[CLUG-tech] PPPoE troubles

Dewet Diener clug-tech at dewet.org
Wed Feb 1 09:04:09 SAST 2006


Chris Nash wrote:
> What port does PPPoE use?

PPPoE is indeed not IP traffic.  Running tcpdump on the ethernet 
interface connected to the ADSL router, you can see PPPoE traffic 
something like this:

08:58:51.5617809:00:17.327613 PPPoE  [ses 0xd445] LCP, Echo-Request 
(0x09), id 112, length 10
09:00:17.338609 PPPoE  [ses 0xd445] LCP, Echo-Reply (0x0a), id 112, 
length 10

After the PPP interface has come up, you can also see the traffic on the 
*ethernet* interface, as tcpdump decodes it from the PPPoE traffic:

09:00:16.024876 PPPoE  [ses 0xd445] IP x.x.x.x.41622 > x.x.x.x.80: S 
134200638:134200638(0) win 5808 <mss 1452,sackOK,timestamp 828945968 
0,nop,wscale 2>
09:00:16.862138 PPPoE  [ses 0xd445] IP x.x.x.x.1556 > x.x.x.x.139: S 
945775641:945775641(0) win 53760 <mss 1440,nop,wscale 
3,nop,nop,timestamp 0 0,nop,nop,sackOK>

Using tcpdump could give you more insight as to what's happening with 
the PPPoE traffic, and why the PPP interface isn't coming up.

d


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