Why do you need to RT’s for Routes in Same VRF?

Good evening,

 

   Why do you need to import/export RTs for sites that have same RD/VRF in order to allow them to communicate with eachother? For example, say that you have two customers, 1 and 2, and two sites per customer, A, B, C, and D. Each site has its own PE-CE connection that goes into the MPLS “core”.

 

Customer 1 has sites A and B with an RD of 1:1.

 

Customer 2 has sites C and D with an RD of 1:2.

 

My understanding is that in order to get sites A/B to communicate with each other, one would need to use RTs to import/export 1:1. The same would be true in order to get sites C/D to communicate with each other.

 

If sites A/B share the same RD, how come routes advertised by each site are not implicitly imported into the VRFs of their respective sites PE routers? For example, when site A advertises a route to its PE, and that PE advertises the route to the site B PE, without the RTs setup, the site B PE will not import the site A PE learned route even though the RD is the same as a locally configured VRF.

 

Are RDs of local significance only and RTs of global significance? That is the only thing I can come up with that makes sense for the aforementioned behavior.

 

 

Thank you for your input.

Comments

  • Hi,

    RD is just to allow the MPLS network to identify the prefixes within the MPLS cloud, with this, you can have different customers, for example, using the same private address space (RFC1918 addressing). The RD is appended into each route, forming the VPNV4 route.

    The RT, on the other hand, controls what get imported and exported. The logic behind RT is that its flexibility, allows you to offer different types of MPLS services, like - Central Services, Hub-and-Spoke, Intranet MPLS VPNs, Extranet MPLS VPNs, etc.

    Sometimes even though you have various customer sites using the same RD, you may want to control the route import/export to create a partial mesh instead of a full mesh. 

    HTH

    Good luck!

     

  • JoeMJoeM ✭✭✭

    Are RDs of local significance only and RTs of global significance? That is the only thing I can come up with that makes sense for the aforementioned behavior.

    This exactly.   RD is only a local identifier.

  • Yes and no. As long as IP prefixes are unique then the RD is arbitrary and locally significant. In practice this is not the case though.

    In reality RD has global significance to the MPLS cloud, as generally lots of customers will have the same overlapping routes (RFC 1918). 

    RD:prefix/len is what the VPNv4/v6 routers in the core make their BGP path selection on, so the combination of both must be unique to allow for separate path selection. 

    Brian McGahan, 4 x CCIE #8593 (R&S/SP/SC/DC), CCDE #2013::13
    [email protected]
     
    Internetwork Expert, Inc.
    http://www.INE.com

    On Oct 22, 2014, at 7:30 PM, "JoeM" <[email protected]> wrote:

    imageHiroProtagonist:
    Are RDs of local significance only and RTs of global significance? That is the only thing I can come up with that makes sense for the aforementioned behavior.

    This exactly.   RD is only a local identifier.




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  • JoeMJoeM ✭✭✭
  • Thank you all for your input and answers!

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