RTB interprets all subnets of 172.16.0.0 according to its misconfigured masks. The consequences, as exhibited in each of the four entries in its route table, are Entry 1: This entry is correct because 172.16.24.0 can be masked with either 22 bits or 23 bits. Entry 2: Subnet 172.16.26.0 is advertised by RTC. Because the 23rd bit of this address is one and RTB is using a 22-bit mask, from RTB's perspective this one appears in the host address space. Therefore, RTB interprets the advertisement of 172.16.26.0 as a host route and marks it with a 32-bit mask in the route table. Entry 3: RTB interprets its interface address 172.16.22.5 as being a member of subnet 172.16.20.0/22, instead of 172.16.22.0/23. When RTB receives RTA's advertisement of subnet 172.16.20.0/23, RTB, thinking it has a directly connected link to that subnet, ignores the advertisement. Notice that subnet 172.16.22.0 is not in the route tables of RTA and RTC for the same reason: RTB is advertising it as 172.16.20.0. Entry 4: RTB interprets its interface address 172.16.18.4 as being a member of subnet 172.16.16.0/22 instead of 172.16.18.0/23. |