Configuring the Media MTU
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 whether the protocol used is IP version 4 (IPv4) or International Organization for Standardization (ISO). Table 9 through Table list the media and protocol MTU sizes by interface type, and Table 15 lists the encapsulation overhead by encapsulation type.
Table 9: Media MTU Sizes by Interface Type for M5, M7i, M10, M10i, M20, and M40 Routers
|
Default Media 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
|
9192 (4-port)
1532 (8-port)
1532 (12-port)
|
1500 (IPv4)
1497 (ISO)
|
Gigabit Ethernet
|
1514
|
9192
|
1500 (IPv4)
1497 (ISO)
|
Serial
|
1504
|
9192
|
1500 (IPv4)
1497 (ISO)
|
SONET/SDH
|
4474
|
9192
|
4470
|
Table 10: Media MTU Sizes by Interface Type for M40e Routers
|
Default Media 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
|
Fast Ethernet
|
1514
|
4500
|
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)
4500 (4-port OC3)
4500 (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
|
Table 11: Media MTU Sizes by Interface Type for M160 Routers
|
Default Media 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
|
Fast Ethernet
|
1514
|
4500
|
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
|
Table 12: Media MTU Sizes by Interface Type for M320 Platforms
|
Default Media 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
|
9192 (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
|
Table 13: Media MTU Sizes by Interface Type for T320 Platforms
|
Default Media 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
|
4500 (4-port)
1532 (12- and 48-port)
|
1500 (IPv4)
1497 (ISO)
|
Gigabit Ethernet
|
1514
|
9192
|
1500 (IPv4)
1497 (ISO)
|
SONET/SDH
|
4474
|
9192
|
4470
|
Table 14: Media MTU Sizes by Interface Type for T640 Platforms
|
Default Media 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
|
Table 15: Encapsulation Overhead by Encapsulation Type
|
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
|
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.
 |
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 interface is specified as 1500 bytes, but the largest possible frame size is actually 1504 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 software 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.
|
For information about configuring the encapsulation on an interface, see Configuring Interface Encapsulation.
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-routers logical-router-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.