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350-401 Practice Question: Is configuring a PPPoE client on a Cisco router…
A network engineer is configuring a PPPoE client on a Cisco router for a DSL connection. The engineer configures the dialer interface with the correct PPPoE profile and authentication credentials. The PPPoE session establishes, but the router cannot ping the ISP's gateway IP address. The engineer checks the routing table and sees that a default route is present via the dialer interface. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the distinction between a successful PPPoE session (Layer 2) and a fully functional IP layer (Layer 3), trapping candidates who assume a session establishment and a default route guarantee connectivity.
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
✓
The dialer interface does not have an IP address negotiated via IPCP.
In a PPPoE setup, the dialer interface typically obtains its IP address dynamically from the ISP via IPCP (IP Control Protocol) during PPP negotiation. If IPCP fails or is not configured to negotiate an address, the dialer interface remains unnumbered or without a usable IP address. Even though the PPPoE session is established and a default route exists, the router cannot route traffic because it lacks a valid source IP address on the dialer interface to send packets to the ISP's gateway.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The dialer interface does not have an IP address negotiated via IPCP.
Why this is correct
The presence of an IP address on the dialer interface is fundamental to PPPoE operation. During PPPoE, the ISP assigns an IPv4 address through IPCP (IP Control Protocol) as part of the PPP negotiation. If the dialer interface lacks an IP address, the router cannot build a usable routing table entry for the default route, and all traffic destined for the Internet is dropped because there is no valid source address. Without this IPCP-assigned address, even though the PPPoE session is established at Layer 2, the router has no IP reachability to the ISP gateway.
- ✗
The physical interface is configured with 'no ip address'.
Why it's wrong here
The physical interface in a PPPoE configuration is intentionally left without an IP address. This interface operates at Layer 2 only and is used to encapsulate PPPoE frames via a dialer pool, while the dialer interface carries all Layer 3 addressing. IPCP assigns the IP address to the dialer interface, not to the physical Ethernet port. Therefore, seeing 'no ip address' on the physical interface is both expected and correct, and it is not the cause of the connectivity failure.
- ✗
The default route is pointing to the wrong next-hop IP.
Why it's wrong here
A default route in a PPPoE environment is typically configured with the dialer interface as the egress, using the form 'ip route 0.0.0.0 0.0.0.0 dialer 0' rather than a concrete next-hop IP. If the default route points to a specific next-hop IP, that IP would normally be the ISP gateway IP assigned during IPCP. However, because the dialer interface lacks an IP address, IPCP has not provided that next-hop value, and even a correctly pointed route cannot function. Thus the issue is not the route's target, but the missing IPCP address that makes any route unusable.
- ✗
The ISP's gateway is not responding to ICMP.
Why it's wrong here
While an unresponsive gateway could cause symptoms like this, it is not the root cause when the PPPoE session is already up. A PPPoE session being established confirms that the Ethernet link, PPP authentication, and LCP negotiation have succeeded, but ICMP requires IP-level routing and addressing. If the dialer interface has no IP address, the router cannot generate an ICMP echo request, so the gateway's response (or lack thereof) is irrelevant. The gateway is not the problem; the router's own missing IP address prevents it from participating in IP communication.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-401 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-401 exam.