This document explains why the maximum transmission unit (MTU) size
must be changed when you implement PPP over Ethernet (PPPoE) Dialin. It also
explains the procedure used to make the change.
There are no specific requirements for this document.
This document is not restricted to specific software and hardware
For more information on document conventions, refer to the
Technical Tips Conventions.
When a user requests a web site, a client/server negotiation occurs
between the PC and the web server that hosts the web site. During the
negotiation, a maximum MTU size is negotiated. Since the PC negotiates and its
default MTU size is 1500 bytes (Windows 3x, 9x, NT, ME, and so forth), the web
server negotiates an MTU size of 1500 bytes. Therefore, regardless of the MTU
size you configure on the router, the web server still sends packets up to 1500
bytes in size.
The reason why some pages do not fully load is that the router
fragments IP packets if the PC MTU is misconfigured and a packet greater than
1492 bytes is sent to the router. This fragmentation does not occur on the
return path through the universal access concentrator (UAC) (Cisco 6400 or
7200). When the UAC receives a packet greater than 1492 bytes, the packet is
dropped, and the UAC generates and sends an Internet Control Message Protocol
(ICMP) message to the web server that sent the oversized packet. The ICMP
informs the web server that it sent an oversized packet and that it needs to
resend the packet with a smaller MTU.
Note: For information about why the MTU size is 1492 bytes, refer to the
Baseline Architecture for the Cisco 6400 white paper.
The problem occurs because many web servers block ICMP messages, which
causes the server to continuously send 1500-byte packets. These packets are
dropped, and as a result, the requested web site does not load. If the web
server is properly configured and ICMP messages are not blocked, the server
adjusts its MTU and retransmits until the page loads completely.
A partially loaded page occurs when the initial data packets sent from
the web server are under the 1492 byte maximum. However, a packet is then sent
that exceeds this maximum. The server continues to retransmit this oversized
packet that results in a partially loaded page and a "waiting for reply..."
message in the status bar.
You can change the MTU size with the help of one of these three
Adjust the PPPoE MTU size on the Cisco DSL
Download the Dr. TCP utility that
automatically modifies the MTU setting.
Add and then modify a "MaxMTU" string-value
to the registry key that contains the PC Ethernet
Note: These configuration commands work only if you run Network Address
Translation (NAT) or Port Address Translation (PAT) on the Cisco DSL router.
The ip adjust-mss command in Cisco IOS®
Software Release 12.2(2)XH has changed to ip tcp adjust-mss
. This change is
documented in the
Notes for the Cisco 800 Series Routers and Cisco 820 Series Routers for Cisco
IOS Release 12.2(2)XH.
ip address <ip address> <subnet mask>
ip adjust-mss 1452
!--- The TCP MSS command requires an MSS of 1452, and not 1492.
ip nat inside
no ip directed-broadcast
The Dr. TCP utility needs to be run only once. The registry change is
saved at the completion of this procedure.
Navigate to the
Reload the page in order to ensure you have the current copy.
Download the latest version of the utility
Run the utility.
From the menu choose your Ethernet adapter.
In the MTU field, type 1492.
Click in another field, but do not type anything.
Click Apply in order to save the change, and then
Reboot the PC.
Use this procedure in order to change the MTU size by modification of
the registry key.
Add a "MaxMTU" string-value to the registry key that contains the
Ethernet adapter of your PC.
Set the "MaxMTU" value-data entry to
Reboot the PC.
If you change the MTU size with Dr. TCP or on the Cisco DSL router and
you are still not able to browse certain web sites, adjust the MTU size again.
Change the MTU size to 1452 in Dr. TCP, or change the MSS
adjust value on the Cisco DSL router to 1412. If these sizes
are too large, continue to lower the MTU sizes until you reach a baseline of
1400 for Dr. TCP or 1360 for MSS adjust on
the Cisco DSL router.