tftp blksize

Description                                                    Change the TFTP block size.

Syntax                                                                  [no] tftp blksize bytes

Replace bytes with the Maximum packet length the ACOS TFTP client can use when sending or receiving files to or from a TFTP server. You can specify from 512-32768 bytes.

Default                                                                512 bytes

Mode                                                                   Configuration mode

Usage                                                                  Increasing the TFTP block size can provide the following benefits:

     TFTP file transfers can occur more quickly, since fewer blocks are required to a send a file.

     File transfer errors due to the server reaching its maximum block size before a file is transferred can be eliminated.

To determine the maximum file size a block size will allow, use the following formula:

1K-blocksize = 64MB-filesize

Here are some examples.

Block Size

Maximum File Size


64 MB


512 MB


2048 MB

Increasing the TFTP block size of the ACOS device only increases the maximum block size supported by the ACOS device. The TFTP server also must support larger block sizes. If the block size is larger than the TFTP server supports, the file transfer will fail and a communication error will be displayed on the CLI terminal.

If the TFTP block size is larger than the IP Maximum Transmission Unit (MTU) on any device involved in the file transfer, the TFTP packets will be fragmented to fit within the MTU. The fragmentation will not increase the number of blocks; however, it can re-add some overhead to the overall file transmission speed.

If the ACOS device is a member of an aVCS virtual chassis, use the device-context command to specify the device in the chassis to which to apply this command.

Example                                                            The following commands display the current TFTP block size, increase it, then verify the change:

ACOS(config)#show tftp

TFTP client block size is set to 512

ACOS(config)#tftp blksize 4096

ACOS(config)#show tftp

TFTP client block size is set to 4096

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