cisco router adsl setting

First we will need to build the VPDN group so we will be able to add our dialer after we configure the Ethernet interface we will use for the WAN connection.

router#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
router(config)#vpdn enable
router(config)#vpdn-group 1
router(config-vpdn)#request-dialin
router(config-vpdn)#protocol pppoe
router(config-vpdn)#exit

Now we configure one Ethernet interface for use as our WAN interface.

router(config)#interface Ethernet1
router(config-if)#description ADSL WAN Interface
router(config-if)#no ip address
router(config-if)#no ip redirects
router(config-if)#no ip unreachables
router(config-if)#no ip proxy-arp
router(config-if)#no ip mroute-cache
router(config-if)#pppoe enable
router(config-if)#no cdp enable
router(config-if)#exit

Now to add your Dialer interface:

router(config)#interface Dialer1
router(config-if)#description ADSL WAN Dialer
router(config-if)#ip address negotiated
router(config-if)#no ip unreachables
router(config-if)#ip nat outside
router(config-if)#encapsulation ppp
router(config-if)#no ip mroute-cache
router(config-if)#dialer pool 1
router(config-if)#dialer-group 1
router(config-if)#no cdp enable

At this point you will need to find out what type of authentication your ISP requires. When you run into problems with this (ISP says one thing, it's actually something else...), you are suggested to turn on the debug ppp packet option to view low level packet output. Generally you will need to either use CHAP or PAP authentication. In some cases ISP requires both type of authentication. Following is how to set both up.

router(config-if)#ppp authentication chap pap callin
router(config-if)#ppp chap hostname ispusername

You want to make sure that username is whatever your ISP requires. Some ISP like the full e-mail address and some just need the username. You may receive a letter or email regarding this info. Consult your ISP if you are unsure. Following is the setup.

router(config-if)#ppp chap password isppassword
router(config-if)#ppp pap sent-username ispusername password isppassword

As you can see, with PAP as opposed to chap you input your username and password all at once in one command.

In some routers running newer IOS image, you may need to enter the password as it is (clear text) or encrypted. When you need to enter them as clear text, then you need to type in 0 (zero) then the password to indicate the password you are about to enter is in clear-text format. Similarly, you need to type in 7 (seven) followed by the password to indicate the password you are about to enter is in encrypted format.

You may notice that the Dialer1 interface is part of dialer group 1. This dialer group controls the Layer-3 protocol traffic that go over the Layer-2 PPP encapsulation. For this dialer group control, you need to specify which Layer-3 protocols the Dialer1 interface is allowed to pass through over the Layer-2 PPP. In this case, you want to pass IP protocols. Following is the setup.

router(config-if)#exit
router(config)#dialer-list 1 protocol ip permit

Another problem you may experience with many providers making constant changes to their network is with packet fragmentation from PCs with MTU Maximum Segment Size (MTU MSS) set too high. The MTU on the dialer interface should be 1492 as PPPoE adds an 8 byte encapsulation header. The key is setting ip tcp adjust-mss 1440 on the inside ethernet interface. You will find many different suggestions and recommendations out there. Some will say 1492 or 1460 MTU size instead of 1440. Some will even say 1452. 1452 MSS is pretty much the standard for DSL with a PPPoE transport. Normal MSS is 1500 bytes. But you have to account for the 40 byte IP header and the 8 byte PPPoe header. That gets you to 1452.

Following is from the mouth of Cisco, "If you have ADSL running PPPoE and run into problems resolving DNS, adjust your MTU on your ethernet interface using the command ip tcp adjust-mss 1452. This is because PPPoE requires more bits in the header packet than any other type of circuit."

The last bit of optimization is a little more subtle and is a debatable topic. As the PPPoE traffic is carried over ATM cells, it has to be chopped up before it can be transmitted. ATM cells are 53 bytes long and have a 5 byte header. So a total of 48 bytes of payload. If you were to take 1452 bytes of data and split it up across 48 byte payloads. You would come up with 30.25 cells. The .25 is a 12 byte remainder that would have to be sent in a separate ATM cell. ATM cells are always 53 bytes. So the payload would have to be stuffed with an additional 36 bytes of null data for that last chunk. So to be completely optimized you would set the MSS to 1440 to eliminate those wasted 36 bytes.

Adjusting MTU size may be news to you, but the minute you do it all network-related problems might be fixed. With this in mind, note that MTU size should not be too big in order to avoid general connectivity issue. Similarly MTU size should not be too small in order to avoid ineffective traffic flow. Feel free to experiment to set MTU size to either 1452 or 1440 to see which size brings you the most suitable result.