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How is the MTU(maximum tranmission unit) determined on a call?
Article Id #: 284

The remote access server has a default MTU of 1524. This is the maximum The MTU of the ethernet media. We recommend that this not be changed.
The MTU will be negotiated during LCP negotation for a dial-in user. During LCP negotiation we will tell the remote end we are capable of 1524.
There are two ways in which a customer can receive an MTU that is lower:

1. The RADIUS software returns a Framed-MTU attribute that specifies a lower value. In releases 2.3.3 and lower, we will change the MTU in response to this attribute. In 2.4.1 and above this RADIUS attribute is ignored.

2. The remote modem indicates that an MTU of 1524 is not acceptable and wants 512. We 'give in' to that request and assign 512 as the MTU.

A lower MTU on a dial-in call is not necessarily a bad thing. Most packets that are larger than the MTU of a connection can be broken down and sent down the connection in smaller chunks.
The problem occurs when the remote access server receives a packet larger than the connection's MTU and the packet has the Do Not Fragment bit set. This bit tells the remote access server that it must not split the packet into smaller chunks. Because the remote access server can not split the packet into smaller chunks AND can not send it as-is over the connection, it drops the packet.
What the dial-in user may see is the inability to load certain web sites.

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