Refer to the topology below and answer the following question.
Why has the Branch3 router lost connectivity with R1? Use only show commands to troubleshoot because usage of the debug command is restricted on the
Branch3 and R1 routers.
First we should check Branch3 (and R1) with the "show ip interface brief" command to find any Layer1/Layer2 issue.
We see interfaces connecting between them are in "up/down" states which indicates a Layer 2 issue so we should check the configuration of these interfaces carefully witch the "show running-config" command and pay attention to these interfaces.
and on Branch3:
We learn from above config is R1 is using CHAP to authenticate Branch3 router (via the "ppp authentication chap" command on R1). Branch3 router is sending
CHAP hostname "Branch_3" and CHAP password "Branch3_Secret!" to R1 to be authenticated. Therefore, we should check if R1 has already been configured with such username and password or not with the "show running-config" command on R1:
On R1 we see the configured username is "Branch3", not "Branch_3" so the usernames here are mismatched and this is the problem -> Answer A is correct.