The MPLS-OPS Archive

[Robert Raszuk <raszuk@cisco.com>]

John,

> How many bits does the RD field have?? :-o are they enuf to put an AS # in
> it? and make it globally unique?

8 bytes = 64 bits ... :).

   The RDs are structured so that every service provider can administer
   its own "numbering space" (i.e., can make its own assignments of
   RDs), without conflicting with the RD assignments made by any other
   service provider.  An RD consists of a two-byte type field, an
   administrator field, and an assigned number field.  The value of the
   type field determines the lengths of the other two fields, as well as
   the semantics of the administrator field.  The administrator field
   identifies an assigned number authority, and the assigned number
   field contains a number which has been assigned, by the identified
   authority, for a particular purpose.  For example, one could have an
   RD whose administrator field contains an Autonomous System number

Rosen & Rekhter              Informational                      [Page 9]
RFC 2547                     BGP/MPLS VPNs                    March 1999

   (ASN), and whose (4-byte) number field contains a number assigned by
   the SP to whom IANA has assigned that ASN.  RDs are given this
   structure in order to ensure that an SP which provides VPN backbone
   service can always create a unique RD when it needs to do so.
   However, the structuring provides no semantics. When BGP compares two
   such address prefixes, it ignores the structure entirely.

> so we still have a label stack, the LDP gives u the LSP to the end PE and
> the inner label to identify the VRF is passed by BGP?
> is this correct?

If you use LDP yes - if you use IP for getting to your remote PE - No.

> now, how do we generate the inner label?

Inner label (VPN label) is generated by BGP in any case.

> and in case we go across multiple service providers, how do we handle it in
> such scenarios?
> 
> as now we would have a "outer label", an "inner label" and put the remote
> AS' router loopback into your IGP? how exciting!

No need. If you use IP for transport of 2547 there is no outer label and
you don't need to put your remote AS's loopacks into any IGP. Most
implementations I know can handle mutli level recursion just fine.

R.

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