350-501 Security and Services Practice Question
A service provider is deploying a Broadband Network Gateway (BNG) for subscriber management. Which protocol is used by the BNG to authenticate subscribers via a RADIUS server?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
RADIUS
The BNG acts as a RADIUS client and uses RADIUS protocol to send authentication requests (Access-Request) for PPPoE or IPoE sessions.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
LDAP
Why it's wrong here
LDAP is not used for real-time authentication in BNG.
- ✗
Diameter
Why it's wrong here
Diameter is used in LTE/4G but not typically in BNG for fixed access.
- ✗
TACACS+
Why it's wrong here
TACACS+ is for device administration, not subscriber authentication.
- ✓
RADIUS
Why this is correct
RADIUS is used for subscriber authentication, authorization, and accounting.
Quick reference
AAA Protocol Comparison
| Protocol | Port(s) | Encryption | Transport | Primary Use |
|---|---|---|---|---|
| RADIUS | 1812 / 1813 | Password only | UDP | Network access control |
| TACACS+ | 49 | Full packet | TCP | Device administration |
| Diameter | 3868 | Full session | TCP / SCTP | Carrier / mobile networks |
| 802.1X | — | EAP-based | Layer 2 | Port-based access control |
TACACS+ encrypts the entire packet; RADIUS only encrypts the password field — a key exam distinction.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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