CDP Behavior When an Interface Has No IP Address

200-301 By islever Editorial Last updated

Configure and verify Layer 2 discovery protocols (Cisco Discovery Protocol and LLDP)

Short answer

B CDP still runs normally and reports the IP address of another interface on the adjacent device when its CDP interface has no address.

Question

In a CDP environment, what happens when the CDP interface on an adjacent device is configured without an IP address?
  1. CDP becomes inoperable on that neighbor
  2. CDP uses the IP address of another interface for that neighbor Correct Answer
  3. CDP operates normally, but it cannot provide IP address information for that neighbor
  4. CDP operates normally, but it cannot provide any information for that neighbor

Key Takeaways

The question tests whether CDP requires an IP address on the discovery interface itself, and the trap is trusting Packet Tracer output over Cisco's documented substitution behavior.

This page examines Cisco Discovery Protocol operation when the CDP-enabled interface of an adjacent device has no IP address configured. Cisco's own CDP configuration documentation states the device substitutes another interface's IP address, which makes option B the defensible answer despite lab simulations showing an empty field.

Common mistake: Option C is the most common wrong pick, because testers in Packet Tracer and physical labs see an empty IP field in 'show cdp neighbors detail' and assume CDP simply omits address information rather than borrowing another interface's address.

Expert Analysis

Why the Answer Is Correct

Cisco's CDP configuration guide states explicitly: "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." CDP is a Layer 2 multicast protocol that does not need an IP address to run, so the neighbor relationship stays fully functional while the address field is populated from a different interface on that device. This directly matches option B: normal CDP operation plus a substituted IP address for the neighbor. Gere and oooMooo both cite this exact documented sentence, and it is vendor-official doctrine for exam purposes.

Why the Other Options Are Wrong

Option A is wrong because CDP never depends on an IP address to function; it sends periodic multicast frames on the wire regardless of addressing. Option C describes what several testers observed in Packet Tracer, but it contradicts the published Cisco behavior and only reflects a simulator limitation rather than the documented protocol result. Option D is clearly false, since CDP still exchanges the device ID, port ID, platform, capabilities, and other TLVs even when no address is present on that interface. The distinction between C and B hinges on whether the address field is merely left blank or back-filled from another interface on the same device.

Community Comment Notes

Gere quoted the Cisco guide verbatim: "the IP address of another interface will be updated as IP address for the non-IP address interface," and oooMooo repeated that same citation to defend B. On the other side, ddban reported "I don't see how you guys say it is B" after testing in Packet Tracer, and MDK94 noted that in "packet tracer or on physical lab equipment" no IP address appears. Nicocisco confirmed the same after lab testing, saying "the switch don't see other interface of my router," while dropspablo countered that GNS3 testing does show the other interface address, suggesting a platform difference rather than a protocol change.

Official Reference

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/cdp/configuration/15-mt/cdp-15-mt-book/nm-cdp-discover.html

This link opens the official vendor documentation in a new tab.

Exam Strategy

When a vendor documentation quote is attached to a question, treat the published behavior as the exam-correct answer even if a simulator prints something different. Recognize that CDP is Layer 2 and always operates without IP addressing, so eliminate options claiming the protocol stops working.

Frequently Asked Questions

Why is C wrong if Packet Tracer shows an empty IP address field?

Packet Tracer simplifies CDP output, but Cisco's configuration guide says another interface's IP is substituted. The exam expects the documented behavior, not the simulator's display.

Does CDP stop working if the interface has no IP address?

No. CDP is a Layer 2 protocol that exchanges multicast frames without any IP addressing, so neighbor discovery continues normally regardless of interface IPs.

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Discussion 94 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 30 of 94

