Hello Hisham I have a doubt about the concept you stated
" *** The opposite is not correct i.e During label disposition (popping) the EXP value is not copied to whatever is below it. The label will be popped and nothing will be propagated downwards. " as I believe that the "uniform mode " is the default one, where the IPP is copied into the mpls-EXP at the imposition and mpls-EXP is copied back at the poping... and whenever the mpls-EXP is changed through the SP cloud it will affect both labels according to the command " set mpls experimental imposition To set the value of the Multiprotocol Label Switching (MPLS) experimental (EXP) field on all imposed label entries, use the *set mpls experimental imposition *command in QoS policy-map class configuration mode. " but when we need to have the "pipe-mode" or "short-pipe mode" we should use the other command " set mpls experimental topmost To set the Multiprotocol Label Switching (MPLS) experimental (EXP) field value in the topmost label on either an input or an output interface, use the *set mpls experimental* *topmost* command in QoS policy-map class configuration mode. " Which will leave the VPN-label Exp unchanged So the IPP at the egress will not be changed as well. Thanks Ahmed On Tue, Jun 24, 2008 at 11:37 PM, hehsam elezaby <[EMAIL PROTECTED]> wrote: > Hello , > > I would like to add a piece of info which helped me so much during my > lab preparation and made me understand MPLS QoS scenarios very well. > > *** The EXP field is copied from whatever there is below it during label > imposition. For example if we have an ingress PE and it is doing MPLS VPN > label imposition , it will apply same value as IP Precedence to both MPLS > labels being added on top of the IP packet. > > *** The opposite is not correct i.e During label disposition (popping) the > EXP value is not copied to whatever is below it. The label will be popped > and nothing will be propagated downwards. > > We always need QoS groups on PE routers when we do policies in the egress > direction to the CE based upon provider markings (EXP bits). The packet > arrives at the PE with only the VPN label and it copies the EXP value into > the QoS group and then performs egress policy based on whatever value is > inside the QoS group. This is what we usually call the "pipe mode". You will > never need such a setup if the customer packet already has DSCP or IP > Precedence marking because we will be simply doing our policy based on the > original customer marking AKA "short pipe" mode. > > But what is for some kind of reason (usually a forced marking policy > configured in the middle of the provider network) the MPLS VPN packet will > have different EXP values on both labels and the PHP router (P router one > hop before PE) will then pop the topmost label and propagate the packet with > an unrealistic label i.e bottom label which should have been changed too but > it actually didn't (again as i explained in the second point , the EXP value > will never be propagated in a pop operation). We need to guarantee that the > policy is based upon the topmost label EXP value so we copy it into the QoS > group on the PHP router and then perform policy based on this value in > egress direction to PE router. This could be needed in both "pipe" and > "short pipe" modes because it is something related to the P router and not > the PE. > > Finally , if we have Explicit null enabled , we will never need any QoS > groups on the P routers because it will always be sending a dual stack label > to the PE router no matter what happens. But definitely we will still need > it on PE routers in egress policies as i explained before for the "pipe > mode". > > I hope this helps. > > Hisham El-Ezaby > CCIE# 21190 (SP) > > > ----- Original Message ---- > From: mohamed hamed <[EMAIL PROTECTED]> > To: OSL CCIE Service Provider Lab Exam <[email protected]> > Sent: Monday, June 23, 2008 11:35:54 PM > Subject: [OSL | CCIE_SP] QoS group > > Hi everyone, > > > > Does anyone know where we should use the QoS group on P or PE Router? > > > > As I understand that we usually use it on PE router, but I found some Labs > on the internetwork expert use it on P router as well > > > > If anyone knows why we should use QoS grouo on P routers , please unicast > me a mail back > > > > *Mohamed Hamed* > > *Network Consulting Engineer*** > > *Mobile (KSA-Riyadh): +966543464502* > > > > > > > ------------------------------ > Introducing Live Search cashback . It's search that pays you back! Try it > Now<http://search.live.com/cashback/?&pkw=form=MIJAAF/publ=HMTGL/crea=introsrchcashback> > >
