Why Ethernet card at half-duplex works well when connected to ADSL modem?

Answered Question
Apr 1st, 2008

Hi,

We have this setup:

LAN - PIX - 1721 - Efficient ADSL & Serial

ADSL is connected to Ethernet card of 1721 and have Dialer setup in the router for PPP.

There is PBR in 1721 such that IP packets from x.x.26.94 (PAT address) from PIX will be routed to ADSL.

For months, downloading large files would just die in the middle.

But last night, I changed the Ethernet interface to "half duplex" from "full duplex" and so far, never had disconnection issue after downloading GIGs of files and also transfer rate is back to normal.

Can someone explain why?

Thanks.

Archie

I have this problem too.
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Correct Answer by Paolo Bevilacqua about 8 years 8 months ago

When the router was in full duplex, it was probably have been causing transmission aborts to the adsl modem in half duplex.

If you have a free wic slot in the 1721, you can install an adsl card in there and further improve the connection. However, an 857 or 877 router would be even more economical.

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Correct Answer
Paolo Bevilacqua Tue, 04/01/2008 - 07:58

When the router was in full duplex, it was probably have been causing transmission aborts to the adsl modem in half duplex.

If you have a free wic slot in the 1721, you can install an adsl card in there and further improve the connection. However, an 857 or 877 router would be even more economical.

agcastle2000 Tue, 04/01/2008 - 20:58

Thanks for your explanation.

I don't know that the Ethernet port in Efficient ADSL modem is in half duplex. Does this apply to all ADSL modems? The downside with the 1721 is that it's not possible to configure the duplex to auto.

1721 has only two WIC slots and the other slot is already populated by WIC-1T (serial). PIX doesn't support Policy-Based Routing so I can't put 857 or 877 next to 1721.

Paolo Bevilacqua Wed, 04/02/2008 - 06:12

Yes, many ADSL modems are half-duplex. The idea is to slow down ethernet traffic before it enters the slow ADSL line.

It is always better to manually configure speed and duplex on cisco devices and not rely on auto-negotiation.

Alos consider that with the right configuration the router does all what a PIX does so you can also consider eliminating the PIX altogether.

Hope this helps, please rate post if it does!

agcastle2000 Sun, 04/27/2008 - 23:09

Hi,

Sorry for the late follow-up reply. While the slow download issue with the ADSL was fixed but we are still experiencing an issue when we download any file(drivers, software,etc.) from HP site. The connection timeouts after it gets redirected to HP FTP site.

This must not be ADSL related as it works when the ADSL box is directly connected to the notebook, but I can say that there is still a delay during the redirection to FTP.

Do you think it's best to post this in another forum?

Thanks.

Archie

Paolo Bevilacqua Tue, 04/29/2008 - 08:42

Is that a PPPoE connection ? If so you will need to configure "ip tcp adjust-mss 1452" on the LAN interface that connects the PC.

Hope this helps, please rate post if it does!

agcastle2000 Tue, 04/29/2008 - 21:22

It's PPPoE.

Yes, I have it in the Ethernet0.

As I said, if the ADSL is directly connected to my notebook and setup PPPoE connection from the notebook, I can happily download from HP site. The HTTP to FTP redirection appears to timeout.

Where do you think the issue lies?

Thanks,

Archie

Paolo Bevilacqua Wed, 04/30/2008 - 13:01

The "ip tcp adjust-mss" command goes on the LAN interface that connects the PC.

That is, if Ethernet0 is used for PPPoE, it would go on some other LAN interface.

Lack of this commands cause some but not all websites to fail strangely.

However, if you also have a PIX, the problem can be in there, try bypassing it.

agcastle2000 Fri, 05/02/2008 - 21:49

Paolo,

See the configuration from our 1721 router. I agree with what you said, the PIX can be the problem

interface Ethernet0

description $CONNECTED TO ADSL MODEM$

no ip address

no ip redirects

no ip unreachables

no ip proxy-arp

ip route-cache flow

ip tcp adjust-mss 1452

half-duplex

pppoe enable group global

pppoe-client dial-pool-number 1

no cdp enable

interface Dialer1

bandwidth 1024

bandwidth inherit 1024

ip address negotiated

no ip redirects

no ip unreachables

no ip proxy-arp

ip mtu 1492

ip nbar protocol-discovery

ip nat outside

ip inspect SDM_LOW out

ip virtual-reassembly

encapsulation ppp

ip route-cache flow

dialer pool 1

dialer-group 1

no cdp enable

ppp authentication chap callin

ppp chap hostname xxxxx

ppp chap password 7 xxxxxxxx

Archie

Paolo Bevilacqua Sat, 05/03/2008 - 08:43

Hi, as I mentioned already, the command "ip tcp adjust-mss 1452" goes on the interface that connects to the pix, not the one that connects the adsl modem.

agcastle2000 Sat, 05/03/2008 - 22:07

It's clear to me now where to add the line.

My router has serial connection also for Internet Leased Line aside from the ADSL.

Will adding the line on the inside interface of the router not affect the packets going/coming to/from the serial interface?

Anyway, I added the line but it didn't help.

In Firefox, I'm getting a dialog that says:

"The connection to the server was reset while the page is loading."

This happens when the "Connect to ftp.hp.com" starts to show in the lower left corner of the Firefox browser.

Thanks.

Paolo Bevilacqua Sun, 05/04/2008 - 10:02

Hi, the command is mandatory when using PPPoE, on the other hand FTP can be being blocked by the PIX, try bypassing it.

agcastle2000 Mon, 05/05/2008 - 05:18

I temporarily solved and found the problem. If you're familiar with PIX, here's what I did:

I temporarily disabled the service policy:

no service-policy global_policy global

And the issue with HTTP to FTP when downloading a file in HP is solved.

There is a line "inspect ftp" under policy-map which I think is the one causing the issue.

Thanks.

agcastle2000 Sun, 04/27/2008 - 23:10

Hi,

Sorry for the late follow-up reply. While the slow download issue with the ADSL was fixed but we are still experiencing an issue when we download any file(drivers, software,etc.) from HP site. The connection timeouts after it gets redirected to HP FTP site.

This must not be ADSL related as it works when the ADSL box is directly connected to the notebook, but I can say that there is still a delay during the redirection to FTP.

Do you think it's best to post this in another forum?

Thanks.

Archie

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