RADIUS

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.