200/4200 switches for certain releases.

The maximum frame size (includes dot1q tag and CRC), which can be received on an interface, is known as MRU (Maximum Receive Unit).

All EX platform switches use the following formula to calculate the MRU (Maximum Receive Unit):

MRU = Media-mtu + 8


Example of MRU calculation when the interface has the default media MTU value (1514) :

{master:0}
root> show interfaces ge-0/0/0 extensive
Physical interface: ge-0/0/0, Enabled, Physical link is Up
    Interface index: 130, SNMP ifIndex: 506, Generation: 133
    Link-level type: Ethernet, MTU: 1514, Speed: Auto, Duplex: Auto,
    BPDU Error: None, MAC-REWRITE Error: None, Loopback: Disabled,

root# set interfaces ge-0/0/0 mtu 920 0

root> show interfaces ge-0/0/0 extensive
Physical interface: ge-0/0/0, Enabled, Physical link is Up
Interface index: 130, SNMP ifIndex: 506, Generation: 133
Link-level type: Ethernet, MTU: 9200, Speed: Auto, Duplex: Auto,

root@:RE:0% lcdd_cmd -A fpc0 -d chassism -c "show mac ge-0/0/0"
chassism<0>#show mac ge-0/0/0
MAC Layer Information
devNum:1, portNum:1
MAC Control Reg0: 0x0000c801 Port Enable  YES
                             Port Type    SGMII
                             MRU          9208    <

root@:RE:0% lcdd_cmd -A fpc0 -d chassism -c "show mac ge-0/0/0"
chassism<0>#show mac ge-0/0/0
MAC Layer Information
devNum:1, portNum:1
MAC Control Reg0: 0x0000c801 Port   Enable YES
                             Port   Type SGMII
                             MRU    1530     <