Question on BGP-4 routing decision case

[Jon 'tex' Boone <[e-mail]>]

Folks,
  
  In the following network diagram, the two routers (router A and
router B) are both in AS 55.  They will both have PVC connections to
the vBNS through the ASX 200 ATM switch.  The interface on Router A
also has a PVC for a RFC 1577 network which Router B is *not*
connected to (it is a research network).

  Both Router A and Router B advertise the same four networks to vBNS:
128.91/16, 130.91/16, 158.130/16 and 165.123/16.  However, we would
like to have the vBNS router prefer to send traffic for 158.130/16
through Router A, while preferring to send traffic for the other three
networks through Router B [this is mainly due to politics -
Administrative control of Router A is done by a different department
than that of Router B.]

  Is it the case that the use of Multi-Exit Discriminator attributes
on each route can be used to enforce this policy?  I imagine that the
Routers would make the following announcements to the vBNS router:

Router    Network      MED
------    ----------   ---
  A       128.91/16     10
          130.91/16     10
          158.130/16     1
          165.123/16    10
  B       128.91/16      1
          130.91/16      1
          158.130/16    10
          165.123/16     1

Since these are Cisco routers, I imagine using route-maps to set the
MED attribute for each of these routes.  Is this a coherent idea?


                     ------------
                     | Router A |
                     ------------
                          |
                          |
  --------           -----------
  | vBNS |-----------| ASX 200 |
  --------           -----------
                          |
                          |
                     ------------
                     | Router B |
                     ------------



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Jon Boone                         Network Engineer
ISC Networking          University of Pennsylvania
[e-mail]                   [phone]