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apooley
Posts: 3
Registered: ‎10-29-2009
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ToS header in L2TP frame

Hi All,

 

I just wondered if anyone could suggest how in an E120 router I could disable the copying of the ToS byte from the IP header into the L2TP header.

 

BT currently drop broadband traffic that is marked as having ToS bits set to EF or AF11 and our subscribers can mark their packets. If I can leave the L2TP header blank (i.e. ToS = 0)  then this solves the issue.

 

Thanks in advance

Al

 

 

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Ulf
Posts: 144
Registered: ‎11-13-2008
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Re: ToS header in L2TP frame

Hi,

 

I didn't even know this was a default behaviour. AFAIK there is no knob for this, but can't you just override these ToS bits in the outgoing int with a policy? Please let me know if I got you right.

 

Best Regards

 

Ulf

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New User
apooley
Posts: 3
Registered: ‎10-29-2009
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Re: ToS header in L2TP frame

The trouble is I don't want to remark the IP header merely the L2TP header. I'm not sure how I can do that in a policy?

 

Appreciate any ideas.

Thanks

Alastair

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Ulf
Posts: 144
Registered: ‎11-13-2008

Re: ToS header in L2TP frame

Hi,

 

not sure that I follow. I wasn't aware that there is a ToS/QoS-field in the L2TP header. Can you please elaborate?

 

Thanks

 

Ulf

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New User
apooley
Posts: 3
Registered: ‎10-29-2009
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Re: ToS header in L2TP frame

L2TP encapsulates the IP frame with a new header as L2TP is a tunneling protocol. The header for the L2TP frame is a new header which is separate from the one in the underlying IP packet.

 

In the Cisco implementation of a BRAS you can choose if the L2TP header copies ToS value from the underlying IP packets header  or not.

 

I actually logged a case with JTAC and they've confirmed that this isn't an option in the E-series. The L2TP header ToS is copied straight from the IP packet and there is no way of disabling that.

 

Appreciate your help but turns out this isn't a fixable issue.

Thanks

Al

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Ulf
Posts: 144
Registered: ‎11-13-2008
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Re: ToS header in L2TP frame

[ Edited ]

Hi,

 

still not sure if we're takling about the same thing here. According to http://tools.ietf.org/html/rfc2661#section-3.1 there is no (besides the single QoS-bit) ToS in the L2TP header. So are you talking about the ToS (in the IP-header):

 

1. of the inner/subcriber IP-paket (ppp-encaps) or

 

2. of  the outer IP-paket (L2TP-encaps PPP-frame)

 

I assume with "The L2TP header ToS is copied straight from the IP packet and there is no way of disabling that" you / JTAC refer to 2. This ToS can be modified in an IP-policy.

 

Please clarify your understanding.

 

Ulf

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Juniper Employee
Bernd
Posts: 9
Registered: ‎08-04-2009
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Re: ToS header in L2TP frame

Hi Al,


I still have issues to understand your setup.

Is your E120 LNS or LAC?

From which header to which the TOS information is copied exactly?


Protocol-Stack (packet encapsulation):


CPE...LAC----LAC...LNS---LNS

L2    L2     L2    L2    L2

pppoe pppoe  ip-o  ip-o  ip

ppp   ppp    l2tp  l2tp

ip    ip     ppp   ppp

             ip    ip


From my understanding the LAC has no knowledge of the inner ip header "ip".

So no way to copy something there directly.

On the LNS you are able to set the TOS of the outer IP header "ip-o" by using policies based on traffic-classes or user-packet-class (which could be set by information of the inner IP header "ip").


Gruss

Bernd

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