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Forum Replies

  1. Hi Kumaran,

    These have to do with PHP (Penultimate Hop Popping).

    Imagine you have a couple of routers like this:

    CE1 - PE1 - P1 - P2 - PE2 - CE2

    Let’s say CE1 sends an IP packet meant for CE2. What happens is that PE1 adds a label and then it gets label switched from PE1 to P1 > P2 > PE2.

    To save PE2 a label lookup, we use PHP. This means that P2 will remove the label before forwarding it to PE2. This will save PE2 a label lookup.

    P2 knows that it has to do PHP because PE2 will tell it to. This is done with the implicit NULL label which has a value of 3. This i

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  2. 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
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  3. 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

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  4. Hi Gareth,


    Each routing protocol has its own “RIB”. The OSPF LSDB can be called the OSPF RIB and the EIGRP topology table is the EIGRP RIB.

    The routing table can be considered the “main” RIB of the router.

    The FIB is your forwarding table, on Cisco routers, this is the CEF table. It doesn’t only contain L3 information like the RIB does but also L2 information (needed to reach the next hop).


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  5. Hello Chris

    In PHP, it is more appropriate to refer to the outer and inner label, although the top and bottom labels are also used as terminology. The top label is the outer and the bottom is the inner. So in PHP, it is the outer label that is removed by the Label Switch Router (LSR) before the packet is passed to the adjacent Label Edge Router (LER).

    In the case of MPLS VPN, a packet consists of two labels, the inner one is the VPN label and the outer one is the common MPLS label. So when PHP will be performed by an LSR it will remove only the outer label.

    ... Continue reading in our forum

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