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Media MTU and Protocol MTU

A maximum transmission unit (MTU) is the largest data unit that can be forwarded without fragmentation. Configure the media MTU for a physical interface and the MTU for a protocol to optimize traffic over your network.

MTU Overview

A maximum transmission unit (MTU) is the largest data unit that can be forwarded on a link without fragmentation. If a packet exceeds the MTU for the interface or protocol it passes through, the device fragments the packet. When a packet is larger than the MTU, the device either drops the packet or fragments it and transmits the fragments. Fragmentation slows down the network and can lead to packet loss.

Some protocols such as IS-IS do not support fragmentation. With these protocols, if a packet exceeds the MTU for a link, the device drops the packet.

Configure the media MTU for a physical interface and the MTU for a protocol to avoid packet loss and optimize traffic over your network.

Use Feature Explorer to confirm platform and release support for specific features.

Review the Platform-Specific MTU Behavior section for notes related to your platform.

Media MTU Overview

The media maximum transmission unit (MTU) for an interface is the largest data unit that can be forwarded through that interface without fragmentation.

The default media MTU depends on the encapsulation used on that interface and the Layer 3 (L3) MTU. In some cases, the L3 MTU depends on whether the protocol used is IP version 4 (IPv4) or International Organization for Standardization (ISO).

The default media MTU for a physical interface depends on the Layer 2 (L2) overhead and is calculated as follows:

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 when you calculate MTUs for interoperability.

Keep the following in mind when configuring the media MTU:

  • The MTU size must be the same on both sides of a point-to-point connection.
  • All interfaces in the subnet of point-to-multipoint connections must use the same MTU size.
  • 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.
  • The maximum number of data-link connection identifiers (DLCIs) is determined by the MTU on the interface. If you have keepalives enabled with the MTU set to 5012, the maximum number of DLCIs is 1000.

Because tunnel services interfaces are considered logical interfaces, you cannot configure the MTU setting for the associated physical interface. This means that you cannot configure the MTU size for the following interface types:

  • Generic routing encapsulation (gr-)
  • IP-IP (ip-)
  • Loopback (lo-)
  • Link services (ls-)
  • Multilink services (ml-)
  • Multicast (pe-, pd-)

Configure the Media MTU

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. In other words:

The maximum media MTU size that you can configure depends on your device and the type of interface.

Note:

Changing the media MTU or protocol MTU causes an interface to be deleted and added again. This causes the link to flap.

To configure the media MTU:

  1. In configuration mode, go to the [edit interfaces interface-name] hierarchy level.
  2. Include the mtu statement.

Protocol MTU

Overview

The default protocol MTU depends on your device and the interface type. When you initially configure an interface, the protocol MTU is calculated automatically. If you subsequently change the media MTU, the protocol MTU on existing address families automatically changes.

If you reduce the media MTU size but one or more address families are already configured and active on the interface, you must also reduce the protocol MTU size. If you increase the size of the protocol MTU, you must ensure that the size of the media MTU is equal to or greater than the sum of the protocol MTU and the encapsulation overhead.

You can configure the protocol MTU on all tunnel interfaces except virtual tunnel (VT) interfaces. Junos OS sets the MTU size for VT interfaces to unlimited by default.

Protocol MTU for MPLS

If you do not configure an MPLS MTU, 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. 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 as follows:

Encapsulation Overhead by Interface Encapsulation Type

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. The following table lists the interface encapsulation and corresponding encapsulation overhead.

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

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. Use this topic to determine the MTU values you can configure on your device.

