An EIGRP stub router advertises only the connected and summary routes by default and doesn't advertise the routes that it learns from its peers. However, they can be configured if we'd like it only receive routes or advertise a combination of the connected summary, already distributed routes.
The EIGRP stub area routing feature provides many advantages when implemented in a hub-and-spoke network as follows:
- It prevents suboptimal EIGRP routing from occurring within the network.
- This prevents stub routers with low-speed links from being used as transit routers within the hub-and-spoke network.
- It significantly limits the number of query packets and the depth of their propagation, resulting in faster convergence of the EIGRP network and avoiding the stuck-in-active (SIA) states.
A suboptimal routing would be if you have a look at the scenario, is when R4 routing packetes to 192.168.42.0/24 trough R7-R8-R6-R5 rather then going directly to R5, which would be the case if you do redistribution without any kind of filtering, modification.. etc as Ramon mentioned.
