Configuring the Juniper Media, IP & MPLS MTU

This is a very useful article for the people who are preparing for their JNCIE. As an engineers we always have the confusions on the what MTU values to be used in the network. Here is an article which explain on how and what value of MTU to be used especially for Juniper devices.


Below we have some information on MTU Juniper. The actual frames transmitted also contain cyclic redundancy check (CRC) bits, which are not part of the media MTU. For example, the media MTU for a Gigabit Ethernet interface is specified as 1500 bytes, but the largest possible frame size is actually 1504 bytes; you need to consider the extra bits in calculations of MTUs for interoperability.

                                                     Encapsulation Overhead by Encapsulation Type

Interface Encapsulation	Encapsulation Overhead (Bytes)
802.1Q/Ethernet 802.3	21
802.1Q/Ethernet Subnetwork Access Protocol (SNAP)	26
802.1Q/Ethernet version 2	18
ATM Cell Relay	4
ATM permanent virtual connection (PVC)	12
Cisco HDLC	4
Ethernet 802.3	17
Ethernet circuit cross-connect (CCC) and virtual private LAN service (VPLS)	4
Ethernet over ATM	32
Ethernet SNAP	22
Ethernet translational cross-connect (TCC)	18
Ethernet version 2	14
Extended virtual local area network (VLAN) CCC and VPLS	4
Extended VLAN TCC	22
Frame Relay	4
PPP	4
VLAN CCC	4
VLAN VPLS	4
VLAN TCC	22

Interface Encapsulation                                      Encapsulation Overhead (Bytes)

802.1Q/Ethernet 802.3                                                       21

802.1Q/Ethernet Subnetwork Access Protocol (SNAP)      26 

802.1Q/Ethernet version 2                                                  18

ATM Cell Relay                                                                   4

ATM permanent virtual connection (PVC)                           12

Cisco HDLC                                                                        4

Ethernet 802.3                                                                     17

Ethernet circuit cross-connect (CCC) and

virtual private LAN service (VPLS)                                      4

Ethernet over ATM                                                             32

Ethernet SNAP                                                                   22

Ethernet translational cross-connect (TCC)                          18

Ethernet version 2                                                               14

Extended virtual local area network (VLAN) CCC &VPLS  4

Extended VLAN TCC                                                        22

Frame Relay                                                                         4

PPP                                                                                     4

VLAN CCC                                                                        4

VLAN VPLS                                                                      4

VLAN TCC                                                                       22

Default media MTU = Default IP MTU + L2 encapsulation overhead
Default IP MTU = Default media MTU -  L2 encapsulation overhead
MPLS MTU = physical interface MTU - L2 encapsulation overhead - 12
If IP MTU is already set, so MPLS MTU = IP MTU + 20 bytes
IP MTU - payload without header!

Note: In other words, the formula used to determine the MPLS MTU is the following:
MPLS MTU = physical interface MTU - encapsulation overhead - 12