[cisco-bba] L2TP tunnel switching not copying dscp bits between session L2TP tunnel switching not copying dscp bits between session headers

Tony Redstone tony.redstone at googlemail.com
Tue Aug 15 11:14:04 EDT 2006


network topology:

We're terminating ppp users on the LNS and using "QoS: Per-Session
Shaping and Queuing on LNS" and "ip tos reflect" under the vpdn-group
to copy the end user payload dscp bits into the encapsulating L2TP
header.  This works fine.

When L2TP traffic is tunnel switched through a cisco 7301 (LTS) the
dscp bits are reset in the outgoing L2TP session headers on egress
from the LTS.  This means we can't apply appropriate congestion
management either directly on egress from the LTS or further upstream.

Various testing suggests the following:
*) the ToS byte is reset during tunnel switching (to whatever the ip
precedence and/or ip tos values are under the vpdn-group).
*) "ip tos reflect" has no effect on tunnel switched traffic (which,
strictly speaking, makes sense given the command description).

It looks like what we want is an extension to the tos reflect command
to allow reflection with multihop (eg "ip tos reflect multihop").

Any suggestions as to how we can preserve the ToS byte during L2TP
tunnel switching?



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