Media MTU Sizes by Interface Type for M7i and M10i Routers with CFEB

Table 2: Media MTU Sizes by Interface Type for M7i and M10i Routers with CFEB

Interface Type

Default Media MTU (Bytes)

Maximum MTU (Bytes)

Default IP Protocol MTU (Bytes)

Adaptive Services (MTU size not configurable)

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 M7i Routers with CFEB-E, M10i Routers with CFEB-E, and M320 and M120 Routers

Table 3: 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 for MX Series Routers

Table 4: Media MTU Sizes for MX Series Routers by Interface Type

Interface Type

Default Media MTU (Bytes)

Maximum MTU (Bytes)

Default IP Protocol MTU (Bytes)

Gigabit Ethernet

1514

9500

1500 (IPv4), 1488 (MPLS), 1497 (ISO)

10-Gigabit Ethernet

1514

9500

1500 (IPv4), 1488 (MPLS), 1497 (ISO)

Multi-Rate Ethernet

1514

9500

1500 (IPv4), 1488 (MPLS), 1497 (ISO)

Tri-Rate Ethernet

1514

9500

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)
Table 5: Media MTU Sizes for MX Series Routers by MPC
MPC Maximum MTU (Bytes)
MPC1 9500 (Ethernet interfaces)
MPC2 9500 (Ethernet interfaces)
MPC2E 9500 (Ethernet interfaces)
MPC2E-NG. MPC3E-NG 9500
MPC3E 9500 (Ethernet interfaces)
MPC4E 9500 (Ethernet interfaces)
MPC5E 9500 (Ethernet interfaces)
MPC6E 9500 (Ethernet interfaces)
MPC7E (MPC7E-MRATE and MP7E-10G) 16,000
MPC8E (MX2K-MPC8E) 16,000
MPC9E (MX2K-MPC9E) 16,000
MPC10E-10C-MRATE 16,000 (Junos OS Release 19.2R1 and later)
MPC10E-15C-MRATE 16,000 (Junos OS Release 19.1R1 and later)
MX2K-MPC11E 16,000 (Junos OS Release 19.3R2 and later)
MX10003 MPC (MX10003-LC2103) 16,000 (Junos OS Release 17.3R1 and later)
Table 6: Media MTU Sizes for MX Series Routers by Platform
Platform Maximum MTU (Bytes)
MX5, MX10, MX40, MX80 9192
MX204 16,000 (Junos OS Release 17.4R1 and later)

MX304

16000

MX10000

16000

Media MTU Sizes by Interface Type for ACX Series Routers and EX and QFX Series Switches

Table 7: Media MTU Sizes by Interface Type for ACX Series Routers

Interface Type

Switch

Default Media MTU (Bytes)

Maximum MTU (Bytes)

Default IP Protocol MTU (Bytes)

Gigabit Ethernet and 10-Gigabit Ethernet

ACX1000, ACX2000, ACX4000, ACX5048, ACX5096 line of routers, and ACX500

1514

9216

1500 (IPv4), 1497 (ISO)

Gigabit Ethernet and 10-Gigabit Ethernet

ACX5448 series and ACX710 Series

1514

10000

1500 (IPv4), 1497 (ISO)

Gigabit Ethernet and 10-Gigabit Ethernet

ACX7000 Series

1514

9996

1500 (IPv4), 1497 (ISO)

Media MTU Sizes by Interface Type for PTX Series Packet Transport Routers

Table 8: Media MTU Sizes by Interface Type for PTX Series Packet Transport Routers

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)

Media MTU Sizes by Interface Type for JRR200 Series Routers

Table 9: Media MTU Sizes by Interface Type for JRR200 Series Routers

Interface Type

Default Media MTU (Bytes)

Maximum MTU (Bytes)

Default IP Protocol MTU (Bytes)

Management Ethernet Interfaces (em0,em2 -em9)

1514

9192

1500 (IPv4), 1497 (ISO)

MTU and MACsec

Media Access Control security (MACsec) is a Layer 2 (L2) security protocol that provides point-to-point security. MACsec adds a header to packets passing through interfaces where MACsec is enabled. If a packet is near the protocol MTU limit, and the MTU is not adjusted to account for the MACsec header, the packet can exceed the interface MTU when the MACsec header is added. In that case, the device drops the packet. Before enabling MACsec, you must ensure your protocol MTU is large enough to accommodate the additional 32 bytes of MACsec overhead.

