02-22-2010 09:44 AM
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
02-23-2010 10:36 AM
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.
02-26-2010 06:43 AM
not sure that I follow. I wasn't aware that there is a ToS/QoS-field in the L2TP header. Can you please elaborate?
03-01-2010 07:14 AM
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.
03-01-2010 08:00 AM - edited 03-01-2010 08:06 AM
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.
03-03-2010 10:03 AM
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):
L2 L2 L2 L2 L2
pppoe pppoe ip-o ip-o ip
ppp ppp l2tp l2tp
ip ip ppp ppp
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").