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Implicit VS Explicit label

Hello Guys,

I have a small confusion about implicit and explicit label.I read this statement on one of the article.

The implicit
NULL label is assigned by the Egress LSR when it wants the upstream LSR to do PHP
– pop the label. Often the egress LSR does not need the outer label on the MPLS stack and for
efficiency reasons will ask the faster upstream LSR to remove it before forwarding the packet.
The problem with the implicit NULL label is that when the label is popped by the LSR, the EXP
field is also removed, and hence all QoS information is lost. If this information is important then a
better label to use is label 0 – this is the explicit NULL label. This time, instead of popping the
label, the penultimate upstream LSR now adds a label of 0. When the egress LSR receives a label
of 0 it knows it has to remove it. Label 0 cannot be swapped to another value. The EXP bits are
still attached.

Now with reference with the above statement I have the below 3 question that I want to ask , Request you to help me to understand this.

1) As per my understanding both implicit and explicit label is assigned by the last LSR(Egress LSR), correct me if I am wrong.

2)As per the above statement when implicit null is assigned and when next to last router(P router) get the info of implicit null it do PHP , while doing so it removes the EXP bits also , hence QOS information lost.

As per the MPLS label stack the label , EXP and TTL are the separate field.So why the QOS getting lost if P router do PHP , it should affect only label filed not the EXP field.Please en light me on this.

3)When the explicit null is included then the label 0 is assigned by P router( confirm if label 0 assigned by p router or Egdge LSR),even though the label 0 is assigned in mpls header , Edge LSR have to do the Pop in this again its efficiency is descreasing.

Is it mean if explicit null label is used then efficiency will decrease but Qos info will be preserved?

Awaiting for your answers .

Thanks,

3 REPLIES 3
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Cisco Employee

Hi,

Hi,

1) As per my understanding both implicit and explicit label is assigned by the last LSR(Egress LSR), correct me if I am wrong.

<Nagendra> Yes right. Depending on how the egress LSR is configured/instructed, it assigns and advertises label 3 (in case of implicit null) or label 0 for locally configured prefixes.

2)As per the above statement when implicit null is assigned and when next to last router(P router) get the info of implicit null it do PHP , while doing so it removes the EXP bits also , hence QOS information lost.

<Nagendra> Yes, but depending on what QoS model is used. Normally when a label is popped, the TTL and the EXP (BTW, TC is the right name after RFC 5462) value will be coped to the underlying label. But if it is IP (the popped label is the last label in the stack) and if the QoS model is short pipe, the egress will treat the packet based on IP Prec/DSCP. 

As per the MPLS label stack the label , EXP and TTL are the separate field.So why the QOS getting lost if P router do PHP , it should affect only label filed not the EXP field.Please en light me on this.

<Nagendra> As mentioned above.

3)When the explicit null is included then the label 0 is assigned by P router( confirm if label 0 assigned by p router or Egdge LSR),even though the label 0 is assigned in mpls header , Edge LSR have to do the Pop in this again its efficiency is descreasing.

<Nagendra> Label 0 is assigned and advertised by egress LSR. You cannot have the PHP LSR to use label 0 if egress LSR does not instruct it to send it with exp-null label.

HTH,

Nagendra

Hi Nagendra,

Hi Nagendra,

Thanks for replying.The information was quite helpful.But I did not get the below line explained by you.

But if it is IP (the popped label is the last label in the stack) and if the QoS model is short pipe, the egress will treat the packet based on IP Prec/DSCP. 

Cisco Employee

Hi,

Hi,

CE1----PE1---P1---P2---PE2---CE2

Assume CE1 is sending packet with PREC 5 marking to CE2. PE1 will copy the QoS marking to MPLS label(s) and all transit nodes (P1 and P2) will treat the packet based accordingly. Now if there is some policy on P1 that remarks the label EXP value to 3, P2 will treat the packet based on QoS value 3. If the packet is VPN, PE2 will receive it with label and so PE2 will also treat it with QoS 3.

But if the traffic is non VPN (PE1 pushes 1 label), PE1 will receive IP packet (with P2 popping the label) and so the way PE1 treats the packet, depends on what QoS model we use. In case of Uniform mode, P2 will remark the IP QoS from the label value. In this case, it remarks IP PREC to 3 (received on had 5). In case of Pipe mode, it wont rewrite the IP header (to avoid disturbing QoS on customer site). In the former mode, PE2 will receive IP packet with QoS 3 while in the latter mode, it receives with 5.

HTH,

Nagendra

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