A routing system is much more complex than routing protocols within it
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A network routing system such as Internet routing  
typically is a distributed system consisting of not only 
routing protocols but also network configurations that 
enable desired operations of routing protocols. 
Furthermore, a network routing system may consist 
of multiple routing protocols in order to take advantage
of salient properties of different protocols. 
This can lead to a routing system whose behavior
is much more complicated than each individual routing
protocol involved. We illustrate this point with the
following two examples:


 * Routing protocols deployed in the Internet do not 
   have persistent forwarding loops. But the Internet 
   routing systems can. Suppose a stub network owns 
   address block 18.1.0.0/16, but uses only a few subnets
   (e.g., 18.1.1.0/24 and 18.1.2.0/24 only). Therefore, the 
   routers in the stub network have route only to the subnets.
   However, the provider of the stub network has a static route
   for traffic to 18.1.0.0/16 to the stub network, and the stub
   network has a default route to the provider. This will lead 
   to a persist forwarding loop for any traffic to address range
   from 18.1.3.0 to 18.1.255.255. 

 * BGP or IGP does not experience loss of connectivity when 
   an additional link becomes available. Yet, it is possible 
   to lose connectivity in the Internet when adding a link or
   a link recovers from failure. Suppose a destination with two
   providers 1 and 2. The link to provider 1 is recovered from 
   a failure. Provider 1 has three routers that are fully
   meshed with iBGP; Router 3 is connected to destination directly
   and Router 2 is connected to provider 2 directly. In this case, 
   Router 1 may loss the connectivity to the destination when 
   the link is recovered. This is because Router 3 announces the
   new route to Router 1 and Router 2 and Router 2 withdraws its
   old route from Router 1. However, Router 1 can get the withdrawal
   earlier than the announcement, thereby leading to temporary loss
   of connectivity to the destination. 


These specific examples illustrate lessons learned from the current 
routing systems, that is, how a routing system can behave
differently than the protocols involved. In designing future routing
systems, we should not only evaluate individual routing protocol 
but also the routing system as a whole.