Related Documentation
- ACX, J, M, MX, PTX, T Series
- Configuring Interface Encapsulation on Physical Interfaces
- EX, M, MX, PTX, T Series
- Setting the Protocol MTU
Configuring the Media MTU
The media maximum transmission unit (MTU) is the largest data unit that can be forwarded without fragmentation.
This topic contains the following sections:
- Media MTU Overview
- How to Configure the Media MTU
- Encapsulation Overhead by Encapsulation Type
- Media MTU Sizes by Interface Type for M5 and M7i Routers with CFEB, M10 and M10i Routers with CFEB, and M20 and M40 Routers
- Media MTU Sizes by Interface Type for M40e Routers
- Media MTU Sizes by Interface Type for M160 Routers
- Media MTU Sizes by Interface Type for M7i Routers with CFEB-E, M10i Routers with CFEB-E, and M320 and M120 Routers
- Media MTU Sizes by Interface Type for MX Series Routers
- Media MTU Sizes by Interface Type for T320 Routers
- Media MTU Sizes by Interface Type for T640 Platforms
- Media MTU Sizes by Interface Type for J2300 Platforms
- Media MTU Sizes by Interface Type for J4300 and J6300 Platforms
- Media MTU Sizes by Interface Type for J4350 and J6350 Platforms
- Media MTU Sizes by Interface Type for EX Series Switches and ACX Series Routers
- Media MTU Sizes by Interface Type for PTX Series Packet Transport Switches
Media MTU Overview
The default media MTU size used on a physical interface depends on the encapsulation used on that interface. In some cases, the default IP Protocol MTU depends on whether the protocol used is IP version 4 (IPv4) or International Organization for Standardization (ISO).
The default media MTU is calculated as follows:
When you are configuring point-to-point connections, the MTU sizes on both sides of the connections must be the same. Also, when you are configuring point-to-multipoint connections, all interfaces in the subnet must use the same MTU size. For details about encapsulation overhead, see Encapsulation Overhead by Encapsulation Type.
![]() | Note: 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 Version 2 interface is specified as 1514 bytes, but the largest possible frame size is actually 1518 bytes; you need to consider the extra bits in calculations of MTUs for interoperability. The physical MTU for Ethernet interfaces does not include the 4-byte frame check sequence (FCS) field of the Ethernet frame. A SONET/SDH interface operating in concatenated mode has a “c” added to the rate descriptor. For example, a concatenated OC48 interface is referred to as OC48c. If you do not configure an MPLS MTU, the Junos OS derives the MPLS MTU from the physical interface MTU. From this value, the software subtracts the encapsulation-specific overhead and space for the maximum number of labels that might be pushed in the Packet Forwarding Engine. Currently, the software provides for three labels of four bytes each, for a total of 12 bytes. In other words, the formula used to determine the MPLS MTU is the following: MPLS MTU = physical interface MTU – encapsulation overhead – 12 If you configure an MTU value by including the mtu statement at the [edit interfaces interface-name unit logical-unit-number family mpls] hierarchy level, the configured value is used. |
How to Configure the Media MTU
To modify the default media MTU size for a physical interface, include the mtu statement at the [edit interfaces interface-name] hierarchy level:
If you change the size of the media MTU, you must ensure that the size is equal to or greater than the sum of the protocol MTU and the encapsulation overhead.
![]() | Note: Changing the media MTU or protocol MTU causes an interface to be deleted and added again. |
You configure the protocol MTU by including the mtu statement at the following hierarchy levels:
- [edit interfaces interface-name unit logical-unit-number family family]
- [edit logical-systems logical-system-name interfaces interface-name unit logical-unit-number family family]
Because tunnel services interfaces are considered logical interfaces, you cannot configure the MTU setting for the physical interface. This means you cannot include the mtu statement at the [edit interfaces interface-name] hierarchy level for the following interface types: generic routing encapsulation (gr-), IP-IP (ip-), loopback (lo-), link services (ls-), multilink services (ml-), and multicast (pe-, pd-). You can, however, configure the protocol MTU on tunnel interfaces, as described in Setting the Protocol MTU.
