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Media MTU and Protocol MTU

SUMMARY 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.

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 default IP MTU. In some cases, the default IP 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 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.
  • 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:

  • Loopback (lo-)

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.

If you do not configure an MPLS MTU, Junos OS Evolved 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:

You can configure the protocol MTU on all tunnel interfaces.

Configure the Protocol MTU

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 protocol MTU:

  1. In configuration mode, go to the [edit interfaces interface-name unit logical-unit-number] hierarchy level.
  2. Include the mtu statement for each family you want to configure with a non-default MTU value.

    If you configure the protocol MTU for any family, the configured value is applied to all families that are configured on the logical interface.

    Note:

    If you are configuring the protocol MTU for both inet and inet6 families on the same logical interface, you must configure the same value for both families. We do not recommend configuring different MTU size values for inet and inet6 families that are configured on the same logical interface.

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