which part does the command "MTU " define size of frame ?

hblfzhangzhiwei · ‎05-12-2010

Does the command "mtu" under the interface configure mode configure all frame of size or payload of size ?

Ganesh Hariharan · ‎05-12-2010

Does the command  "mtu" under the interface configure mode configure all frame of size  or payload of size ?

Hi,

To support large frames, you only need to change the interface MTU value. If SVI is used, then in order to support large frames, you must increase the MTU on the SVI to route the traffic between VLANs.

Command to configure MTU under interface is

7609(config)#int gigabitEthernet 1/1
7609(config-if)#mtu ?
<1500-9216> MTU size in bytes

7609(config-if)#mtu 9216

Hope to help !!

Ganesh.H

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hblfzhangzhiwei · ‎05-15-2010

thanks, but i hope to know the mtu is size of all frame or the payload size of the frame

Ganesh Hariharan · ‎05-15-2010

thanks, but i hope to know the mtu is size of all frame or the payload size of the frame

Hi,

Ethernet traffic moves in units called frames. The maximum size of frames is called the Maximum Transmission Unit (MTU). When a network device gets a frame larger than its MTU, the data is fragmented (broken into smaller frames) or dropped. Historically, Ethernet has a maximum frame size of 1500 bytes, so most devices use 1500 as their default MTU. An Ethernet packet larger than 1500 bytes is called a Jumbo Frame.

Hope to Help !!

Ganesh.H

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narendrakumar1987in · ‎05-15-2010

Just adding this suplemental info to the good response from Ganesh.

These Jumbo Frames are often reffered to as "GIANTS" as opposed to the "RUNTS",

the name given to frames which has size lesser than 64bytes

Happy Networking !!

Regards

Narendrakumar B.

Nagendra Kumar Nainar · ‎05-15-2010

Hi Zhang,

MTU value specified in "show interface" output is the maximum frame size (including the layer2 header) that can traverse without fragmentation. For example, if you have the below topology

R1------------------R2-----------------------R3

MTU=1000

Assume all connections are fastethernet with MTU size 1000 on R2's outgoing interface (I assume FE MTU cannot be changed from default and I am using this value 1000 only for simplicity). When IP packet of size 986 with DF bit is sent from R1 to R3, it will include 14 bytes layer2 header in R2 which makes the frame of size 1000 bytes and will pass through without any issue. But when IP packet of size 987 with DF but is sent from R1 to R3, including 14 bytes of layer 2 header will make the frame of size 1001 bytes which is more than the outgoing MTU on R2. As we have DF bit, R2 will drop the packet and send ICMP error message to R1.

In turn, if you change "ip mtu" under any interface, it affects the IP size including the IP header.

HTH,

Nagendra