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 1 through Table 7 list the media and protocol MTU sizes by interface type, and Table 10 lists the encapsulation overhead by encapsulation
type.
Table 1: Media
MTU Sizes by Interface Type for M5, M7i with CFEB, M10, M10i with
CFEB, 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 2: 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 3: 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 4: Media
MTU Sizes by Interface Type for M7i with CFEB-E, M10i with CFEB-E,
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 5: 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 6: 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 7: 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 8: 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 9: 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 10: 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 on Physical Interfaces.
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.
Updated: 2009-04-07
