Cisco Certified Network Associate (CCNA) Practice Exam

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What must match on both routers when configuring CHAP authentication?

Username
IP Address
Password
Serial Interface

The correct answer is: Password

CHAP (Challenge Handshake Authentication Protocol) authentication relies on a shared secret to establish the identity of a device attempting to connect over a network. In this context, the shared secret is what is commonly referred to as the password. When configuring CHAP authentication, both routers must have the same password configured. This is because CHAP works by having the connecting router send a hash of the password combined with a unique challenge from the other router. If both routers are using the same password, they will generate the same hash value and be able to verify each other's identity successfully. Matching usernames does not play a role in CHAP authentication, as the authentication process is based solely on the password. IP addresses and specific interface names, such as serial interfaces, do not need to match for CHAP to function, as the authentication relies on the shared secret rather than on the configuration of interfaces or the addresses assigned to them. Therefore, the correct mechanism ensuring that CHAP functions properly and securely is indeed the matching of the password on both routers.