Help us improve your experience.

Let us know what you think.

Do you have time for a two-minute survey?

TechLibrary
Navigation

Configuring the Media MTU on ACX Series Routers

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.

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]

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 ACX Series Routers

Table 2: Media MTU Sizes by Interface Type for 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)

 

Related Documentation

 

Published: 2013-01-11

Previous Page Next Page