Encapsulation Overhead by Encapsulation Type
Table 1: 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 |
Media MTU Sizes by Interface Type for M5 and M7i Routers with CFEB, M10 and M10i Routers with CFEB, and M20 and M40 Routers
Table 2: Media MTU Sizes by Interface Type for M5 and M7i Routers with CFEB, M10 and M10i Routers with CFEB, and M20 and M40 Routers
Interface Type | Default Media MTU (Bytes) | Maximum MTU (Bytes) | Default IP Protocol MTU (Bytes) |
---|---|---|---|
Adaptive Services | 9192 | N/A | N/A |
ATM | 4482 | 9192 | 4470 |
E1/T1 | 1504 | 9192 | 1500 |
E3/T3 | 4474 | 9192 | 4470 |
Fast Ethernet | 1514 | 1533 (4-port) 1532 (8-port) 1532 (12-port) Note: The maximum MTU for two 100Base-TX Fast Ethernet port FIC is 9192 bytes. | 1500 (IPv4), 1497 (ISO) |
Gigabit Ethernet | 1514 | 9192 Note: The maximum MTU for one Gigabit Ethernet port FIC is 9192 bytes. | 1500 (IPv4), 1497 (ISO) |
Serial | 1504 | 9192 | 1500 (IPv4), 1497 (ISO) |
SONET/SDH | 4474 | 9192 | 4470 |
Media MTU Sizes by Interface Type for M40e Routers
Table 3: Media MTU Sizes by Interface Type for M40e Routers
Interface Type | Default Media MTU (Bytes) | Maximum MTU (Bytes) | Default IP Protocol MTU (Bytes) |
---|---|---|---|
Adaptive Services | 9192 | N/A | N/A |
ATM | 4482 | 9192 | 4470 |
E1/T1 | 1504 | 4500 | 1500 |
E3/T3 | 4474 | 4500 9192 (4-port) | 4470 |
E3/DS3 IQ | 4474 | 9192 | 4470 |
Fast Ethernet | 1514 | 1533 | 1500 (IPv4), 1497 (ISO) |
Gigabit Ethernet | 1514 | 9192 (1- or 2-port) 9192 (4-port) | 1500 (IPv4), 1497 (ISO) |
Serial | 1504 | 9192 | 1500 (IPv4), 1497 (ISO) |
SONET/SDH | 4474 | 4500 (1-port nonconcatenated) 9192 (4-port OC3) 9192 (4-port OC3c) 4500 (1-port OC12) 4500 (4-port OC12) 4500 (4-port OC12c) 4500 (1-port OC48) 9192 (2-port OC3) 9192 (2-port OC3c) 9192 (1-port OC12c) 9192 (1-port OC48c) 4500 (1-port OC192) 9192 (1-port OC192c) | 4470 |
Media MTU Sizes by Interface Type for M160 Routers
Table 4: Media MTU Sizes by Interface Type for M160 Routers
Interface Type | Default Media MTU (Bytes) | Maximum MTU (Bytes) | Default IP Protocol MTU (Bytes) |
---|---|---|---|
Adaptive Services | 9192 | N/A | N/A |
ATM | 4482 | 9192 | 4470 |
E1/T1 | 1504 | 4500 | 1500 |
E3/T3 | 4474 | 4500 | 4470 |
E3/DS3 IQ | 4474 | 9192 | 4470 |
Fast Ethernet | 1514 | 1533 | 1500 (IPv4), 1497 (ISO) |
Gigabit Ethernet | 1514 | 9192 (1- or 2-port) 4500 (4-port) | 1500 (IPv4), 1497 (ISO) |
Serial | 1504 | 9192 | 1500 (IPv4), 1497 (ISO) |
SONET/SDH | 4474 | 4500 (1-port nonconcatenated) 9192 (1- or 2-port) 4500 (4-port) | 4470 |
Media MTU Sizes by Interface Type for M7i Routers with CFEB-E, M10i Routers with CFEB-E, and M320 and M120 Routers
Table 5: Media MTU Sizes by Interface Type for M7i Routers with CFEB-E, M10i Routers with CFEB-E, and M320 and M120 Routers
Interface Type | Default Media MTU (Bytes) | Maximum MTU (Bytes) | Default IP Protocol MTU (Bytes) |
---|---|---|---|
ATM2 IQ | 4482 | 9192 | 4470 |
Channelized DS3 IQ | 4471 | 4500 | 4470 |
Channelized E1 IQ | 1504 | 4500 | 1500 |
Channelized OC12 IQ | 4474 | 9192 | 4470 |
Channelized STM1 IQ | 4474 | 9192 | 4470 |
DS3 | 4471 | 4500 | 4470 |
E1 | 1504 | 4500 | 1500 |
E3 IQ | 4471 | 4500 | 4470 |
