* Yeah Hisham, Thanks for a nice explanation,
That's quite correctly said, * - *Always the values from the IP precedence field are propagated through the labels stacks during the label imposition (case of an MPLS-PE ingress router).* * * - *EXP values never propagated to the newly exposed label during the POP operation. <<< use QOS-GRP/Explicit-Null here* * * - *Whenever we need to make decision on the exit interface based on incoming MPLS label which is popped as per the current forwarding-table <<< use QOS-GRP* * Mostly we @ ingress set the label imposition stack, iff we set the topmost only then again we will have to take care of the popping operation and the EXP value. Using Explicit-Null achieves the purpose of exp value retention as the Null value is used to carry the EXP bits. and we don't have to worry about the qos-grp usage. Thanks, Dara * On Wed, Jun 25, 2008 at 2:07 AM, 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> > >
