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, M10, M20, and M40 Routers
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Default Media MTU (Bytes) |
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Default IP Protocol MTU (Bytes) |
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Table 4: Media MTU Sizes by Interface Type for M40e Routers
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Default Media MTU (Bytes) |
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Default IP Protocol MTU (Bytes) |
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- 4-port OC-3
- 4-port OC-3c
- 1-port OC-12
- 4-port OC-12
- 4-port OC-12c
- 1-port OC-48
- 2-port OC-3
- 2-port OC-3c
- 1-port OC-12c
- 1-port OC-48c
- 1-port OC-192
- 1-port OC-192c
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Table 5: Media MTU Sizes by Interface Type for M160 Routers
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Default Media MTU (Bytes) |
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Default IP Protocol MTU (Bytes) |
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Table 6: Media MTU Sizes by Interface Type for T320 Platforms
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Default Media MTU (Bytes) |
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Default IP Protocol MTU (Bytes) |
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Table 7: Media MTU Sizes by Interface Type for T640 Platforms
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Default Media MTU (Bytes) |
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Default IP Protocol MTU (Bytes) |
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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.
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Table 8: Encapsulation Overhead by Encapsulation Type
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Encapsulation Overhead (Bytes) |
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802.1Q/Ethernet version 2 |
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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.
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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.
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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 [edit interfaces interface-name unit logical-unit-number family family] hierarchy level, as discussed in Set the Protocol MTU.