HP ProCurve Secure 7000dl Series Basic Management And Configuration Manual page 269

Configuring the Data Link Layer Protocol for E1, T1, and Serial Interfaces
Set the MTU. The maximum transmission unit (MTU) defines the largest
size that a PPP frame can be. If a frame exceeds this size, it must be
fragmented. By default, the MTU for PPP interfaces is 1500 bytes. To change
this setting, enter:
Syntax: mtu <size>
Replace <size> with a number between 64 and 1520.
For most environments, you should leave the MTU at 1500. In some cases,
however, you may need to adjust the MTU size. For example, you need to
evaluate MTU size if:
The interface is connected to another router that uses a different
MTU size.
The interface is used in a PPP over Ethernet (PPPoE) environment. (For
more information about PPPoE, see Chapter 7: ADSL WAN Connections.)
If two PPP peers use different MTU sizes, this mismatch can affect transmis-
sions and routing. For example, if the PPP peer has a smaller MTU and your
router sends a frame that exceeds that size, the PPP peer will have to fragment
the frame. If the frame is tagged with the "do not fragment" field, then the
router cannot forward the frame.
If you have enabled Open Shortest Path First (OSPF) routing on the ProCurve
Secure Router, you should be especially careful when setting the MTU. OSPF
routers cannot become adjacent if their MTU sizes do not match. You should
ensure that the MTU on the router at the far-end of the PPP connection is using
the same MTU as the router you are configuring.
You may also need to configure the MTU for PPPoE. When two devices initiate
a PPPoE session, they negotiate an MTU of 1492 bytes because the payload
of an Ethernet frame cannot exceed 1500 bytes. With the overhead created by
PPP, the PPPoE frame is 1500 bytes.
Typically, the two PPPoE devices will negotiate the MTU size of 1492. If there
are problems, however, you may need to manually configure the MTU.
Adding a Description. You can add a description to the PPP interface if you
want to document information about it. For example, if you have configured
multiple PPP interfaces, you may want to document how each PPP interface
is being used. To create a description, enter:
Syntax: description <line>
Configuring the Logical Interface
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