Name

Framed-MTU

Synopsis

Attribute Number

12

Length

6

Value

INTEGER

Allowed in

Access-Request, Access-Accept

Prohibited in

Access-Reject, Access-Challenge

Presence in Packet

Not required

Maximum Iterations

1

This attribute allows the RADIUS server to set the connection’s maximum transfer unit (MTU) size to align communications with the way an administrator has configured his network equipment. Briefly, the MTU setting is the largest packet size that can be transmitted over a connection. If the packet size is larger than this value, then the packet is broken up by routers along the path to the destination and then reassembled at the end point.

The RADIUS server will expect the value of this attribute to be somewhere between 64 and 65,535. Most clients or servers will send this attribute, containing a default MTU size of 1,500, inside an Access-Accept packet.

Tip

It’s important to note that only implementations that strictly support RFC 2138 prohibit the Framed-MTU attribute from being passed in Access-Request. The new RADIUS draft supports this, but some implementations may not have been upgraded to support this change.

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