  1. ddban 👍 73
    I tested in Packet tracer and I don't see that CDP uses an IP address of another interface for the CDP neighbor, it just leaves it empty but still works as usual. I don't see how you guys say it is B, but the simulation on PT says otherwise. I'm going with C.
  2. Gere 👍 35
    The Right answer is B. 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. Reference: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/cdp/configuration/15-mt/cdp-15-mt-book/nm-cdp-discover.html
  3. Nicocisco 👍 10
    When tested in the lab, the behaviour described in the link does not occur
  4. Aval0n1 👍 9
    What is the another interface? The C is correct. CDP does not need IPs to works normally https://learningnetwork.cisco.com/s/question/0D53i00000Kt3Mp/how-does-cdp-work-without-l3-addresses
  5. Cristy 👍 9
    I think the right answers should be B
Show 25 more comments
  1. Brocolee 👍 7
    Dude, I got confused with you here. You said Packet Tracer isn't reliable and therefore you disagree with his reasoning..... But then you agree with his answer. Lol wtf?
  2. oooMooo 👍 6
    Read his link! "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." B is correct, as stated by Gere.
  3. dropspablo 👍 5
    According to 4aynick who tested it through GNS3 and the CISCO documentation, answer B is correct, perhaps in Packet Tracer there is indeed this limitation, but it should include the ip address of another interface in an example of a #show cdp neighbors detail command, which does not happen leaving the ip address field empty (as I tested), but in GNS3 it manages to acquire the ip address of another interface (as mentioned by 4aynick), as well as also mentions the CISCO documentation, in this case, taking this into account limitation of Packet Tracer, I agree with answer B “updates the ip address of another interface”. https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/cdp/configuration/15-mt/cdp-15-mt-book/nm-cdp-discover.html#:~:text=Restrictions%20for%20Using%20Cisco%20Discovery%20Protocol,-Cisco%20Discovery%20Protocol&text=Cisco%20Discovery%20Protocol%20is%20not,the%20non%2DIP%20address%20interface
  4. MDK94 👍 5
    This is a terrible question. In Cisco documentation it states that "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." But when you actually test this in packet tracer or on physical lab equipment (I tested on both) NO IP address will be shown for the other routers interfaces, not loopback not physical interface, nothing. Meaning that we cannot be sure what the actual answer is. I think the best hope is to go with the documentation and hope that it's the correct answer when in reality it isn't.
  5. Nicocisco 👍 5
    yeah tested in lab to, and the switch don't see other interface of my router
  6. tumajay 👍 4 Answered C
    It is not B. Why majority picked B is beyond my networking comprehension. Cdp is a layer 2 protocol but can get ip info of an interface IF it has. Not picking the IP of another interface. Why would it even do that? To achieve what? This one i avoid the majority vote
  7. ZUMY 👍 4
    B is correct! 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 addres
  8. daddydagoth 👍 4 Answered B
    Bruh, Cisco's own documentation says it's B then it's bloody B! Reference: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/cdp/configuration/15-mt/cdp-15-mt-book/nm-cdp-discover.html
  9. Bhrino 👍 4
    Packet tracer isn’t really reliable for something so in theory I believe it should be c
  10. 4aynick 👍 4 Answered B
    I was check it in gns3. Correct B
  11. Joshua25 👍 3 Answered C
    Definitely C. Verified on real Cisco hardware - a L3 switch and a router. SW1 port #5: 10.0.0.1 R1 port #1: no ip addr port #3: 172.16.1.1 SW1#5 connected to R1 #1 SW1# show cdp neighbors detail Information about R1 is displayed, without any IP address Particularly, 172.16.1.1 is not displayed at all. R1#show cdp neighbors detail It displays SW1 information with IP address 10.0.0.1
  12. MaxaMillion 👍 3
    Answer is C. CDP Works on Layer 2 the Data Link layer and does not care if there is an IP configured or not. It will only list the IP address with show cdp neighbors detail command or show cdp interfaces or show cdp interface f0/0. The IP address field will be blank if no IP is assigned.
  13. rivera82 👍 3 Answered C
    Although CDP is a Layer 2 protocol but we can check the neighbor IP address with the “show cdp neighbor detail” command. If the neighbor does not has an IP address then CDP still operates without any problem. But the IP address of that neighbor is not provided.
  14. youtri 👍 3
    the correct is B Restrictions for Using Cisco Discovery Protocol • 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.
  15. Mahmood4IPEng 👍 3
    The question says that the CDP "uses" the ip address of another interface and this is incorrect as CDP doesn't need IP address to work so option C is the correct answer
  16. 1stAid 👍 3 Answered C
    C. It will only show info of the interface it's connected to, not another interface info like answer B. Other interface might be connected to something else.
  17. elixirwell 👍 3 Answered B
    https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/cdp/configuration/15-mt/cdp-15-mt-book/nm-cdp-discover.htm
  18. daddydagoth 👍 3
    This is absoultely awful but true. This exam is a mess
  19. Marcos9410 👍 3 Answered B
    B is correct
  20. i_am_confused 👍 2 Answered C
    I just tried this in packet tracer with 2911 routers. When no ip address was configured, the neighbor router just showed a blank when issued the "show cdp neighbors detail" command. However, if an ip address is previously configured, and then a no ip address configured after to remove the ip address, the old ip address stayed when the neighbor used "show cdp neighbors detail". Yes I fast forwarded to make sure timers weren't the issue. At no point did the neighbor advertise another interface's ip address. I think the answer is C.
  21. Sant11 👍 2 Answered B
    Restrictions for Using Cisco Discovery Protocol Cisco Discovery Protocol functions only on Cisco devices. Cisco Discovery Protocol is not supported on Frame Relay multipoint subinterfaces. 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. https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/cdp/configuration/15-mt/cdp-15-mt-book/nm-cdp-discover.html#GUID-3C4F2637-ED36-4EC0-9CD5-B73C0CF9DBEC
  22. JulesAZ 👍 2 Answered B
    Thought it was C Researched and Read the question ....... its B...... 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
  23. ZUMY 👍 2
    Going with B Restrictions for Using Cisco Discovery Protocol Cisco Discovery Protocol functions only on Cisco devices. Cisco Discovery Protocol is not supported on Frame Relay multipoint subinterfaces. 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.
  24. AllyK 👍 2
    I came across this issue real life. I was having an issue with being unable to ssh into a switch. I used 'cdp neighbor detail' on a switch I knew was a neighbor. The output was the same as all the others, except there was no IP address. The correct answer is C.
  25. Dutch012 👍 2 Answered C
    the answer would be B, in the assumption that the adjacent device has another interface with an IP address, but the thing is the question didn't mention another IP interface for the adjacent device, so it's safer to choose C.

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