The difficulty with the internetwork of Figure 11.37, when running classful routing protocols on RTB, is that network 172.16.0.0 is variably subnetted. The solution is to use a 28-bit mask on RTB's E1 interface while RTC still uses a 27-bit mask for the same subnet. This solution works because 172.16.1.96 remains distinct in the internetwork of Figure 11.37, whether it is subnetted with a 27-bit mask or a 28-bit mask.
This configuration changes RTB's E1 mask back to a 27-bit mask. However, RIP does not advertise subnet 172.16.1.144/28. To remedy this problem, a static route is entered to that subnet but with a 27-bit mask. Because the mask references RTB's E0 interface, it is redistributed into RIP automatically.