Cisco Dialer0 – Deutsche Telekom (T-online) DSL connection

Sometimes it is getting very complicate to get your business account connected to the internet. especially if you are not using one of the end user routers like a firtz!box or any other kind of end user router.

To get your Cisco router connected you need a modem and dialer interface with a proper configuration.

Not comes the clue. I found plenty of howtos but I encountered that not all websites were shown correctly.

This was due to the fact that “ip mtu 1492″ and ” ip tcp adjust-mss 1452″ was not set correctly.
With this two settings on dialer0 interface everything works perfect. Took me a while to figure out.

To get you mtu execute an extenden ping:

Protocol [ip]:
Target IP address:
Repeat count [5]: 1
Datagram size [100]:
Timeout in seconds [2]:
Extended commands [n]: y
Source address or interface: dialer0
Type of service [0]:
Set DF bit in IP header? [no]: yes
Validate reply data? [no]:
Data pattern [0xABCD]:
Loose, Strict, Record, Timestamp, Verbose[none]:
Sweep range of sizes [n]: y
Sweep min size [36]: 1401
Sweep max size [18024]: 1500
Sweep interval [1]:
Type escape sequence to abort.
Sending 100, [1401..1500]-byte ICMP Echos to, timeout is 2 seconds:
Packet sent with a source address of
Packet sent with the DF bit set
Success rate is 92 percent (92/100), round-trip min/avg/max = 84/89/124 ms

To find your MTU start a ping with an MTU sweep from 1401 to 1500. The statistics will give you the successful and failed: “Success rate is 92 percent (92/100)” meaning 8 packets failed = 1500 – 8 = 1492

With this information setup your dialer interface:

Configure interface to WAN

interface Ethernet0/0
description EXT_WAN_T_ONLINE
no ip address
pppoe enable group global
pppoe-client dial-pool-number 1
no cdp enable

Configure now the dialer interface

interface Dialer0
 description T-ONLINE-DIALUP
 bandwidth 364
 bandwidth receive 3000
 ip address negotiated
 no ip proxy-arp
 ip mtu 1492
 encapsulation ppp
 ip tcp adjust-mss 1452
 dialer pool 1
 dialer-group 1
 no cdp enable
 ppp authentication chap callin
 ppp chap hostname feste-ip7/
 ppp chap password secretpassword

To get all of you computers connected to the internet NAT is needed. This will be topic of a different post.


Sources for my research:


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