# A Survey on Efforts to Evolve the Control Plane of Inter-Domain Routing
## 4 Issues in the Evolution of the Inter-Domain Routing Control Plane
### Complexity Introduced by the Distributed Configuration  
The traditional approach for the inter-domain routing is through a fully-distributed path selection computation in IP routers that limits the capacity of individual ASs in terms of the management scalability of path selection [38]. A network configuration that changes frequently, to suit the business and organization's desires, is a counterproductive challenge.  
Thus, the big challenge of distributed configuration concerns the complexity of keeping a consistent network policy through all routers. For example, Figure 2 presents the problem of applying the following policy: allow the learning routes advertised by ISP_B to be installed into the customer network routers whilst not allowing those routes to be re-advertised to ISP_A.  
One way to implement this policy is to use communities of the BGP. Then, suppose that a network operator installs the community value of 1000 to routes learned from ISP_B, and for every board router (in the case of Figure 2, just R1), configure an outbound filter to deny all prefixes with the community value of 1000 (indicating that those prefixes come from ISP_B). In Figure 2a, routers R1 and R2 have the correct values of the BGP Community. However, when the number of board routers is several and there is a high frequency of changes/updates in routing policies, a distributed configuration may be a bad idea for the control plane of the routing system due to it being hard to trace incorrect configurations that violate the routing policies. Figure 2b presents a scenario where the given routing policy was violated because of the incorrect value of the BGP community used in R1 (1001 instead of 1000), which caused a route leak of the ISP_B prefix to reach ISP_A.  
Figure 2: Example of the distributed configuration for the deployment of a routing policy. (**a**) Distributed configuration using the _Communities_ of the BGP; (**b**) different values cause the violation of the routing policy (route leak).  
The complexity of managing the configuration in distributed devices to make them operate with the acceptable network behavior can be a challenging task. Additionally, network management is a complex activity due to it requiring the enforcement of the high-level administration network policies in the network.  
To impose those requirements, it is necessary to individually visit each network device and perform low-level instructions/commands (often vendor-specific) on them. Depending on the number of network devices, carrying out such procedures is slow and error-prone [39], and a local network error can affect all the other networks. For example, for the famous case of the Pakistan Telecom incident BGP misconfiguration, in which the access to YouTube in that country was restricted, the company advertised an unauthorized prefix causing many ASs to lose access to the site. Hence, the complexity introduced by a distributed configuration can affect not only the specific network where a misconfiguration occurs, but also the whole inter-domain routing environment.