Fast Ethernet | 1514 | 1533 (4-port) 1532 (8-, 12- and 48-port) | 1500 (IPv4), 1497 (ISO) |
Gigabit Ethernet | 1514 | 9192 | 1500 (IPv4), 1497 (ISO) |
SONET/SDH | 4474 | 9192 | 4470 |
T1 | 1504 | 4500 | 1500 |
CT3 IQ (excluding M120) | 4474 | 9192 | 4470 |
Media MTU Sizes by Interface Type for MX Series Routers
Table 6: Media MTU Sizes by Interface Type for MX Series Routers
Interface Type | Default Media MTU (Bytes) | Maximum MTU (Bytes) | Default IP Protocol MTU (Bytes) |
---|---|---|---|
Gigabit Ethernet | 1514 | 9192 | 1500 (IPv4), 1488 (MPLS), 1497 (ISO) |
10-Gigabit Ethernet | 1514 | 9192 | 1500 (IPv4), 1488 (MPLS), 1497 (ISO) |
Multi-Rate Ethernet | 1514 | 9192 | 1500 (IPv4), 1488 (MPLS), 1497 (ISO) |
Tri-Rate Ethernet | 1514 | 9192 | 1500 (IPv4), 1488 (MPLS), 1497 (ISO) |
Channelized SONET/SDH OC3/STM1 (Multi-Rate) | 1514 | 9192 | 1500 (IPv4), 1488 (MPLS), 1497 (ISO) |
DS3/E3 (Multi-Rate) | 1514 | 9192 | 1500 (IPv4), 1488 (MPLS), 1497 (ISO) |
Media MTU Sizes by Interface Type for T320 Routers
Table 7: Media MTU Sizes by Interface Type for T320 Routers
Interface Type | Default Media MTU (Bytes) | Maximum MTU (Bytes) | Default IP Protocol MTU (Bytes) |
---|---|---|---|
ATM | 4482 | 9192 | 4470 |
ATM2 IQ | 4482 | 9192 | 4470 |
Channelized OC12 IQ | 4474 | 9192 | 4470 |
Channelized STM1 IQ | 4474 | 9192 | 4470 |
DS3 | 4471 | 4500 | 4470 |
Fast Ethernet | 1514 | 1533 (4-port) 1532 (12- and 48-port) | 1500 (IPv4), 1497 (ISO) |
Gigabit Ethernet | 1514 | 9192 | 1500 (IPv4), 1497 (ISO) |
SONET/SDH | 4474 | 9192 | 4470 |
CT3 IQ | 4474 | 9192 | 4470 |
Media MTU Sizes by Interface Type for T640 Platforms
Table 8: Media MTU Sizes by Interface Type for T640 Platforms
Interface Type | Default Media MTU (Bytes) | Maximum MTU (Bytes) | Default IP Protocol MTU (Bytes) |
---|---|---|---|
ATM2 IQ | 4482 | 9192 | 4470 |
48-port Fast Ethernet | 1514 | 1532 | 1500 (IPv4), 1497 (ISO) |
Gigabit Ethernet | 1514 | 9192 | 1500 (IPv4), 1497 (ISO) |
SONET/SDH | 4474 | 9192 | 4470 |
CT3 IQ | 4474 | 9192 | 4470 |
Media MTU Sizes by Interface Type for J2300 Platforms
Table 9: Media MTU Sizes by Interface Type for J2300 Platforms
Interface Type | Default Media MTU (Bytes) | Maximum MTU (Bytes) | Default IP Protocol MTU (Bytes) |
---|---|---|---|
Fast Ethernet (10/100) | 1514 | 9192 | 1500 |
G.SHDSL | 4482 | 9150 | 4470 |
ISDN BRI | 1504 | 4092 | 1500 |
Serial | 1504 | 9150 | 1500 |
T1 or E1 | 1504 | 9150 | 1500 |
Media MTU Sizes by Interface Type for J4300 and J6300 Platforms
Table 10: Media MTU Sizes by Interface Type for J4300 and J6300 Platforms
Interface Type | Default Media MTU (Bytes) | Maximum MTU (Bytes) | Default IP Protocol MTU (Bytes) |
---|---|---|---|
ADSL2+ PIM | 4482 | 9150 | 4470 |
Dual-port Fast Ethernet (10/100) PIM | 1514 | 9192 | 1500 |
Dual-port Serial PIM | 1504 | 9150 | 1500 |
Dual-port T1 or E1 PIM | 1504 | 9150 | 1500 |
Dual-port Channelized T1/E1 PIM | 1504 | 4500 | 1500 |
Dual-port Channelized T1/E1 PIM | 1504 | 9150 | 1500 |
Fast Ethernet (10/100) built-in interface | 1514 | 9192 | 1500 |
G.SHDSL PIM | 4482 | 9150 | 4470 |
4-port ISDN BRI PIM | 1504 | 4092 | 1500 |
T3 (DS3) or E3 PIM | 4474 | 9192 | 4470 |
