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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 on whether the protocol used is
IP version 4 (IPv4) or International Organization for Standardization
(ISO). Table 11 through Table 17 list the media and protocol MTU sizes
by interface type, and Table 20 lists the
encapsulation overhead by encapsulation type.
Table 11: Media
MTU Sizes by Interface Type for M5, M7i, M10, M10i, 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
|
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 12: 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
|
4500
|
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
|
Table 13: 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
|
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 14: Media
MTU Sizes by Interface Type for M320 and M120 Platforms
|
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
|
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 15: Media MTU Sizes by Interface
Type for T320 Platforms
|
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
|
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 16: 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
|
Table 17: 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
|
Table 18: 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
|
Table 19: 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
|
Table 20: 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
|
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 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
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-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.
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