Two Reasons to Use CDP: Native VLAN and Layer 2 Verification
Short answer
CD Use CDP to read a directly connected switch's native VLAN and to confirm Layer 2 adjacency when IP connectivity is down.
Question
Key Takeaways
This item tests CDP's Layer 2, Cisco-proprietary neighbor-discovery role, especially native-VLAN visibility and Layer 2 adjacency proof; the trap is confusing a real CDP field, such as a neighbor IP, with one of the question's two intended uses.
Cisco Discovery Protocol (CDP) is a Layer 2 neighbor-discovery protocol; the two valid uses here are obtaining VLAN information from directly connected switches (C) and verifying Layer 2 connectivity when Layer 3 fails (D).
Common mistake: Choosing E because CDP detail can display a neighbor's IP address; that field exists, but using it to Telnet is not the intended CDP use in this question, and Telnet is an insecure management action.
Expert Analysis
What the stem is really asking
The question asks for two reasons a network administrator would use CDP, not for every field that can appear in show cdp neighbors detail. The word two forces a best-pair choice, and the phrase use CDP points to neighbor discovery at Layer 2 rather than general troubleshooting of services or routing. This is why several options sound like useful network tasks but are outside CDP's role.
Layer 2 scope eliminates A, B, and F CDP is a Cisco-proprietary Layer 2 discovery protocol. It does not inspect the physical cable type, so A is out; cable verification needs interface or transceiver diagnostics. It does not monitor remote network services, so B is out; that is a service-monitoring function. It does not report routing-protocol status, so F is out; CDP may show device capabilities, but it does not show OSPF or EIGRP neighbor state. Those three are attractive because CDP reveals platform and capability information, but none is a CDP use.
The defensible pair is C and D Option C fits because CDP advertisements from directly connected Cisco switches can expose VLAN-related details, most commonly the native VLAN, and this helps detect a VLAN mismatch. The discussion rightly cautions that CDP does not hand over a complete VLAN database, but the exam treats native-VLAN information as VLAN information. Option D fits because CDP runs at Layer 2, so a received neighbor advertisement can prove that frames are crossing the link even when IP or Layer 3 connectivity is broken. That is a classic CDP troubleshooting use.
Why E is the trap Option E is tempting: CDP detail can show an entry or management IP address, and discussion notes that a switch management address may be visible. However, the question asks for the intended two reasons to use CDP, and using that address to Telnet is not one of them; the option combines a real CDP field with an outdated management action. The advertised address may also be 0.0.0.0 or unreachable. The correct pair remains C and D.
Exam Strategy
On multi-select CCNA items, classify each option against the protocol's OSI layer and its documented payload before choosing. For CDP, ask whether the option describes a Layer 2 neighbor fact such as native VLAN, platform, or adjacency, or something CDP cannot see such as cable type, service status, or routing-protocol state. Treat plausible add-ons about Telnet as distractors unless the question explicitly asks about management access; the exam usually wants the two canonical uses.
Frequently Asked Questions
Does CDP provide a complete VLAN list from a neighboring switch?
Why isn't option E correct if CDP can show a neighbor's IP address?
How can CDP verify Layer 2 connectivity when Layer 3 fails?
Discussion 15 contributions
Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.
From public community threads 15
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Rether16 👍 7Never use Telnet! :-)
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wakaish 👍 6Two reasons a network administrator would use CDP (Cisco Discovery Protocol) are: C. To obtain VLAN information from directly connected switches: CDP can provide information about the directly connected switches, including their device type, platform, and VLAN information. This can be helpful for understanding the network topology and configuration. D. To verify Layer 2 connectivity between two devices when Layer 3 fails: CDP can be used to verify the physical and data link layer connectivity between two Cisco devices. When Layer 3 connectivity fails, CDP can help in identifying and diagnosing Layer 2 issues that may be causing the problem. The other options (A, B, E, F) do not directly relate to the typical use cases of CDP.
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Targaryen 👍 4C. to obtain VLAN information from directly connected switches - You can get the Native VLAN. D. to verify Layer 2 connectivity between two devices when Layer 3 fails - Can get information even without L3. E. to obtain the IP address of a connected device in order to telnet to the device - You can get the Management Address of a Switch. I guess D and E are the best options.
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JulesAZ 👍 3C - CDP allows network engineers to gather information about neighboring network devices, determining the type of hardware or equipment, software version, active interfaces the device is using (whether physical or VLAN), how they are configured, and other useful information D - CDP is a Layer 2, media-independent, and network-independent protocol that runs on Cisco devices and enables networking applications to learn about directly connected devices nearby2. If a neighbor has no IP address on an interface enabled with Cisco Discovery Protocol, the IP address of another interface will be updated as IP address for the non-IP address interface
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Pierre_Mon_Frere 👍 3When it comes to these types of multiple choice exams, avoid playing the "what if" game. Read the question, the whole question, and nothing but the question. Do not add to and do not take away. Telnet is not a secure protocol, but the question is not testing you on that, it is asking to find out if you know what information you can get using CDP. What you do with that information later, whether it works or not, is irrelevant to what information you got from CDP.
Show 10 more comments
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c97fbbc 👍 3CDP does not display other VLANs, just Native VLAN IDs of connected interfaces.
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Giuseppe_001 👍 2zumy aspettiamo la tua conferma
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ZUMY 👍 2Going with D & E
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Besxp 👍 1Why C and D? C. To obtain VLAN information from directly connected switches. ✅ CDP can advertise information such as the native VLAN on a connected switch. This helps administrators troubleshoot VLAN configuration mismatches. D. To verify Layer 2 connectivity between two devices when Layer 3 fails. ✅ CDP operates at Layer 2, so it can still discover a directly connected Cisco neighbor even when IP connectivity is broken. Why not E? E is also technically valid: CDP can advertise a neighbor's management IP address, which an administrator could use to attempt Telnet access. However, this question is commonly presented in CCNA practice exams with C and D as the expected answers. The wording of C is broad, but CDP can indeed provide certain VLAN-related information, particularly the native VLAN.
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RahmNn 👍 1C. to obtain VLAN information from directly connected switches. CDP can provide details about VLANs on directly connected devices, which helps in understanding the network topology and configuration. E. to obtain the IP address of a connected device in order to telnet to the device. CDP allows administrators to gather information about neighboring devices, including their IP addresses. This information can be useful for management purposes, such as remotely accessing the device via Telnet or SSH.
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Kirby87 👍 1C. Obtain VLAN Information CDP can reveal the native VLAN and VTP domain of directly connected Cisco switches. This helps administrators detect VLAN mismatches and troubleshoot segmentation issues. D. Verify Layer 2 Connectivity Because CDP operates at Layer 2, it works even when Layer 3 (IP) is down. This makes it a powerful tool for diagnosing physical and data link layer problems, especially when ping or traceroute fails.
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croftg1 👍 1What if telnet is blocked on that IP address. Would you allow telnet on a CDP advertised IP address? That sounds like bad security practice.
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croftg1 👍 1What if telnet and SSH is blocked on the CDP advertised IP address, and only allowed through a secure management VLAN?
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croftg1 👍 1Or perhaps in a tightly secured environment. Even then it is a risk.
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croftg1 👍 1It is sill important vlan information
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