Configure the Media MTU

The default media MTU size used on a physical interface depends on the encapsulation used on that interface. Table 3, Table 4, Table 5, Table 6, and Table 7 list the media MTU sizes by interface type, and Table 8 lists the encapsulation overhead by encapsulation type.

Table 3: 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)
ATM	4482	9192	4470
E1/T1	1504	9192	1500
E3/T3	4474	9192	4470
Fast Ethernet	1514	4-port-9192 8-port-1532 12-port-1532	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 4: Media MTU Sizes by Interface Type for M40e Routers

Interface Type	Default Media MTU (Bytes)	Maximum MTU (Bytes)	Default IP Protocol MTU (Bytes)
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	1- or 2-port-9192 4-port-4500	1500 (IPv4) 1497 (ISO)
Serial	1504	9192	1500 (IPv4) 1497 (ISO)
SONET/SDH	4474	1-port nonconcatenated-4500 4-port OC-3-4500 4-port OC-3c-4500 1-port OC-12-4500 4-port OC-12-4500 4-port OC-12c-4500 1-port OC-48-4500 2-port OC-3-9192 2-port OC-3c-9192 1-port OC-12c -9192 1-port OC-48c-9192 1-port OC-192-4500 1-port OC-192c-9192	4470

Table 5: Media MTU Sizes by Interface Type for M160 Routers

Interface Type	Default Media MTU (Bytes)	Maximum MTU (Bytes)	Default IP Protocol MTU (Bytes)
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	1- or 2-port-9192 4-port-4500	1500 (IPv4) 1497 (ISO)
Serial	1504	9192	1500 (IPv4) 1497 (ISO)
SONET/SDH	4474	1-port nonconcatenated-4500 1- or 2-port-9192 4-port-4500	4470

Table 6: 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
Fast Ethernet	1514	4-port-4500 48-port-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 T640 Platforms

Interface Type	Default Media MTU (Bytes)	Maximum MTU (Bytes)	Default IP Protocol MTU (Bytes)
48-port Fast Ethernet	1514	1532	1500 (IPv4) 1497 (ISO)
Gigabit Ethernet	1514	9192	1500 (IPv4) 1497 (ISO)
SONET/SDH	4474	9192	4470

Table 8: Encapsulation Overhead by Encapsulation Type

Interface Encapsulation	Encapsulation Overhead (Bytes)
ATM Cell Relay	4
ATM PVC	12
Cisco HDLC	4
Frame Relay	4
Point-to-Point Protocol	4
Ethernet over ATM	28
Ethernet version 2	14
Ethernet 802.3	17
802.1Q/Ethernet version 2	18
802.1Q/Ethernet 802.3	21
Ethernet CCC and VPLS	4
Ethernet TCC	18
Ethernet SNAP	22
802.1Q/Ethernet SNAP	26
VLAN CCC and VPLS	4
Extended VLAN CCC and VPLS	4
Extended 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.

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 FCS field of the Ethernet frame. A SONET interface operating in concatenated mode has a "c" added to the rate descriptor. For example, a concatenated OC-48 interface is referred to as OC-48c.

For information about configuring the encapsulation on an interface, see Configure 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. 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]

For more information, see Set the Protocol MTU.