Hello Kannan,
the router should check if the proposed prefix address is in accordance with the proposed subnet mask/prefix length
A subnet mask of 255.255.255.240.0 means a prefix length of 20 -> 12 host bits from rightmost less significant bit
The prefix base address must have all host bits set to 0, that is last 12 bits of the address must be zero.
If you look at the two proposed subnet addresses
10.122.171.0 and 10.122.170.0 you see that in the third octect the first violates the rule because there multiple bits set to 1 in 171 but also the second one violates the rule!
171 in binary is
1010||1011
170 in binary is
1010||1010
note : with || I'm indicating the network part to host part boundary that is in the middle of the third byte (the interesting octect)
the prefix base address is 10.122.160.0/20 and so also 10.122.170/20 should be rejected by router CLI.
This looks like to be a bug in my opinion. Are you sure you have typed 10.122.170.0 in second attempt?
Hope to help
Giuseppe
