What is Core MPLS LSP

[ayushmaanbhav]

Hi ,

Could any one explain me what is Core MPLS LSP (It seems that it do not support any TE, LDP or other protocol) ?

Thanks

[Vignesh Rajendran Praveen]

Hi Ayush,

The PE routers utilize label switching to switch packets across the backbone. The LSP is a specific path between PE routers on the MPLS core that the traffic will traverse. Traffic labeled in a specific way is forwarded along a specific LSPs.

MPLS core LSPs may have been manually provisioned, or may have been established by one of the other MPLS control plane mechanisms.

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Thanks & Regards,

Vignesh R P

[ayushmaanbhav]

Hi Vignesh,

     As it is mention Core MPLS, it understood that it do not use any latest protocol to manage the path or Traffic Engineering.

What I am not sure is how these LSP are created (if created manually) or it is just plain LDP Protocol which distribute the labels across each Hop (LSR) and based on which the packets moves to next destination.

I am bit confuse on the above concept, when we have pre defined protocols LDP, which help to generate Label for moving the packet to its destination, why we have to create / how we create LSP . How LSP overtake the labels defined by LDP at each Hop ?

[Nagendra Kumar Nainar]

Hi Ayush,

What I am not sure is how these LSP are created (if created manually) or it is just plain LDP Protocol which distribute the labels across each Hop (LSR) and based on which the packets moves to next destination.

Partially Yes .

LDP is one of the protocol used to build LSP. IGP like OSPF or ISIS build shortest path to a destination whcih can simply be thought as IP path. Without LDP or any otehr label signaling protocol, forwarding decision in all nodes will be based on IP header.

By using LDP, you exchange the label details between nodes and the forwarding decision will be based on label and not IP header. So this is known as LSP - Label switched Path.

While LDP already helps you with building LSP, it relies on shortest path and cannot take any explicit path.

Traffic Engineering helps build LSP to a destination via explicit path (non-shortest path) using RSVP.

HTH,

Nagendra

[ayushmaanbhav]

Hi Nagendra,

Thank you for reply.

If I understood correctly, then there will be only one Path (LSP) for each destination , if LDP alone is used to forwarding packets. And does this can be called as Core MPLS LSP ?

What are the other protocols which are similar to LDP ? Helping to generate Path automatically without giving any inputs.

Thanks

[Nagendra Kumar Nainar]

Hi,

If I understood correctly, then there will be only one Path (LSP) for each destination ,

In case of ECMP, you will see multiple path. Ideally with LDP, you will see one LSP per IP path.

               R3-------R4

               /                 \

R1---R2                   R6--R7

             \                  /

              R8--------R9

In the above topology, R1 will have one egress interface to reach R7. But R2 will hvae 2 egress interface (equal cost multi path) an can load balance between nexthops R3 and R8. So we can assume 2 LSPs between R1 to R7 as below,

LSP1 --> R1-R2-R3-R4-R6-R7

LSP2 -> R1-R2-R8-R9-R6-R7

if LDP alone is used to forwarding packets. And does this can be called as Core MPLS LSP ?

Normally yes. But Terminologies defers per person

You can simply refer it as LSP.

What are the other protocols which are similar to LDP ? Helping to generate Path automatically without giving any inputs.

Currently LDP and RSVP are the predominant protocol used for label signaling. There is a new concept coming as Segment Routing that will have IGP (OSPF and ISIS) itself to carry the labels and this eliminate the need of a separate label signaling protocol.

-Nagendra