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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

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:

Default media MTU = Default IP MTU + encapsulation overhead

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:

[edit interfaces interface-name]mtu bytes;

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
(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 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
(MTU size not configurable)

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
(MTU size not configurable)

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
(channelized to DS0s)

1504

4500

1500

Dual-port Channelized T1/E1 PIM
(clear channel T1 or E1)

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
(channelized to DS0s)

1504

4500

1500

Dual-port Channelized T1/E1 PIM
(clear channel T1 or E1)

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

 

Published: 2012-12-11

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