Media MTU Sizes by Interface Type for J4350 and J6350 Platforms
Table 11: Media MTU Sizes by Interface Type for J4350 and J6350 Platforms
Interface Type | Default Media MTU (Bytes) | Maximum MTU (Bytes) | Default IP Protocol MTU (Bytes) |
---|---|---|---|
4-port ISDN BRI PIM | 1504 | 4092 | 1500 |
ADSL2+ PIM | 4482 | 9150 | 4470 |
Dual-port Fast Ethernet (10/100) PIM | 1514 | 9192 | 1500 |
Dual-port Serial PIM | 1504 | 9150 | 1500 |
Dual-port T1 or E1 PIM | 1504 | 9150 | 1500 |
Dual-port Channelized T1/E1 PIM | 1504 | 4500 | 1500 |
Dual-port Channelized T1/E1 PIM | 1504 | 9150 | 1500 |
4-port Fast Ethernet (10/100) ePIM | 1518 | 1518 | 1500 |
Gigabit Ethernet (10/100/1000) built-in interface | 1514 | 9018 | 1500 |
Gigabit Ethernet (10/100/1000) Enhanced Physical Interface Module (ePIM) | 1514 | 9018 | 1500 |
Gigabit Ethernet (10/100/1000) SFP ePIM | 1514 | 9018 | 1500 |
G.SHDSL PIM | 4482 | 9150 | 4470 |
T3 (DS3) or E3 PIM | 4474 | 9192 | 4470 |
![]() | Note: On Gigabit Ethernet ePIMs in J4350 and J6350 Services Routers, you can configure a maximum transmission unit (MTU) size of only 9018 bytes even though the CLI indicates that you can configure an MTU of up to 9192 bytes. If you configure an MTU greater than 9018 bytes, the router does not accept the configuration and generates a system log error message similar to the following: /kernel: ge-0/0/0: Illegal media change. MTU invalid: 9192. Max MTU supported on this PIC: 9018 On 4-port Fast Ethernet ePIMs in J4350 and J6350 Services Routers, you can configure a maximum transmission unit (MTU) size of only 1518 bytes even though the CLI indicates that you can configure an MTU of up to 9192 bytes. If you configure an MTU greater than 1518 bytes, the router does not accept the configuration and generates a system log error message similar to the following: /kernel: fe-3/0/1: Illegal media change. MTU invalid: 9192. Max MTU supported on this PIC: 1518 |
Media MTU Sizes by Interface Type for EX Series Switches and ACX Series Routers
Table 12: Media MTU Sizes by Interface Type for EX Series Switches and ACX Series Routers
Interface Type | Default Media MTU (Bytes) | Maximum MTU (Bytes) | Default IP Protocol MTU (Bytes) |
---|---|---|---|
Gigabit Ethernet | 1514 | 9192 | 1500 (IPv4), 1497 (ISO) |
10-Gigabit Ethernet | 1514 | 9192 | 1500 (IPv4), 1497 (ISO) |
Media MTU Sizes by Interface Type for PTX Series Packet Transport Switches
Table 13: Media MTU Sizes by Interface Type for PTX Series Packet Transport Switches
Interface Type | Default Media MTU (Bytes) | Maximum MTU (Bytes) | Default IP Protocol MTU (Bytes) |
---|---|---|---|
10-Gigabit Ethernet | 1514 | 9500 | 1500 (IPv4), 1488 (MPLS), 1497 (ISO) |
40-Gigabit Ethernet | 1514 | 9500 | 1500 (IPv4), 1488 (MPLS), 1497 (ISO) |
100-Gigabit Ethernet | 1514 | 9500 | 1500 (IPv4), 1488 (MPLS), 1497 (ISO) |
Related Documentation
- ACX, J, M, MX, PTX, T Series
- Configuring Interface Encapsulation on Physical Interfaces
- EX, M, MX, PTX, T Series
- Setting the Protocol MTU
Published: 2012-12-11
Related Documentation
- ACX, J, M, MX, PTX, T Series
- Configuring Interface Encapsulation on Physical Interfaces
- EX, M, MX, PTX, T Series
- Setting the Protocol MTU