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Forum Replies

  1. Rene,

    Great lesson however, I have a question about MPLS TTL =254. It means there are 254 hops from CE1 to CE2 for each hop TTL decreases by 1 is that correct? How do we determine or define TTL value?



  2. Hi Hamood,

    That's right, the TTL for the MPLS label works the same as the TTL for the IP packet.

    When the PE router receives an IP packet then it will copy the TTL of the IP packet into the MPLS label TTL. Within the MPLS Core, the TTL of the label decreases at each hop. When the MPLS label is popped, it's TTL value is copied into the TTL of the IP packet.


  3. Hi Rene,

    Hope you are doing great ......

    I have some questions ......

    1. What is the advantage Using PHP over Ultimate HOP popping . I am facing some confusion regarding why we will use PHP.
    2. Suppose 3 ldp Router connected serially .. R1+R2+R3 , R1 has a prefix that tag: imp-null and advertise to R2 . R2 also created Tag :16 for and advertise to R1, R3 . Also R3 created Tag:20 for and advertise to R2. So, R2 got those Tags for : 1. Tag :imp-null from R1, 2. Tag: 16 (own creation), 3. Tag:20 from R3 . So my questions is how R2 create LFIB where 3 tags available for same Prefix .
    3. When PE will receive regular IP packect ,then it will add Label, right ?My questions is why MPLS will kick in . It can forward packet based on IP . What is the lookup process behind this .
  4. Hi @Zaman.rubd

    Let's look at an example of PHP first. Take a look at this picture:

    In this example, P3 pops the label and forwards only the IP packet to PE2. The advantage here is that we saved a lookup for PE2. It only has to route the packet, not another lookup for the label.

    Without PHP, (that's Ultimate Hop Popping), the egress router (PE2) has to pop the label and do a lookup in the IP routing table.

    PHP helps to move some of the load (label lookup / pop) from the PE2 to the P3 router. Keep in mind the PE routers have more work to do than the P routers....

    About the LFIB, keep this picture in mind:

    R2 will have the IP address of R1 installed as the next hop for, that won't change:

    R2#show ip route
    Routing entry for
      Known via "ospf 1", distance 110, metric 2, type intra area
      Last update from on GigabitEthernet0/1, 00:03:17 ago
      Routing Descriptor Blocks:
      *, from, 00:03:17 ago, via GigabitEthernet0/1
          Route metric is 2, traffic share count is 1

    Here you can see the labels that were created/advertised:

    R2#show mpls ldp bindings     
      lib entry:, rev 16
            local binding:  label: 19
            remote binding: lsr:, label: imp-null
            remote binding: lsr:, label: 20

    And the LFIB:

    R2#show mpls forwarding-table 
    Local      Outgoing   Prefix           Bytes Label   Outgoing   Next Hop    
    Label      Label      or Tunnel Id     Switched      interface              
    16         Pop Label   0             Gi0/2
    17         Pop Label       0             Gi0/2
    18         Pop Label   0             Gi0/1
    19         Pop Label       0             Gi0/1


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