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As far as I know, in L3VPNs which are implemented on top of MPLS-TE - traffic is routed automatically via the tunnel, since the route's NH information is learned via MP-BGP and this NH is also the head end of the tunnel - hence the VPN traffic will flow automatically via the tunnel(correct me if I'm wrong).

Are there some scenarios in which one has to manually route traffic via MPLS-TE tunnels? or use some other methods to achieve this ?

I heard about IGP shortcut, and I'm not sure what it does and what are the use cases of it in regard to MPLS-TE tunnels.

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As far as I know, in L3VPNs which are implemented on top of MPLS-TE

MPLS-TE and MPLS L3VPN are two independent features and L3VPN does not require the use of MPLS-TE. Some companies use MPLS-TE to achive specific routing or failover qualities for their L3VPNs, but MPLS-TE is only related to MPLS L3VPN by the fact that it uses MPLS labels.

Are there some scenarios in which one has to manually route traffic via MPLS-TE tunnels?

Yes, if you're not enabling certain MPLS-TE features such as forwarding adjacencies, then you may simply be static-routing traffic into the tunnel. YMMV, this is part of engineering an MPLS-TE service; no two MPLS-TE networks have to be alike.

I heard about IGP shortcut, and I'm not sure what it does and what are the use cases of it in regard to MPLS-TE tunnels.

forwarding adjacencies are an example of an MPLS-TE IGP shortcut. IGP shortcut refers to a mechanism to make the MPLS-TE tunnel appear as a "link" in your IGP. By default, MPLS-TE is an overlay on top the default IGP path (that does not imply that MPLS-TE LSPs must take the same path as the IGP traffic though).

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  • Thanks for your explanation! but , I still don't quite get it. how is this "link" represented in IGP? what is the ip prefix it represents? does it somehow represent all IP addresses beyond the tunnel ? or just the tunnel tail-end's prefix ?
    – manish ma
    Jun 29 '18 at 15:04
  • The IGP doesn't form a direct adjacency over the forwarding adjacency; however, it appears in the IGP database (as well as all subnets "downstream" of this forwarding adjacency, assuming the MPLS-TE tunnel is chosen by SPF as the best-path)
    – This
    Jun 29 '18 at 15:13

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