The MPLS WG Archive

[Eric Rosen <erosen@cisco.com>]

In  the  "hub  and spoke"  case  that  Ajay  mentioned,  the intent  is  the
following. 

Let H be the  hub, S1 and S2 the spokes.  Let A be  the address prefix for a
subnet that attaches to S2.  What we want is:

- S1 thinks the route to A is via H. 

- H thinks the route to A is via S2. 

Thus traffic from S1 to A goes via H and then on to S2. 

To make this work, S2 must distribute a route RD1:A, and H must distribute a 
route RD2:A.   Both these  routes have to  be able  to pass through  a route
reflector, since  RD1:A must make  it to  H, and RD2:A  must make it  to S1.
This  implies that  RD1  must be  different  than RD2;  otherwise the  route
reflector could not pass on both routes. 

Given that S1,  S2, and H are in the  same VPN, this is a  case in which you
couldn't just replace the RD with a VPN-id.  There might also be other cases
in which  you want to pass around  two different routes to  the same address
prefix, using  policy to decide which  to install where  (presumably in some
manner that ensures loop-freedom).