Note: The MACsec header can be smaller than 32 bytes when there is no Secure Channel Identifier (SCI) field. We recommend assuming the MACsec header is 32 bytes to ensure the device transmits the MACsec packet.

Automatic MTU Adjustment for MACsec

Some devices can adjust the MTU to include the MACsec header automatically. Use this feature to ensure the interface or protocol MTU is adjusted properly to account for the MACsec overhead. Without this feature, you (the network administrator) need to adjust the interface and protocol MTU manually.

When MACsec is enabled on a physical interface or a logical interface, these devices can automatically adjust the MTU to include the MACsec header for that interface. If the device is using the default interface MTU when this feature is enabled, the device automatically increases the interface MTU to accommodate the MACsec header. If you (the network administrator) have configured a custom interface MTU, the device automatically reduces the protocol MTU to make space for the MACsec header instead. When MACsec is enabled on a specific logical interface, the protocol families under that logical interface use an adjusted MTU that accommodates the MACsec header.

The following table shows how devices that support automatic MTU adjustment react when you have not already configured the MTU.

Table 10: Automatic MTU Adjustment for MACsec
MACsec Enabled At: Physical Interface (IFD) MTU Protocol MTU
Physical interface (IFD) level Device increases the IFD MTU by 32 bytes to account for the MACSec overhead. (Adjusted IFD MTU) − (32 + L2 overhead)
Logical interface (IFL) level IFD MTU remains unchanged. (IFD MTU) − (32 + L2 overhead)

To enable automatic MTU adjustment for MACsec:

  1. Configure MACsec at both the [edit interfaces interface-name] and the [edit security macsec interfaces interface-name] hierarchy levels. See Configuring MACsec for more information.

  2. Configure the enable-auto-mtu-update statement at the [edit security macsec] hierarchy level.

Note:

  • This feature is not supported on aggregated Ethernet interfaces or interfaces belonging to link aggregation groups (LAGs).

  • This feature is not supported for the CCC, VPLS, BRIDGE, or TCC protocol families.

    • When MACsec is enabled at the physical interface level—If any logical interface under it uses any of these protocols, the device skips MTU adjustment for all logical interfaces under that physical interface hierarchy.

    • When MACsec is enabled at the logical interface level—If the logical interface uses any of these protocols, the device skips MTU adjustment only for the protocol configured under that logical interface.

Platform-Specific MTU Behavior

Use Feature Explorer to confirm platform and release support for specific features.

Use the following table to review platform-specific behavior for your platform:

Platform Difference

ACX Series

  • ACX Series routers that support protocol MTU need to explicitly configure MTU at the family level for IPv4 and IPv6 make MTU exception work in egress.

    Follow the guidelines below while configuring MTUs. If you configure MTUs:

    • If you configure MTUs for both inet and inet6 families, inet MTU gets precedence.

    • If you configure MTU only at inet level, the same value applies to inet6 as well.

    • If you configure MTU only for inet6 level, the same value applies to inet as well.

MX Series

  • MX304, MX960, MX2020, MX10003, MX10008: When MACsec is enabled on the interfaces of these devices, you can enable the device to automatically increase the MTU as described in the Automatic MTU Adjustment for MACsec section above.

Change History Table

Feature support is determined by the platform and release you are using. Use Feature Explorer to determine if a feature is supported on your platform.

Release
Description
19.3R2
Starting in Junos OS Release 19.3R2, the maximum configurable MTU size for MX2K-MPC11E is 16,000 bytes.
19.2R1
Starting in Junos OS Release 19.2R1, the maximum configurable MTU size for MPC10E-10C-MRATE is 16,000 bytes.
19.1R1
Starting in Junos OS Release 19.1R1, the maximum configurable MTU size for MPC10E-15C-MRATE is 16,000 bytes.
17.4R1
Starting in Junos OS Release 17.4R1, the MTU size for MX204 is 16,000 bytes.
17.3R1
Starting in Junos OS Release 17.3R1, the MTU size for MX10003 MPC is 16,000 bytes.