It may be rare to find an IOS that has its BGP's auto-summary command enabled by default, but I think it is very important to touch on this BGP quirky feature and its impact on how BGP injects routes from IP routing table to its BGP table and how it behaves differently when redistribution or network command is used. Let kick things off by a really simple network topology. Two routers, in the same AS 10 with iBGP neighbor relationship established between the two. R1 has 5 subnets that it will advertise to R2. I will start with no auto-summary configured first, since this is the default in the recent IOS versions. R1: interface Loopback10 ip address 10.10.10.1 255.255.255.0 !  interface Loopback20 ip address 10.10.20.1 255.255.255.0 !  interface Loopback30 ip address