Originally, I was thinking on the same lines as you are but after reading the following from Cisco book. I got kind of confused. I might have misunderstood it. However, I have included it here for your reading and to help me out with this issue.
The cisco CCIE third edition book has an explanation on page 459 that reads as follows. It is the second para from the top. +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ "These switches make most internal QOS decisions based on internal DSCP setting. The internal DSCP has been determined when the frame is forwarded. So, when a frame has been assigned an internal DSCP and an egress interface, the following logic determines into which of the four interface output queues the frame is placed. 1. The frame's internal DSCP is compared to a global DSCP-COS map to determine a COS vlaue. 2. The per interface COS-queue map determines the queue for a frame based on the assigned CoS ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ My interpretation of the above paragraph. Throughout this paragraph there is no mention of packet anywhere. The DSCP value is assigned to the frame and not the IP header. It is assumed to be internal to the switching logic and is not assigned to the packet header. If a switch were to convert a COS value into a DSCP value, in that an incorrect mapping in any switch will alter the DSCP in the IP header thus effectively becoming a marking tool. As far as I know switches do not change the DSCP value in the IP header for a packet received over a tunnel interface. Let me know if you think it is something incorrectly printed in the book that we need to refer to the author. Thanks Suresh On Thu, Jun 5, 2008 at 8:14 PM, Scott Morris <[EMAIL PROTECTED]> wrote: > Please let me know where you were reading that, because that's not the > behavior I've observed in real life! > > If it comes in as CoS 5, the DSCP will get rewritten (provided you have mls > qos enabled and the trust level appropriately set). That will indeed affect > other devices. > > Scott > > -----Original Message----- > From: Suresh Mishra [mailto:[EMAIL PROTECTED] > Sent: Thursday, June 05, 2008 5:15 PM > To: Scott Morris; OSL CCIE Routing and Switching Lab Exam > Subject: LAB 12 task 7 > > Hello all, > > This LAB requires that we change the cos-dscp map to map cos 5 to dscp 46. > Now this is required for dwonstream routers so that they see DSCP > 46 in the IP packet. > > However i was reading the document , it says that cos-dscp mapping is used > internally by the switch and has nothing to do with changing the IP packets > DSCP field. In that case downstream routers will not see the dscp value. > > Can somebody help me out on this issue? > > Thanks > Suresh > >
