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[EX] Relation between media MTU and the maximum allowed frame size on an interface

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Article ID: KB23906 KB Last Updated: 04 Mar 2017Version: 2.0
Summary:
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.
Symptoms:
The maximum frame size (includes dot1q tag and CRC), which can be received on an interface, is known as MRU (Maximum Receive Unit).


Cause:
 
Solution:
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,

MRU = 1514 + 8 = 1522

Example of MRU calculation when the interface has the media MTU configured as 9200:
root# set interfaces ge-0/0/0 mtu 9200

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,
MRU = 9200 + 8 = 9208

The following output confirms this:
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   <
The default behavior changed for EX 3200/4200 switches in Junos 10.3, 10.4, and 11.1 releases. This behavior was fixed in 11.2 onwards and reverted to using MRU = Media-mtu + 8.

The MRU calculation formula used in these releases (10.3, 10.4, 11.1):

MRU = Media MTU + 8 + 8

For default media MTU (1514)

MRU = 1514 + 8 + 8 = 1530

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


The following table lists this behavior for different Junos versions:



MRU Calculation
Junos CodeMedia MTUNumber of bytes to addMRU
10.0R3151481522
10.1S1.3151481522
10.2R3.1151481522
10.3R1.91514161530
10.4R1.91514161530
10.4R2.61514161530
10.4R8.51514161530
11.1R1.21514161530
11.2R4.3151481522
11.2R5.4151481522
11.3R1.7151481522


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