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DHCP & DHCPv6 Questions

July 22nd, 2019 in ROUTE 300-101 Go to comments

Question 1


DHCP options 3, 66, and 150 are used to configure Cisco IP Phones. Cisco IP Phones download their configuration from a TFTP server. When a Cisco IP Phone starts, if it does not have both the IP address and TFTP server IP address preconfigured, it sends a request with option 150 or 66 to the DHCP server to obtain this information.
+ DHCP option 150 provides the IP addresses of a list of TFTP servers.
+ DHCP option 66 gives the IP address or the hostname of a single TFTP server.

Reference: http://www.cisco.com/c/en/us/td/docs/security/asa/asa84/configuration/guide/asa_84_cli_config/basic_dhcp.pdf

Question 2


Most vendor’s routers/switches have the ability to function as:
+ A DHCP client and obtain an interface IPv4 address from an upstream DHCP service
+ A DHCP relay and forward UDP DHCP messages from clients on a LAN to and from a DHCP server
+ A DHCP server whereby the router/switch services DHCP requests directly

Question 3


Extended Unique Identifier (EUI) allows a host to assign itself a unique 64-Bit IPv6 interface identifier (EUI-64). This feature is a key benefit over IPv4 as it eliminates the need of manual configuration or DHCP as in the world of IPv4. The IPv6 EUI-64 format address is obtained through the 48-bit MAC address. The MAC address is first separated into two 24-bits, with one being OUI (Organizationally Unique Identifier) and the other being NIC specific. The 16-bit 0xFFFE is then inserted between these two 24-bits for the 64-bit EUI address. IEEE has chosen FFFE as a reserved value which can only appear in EUI-64 generated from the an EUI-48 MAC address.

Question 4


Please notice that the “ipv6 address autoconfig” is configured on the DHCP Relay Agent (not DHCP Server). A configuration example can be found at https://community.cisco.com/t5/networking-documents/stateful-dhcpv6-relay-configuration-example/ta-p/3149338

Question 5


A DHCPv6 configuration information pool is a named entity that includes information about available configuration parameters and policies that control assignment of the parameters to clients from the pool. A pool is configured independently of the DHCPv6 service and is associated with the DHCPv6 service through the command-line interface (CLI).
Each configuration pool can contain the following configuration parameters and operational information:
Prefix delegation information, which could include:
+ A prefix pool name and associated preferred and valid lifetimes
+ A list of available prefixes for a particular client and associated preferred and valid lifetimes
– A list of IPv6 addresses of DNS servers
– A domain search list, which is a string containing domain names for DNS resolution

Reference: http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipv6/configuration/15-2mt/ipv6-15-2mt-book/ip6-dhcp.html

This is how to configure a DHCPv6 pool:

ipv6 unciast-routing
ipv6 dhcp pool <pool name>
address prefix <specify address prefix> lifetime <infinite> <infinite>
dns-server <specify the dns server address>
domain-name <specify the domain name>

For example:

ipv6 dhcp pool test
address prefix 2010:AA01:10::/64 lifetime infinite infinite
dns-server AAAA:BBBB:10FE:100::15
dns-server 2010:AA01::15
domain-name example.com

Reference: https://supportforums.cisco.com/document/116221/part-1-implementing-dhcpv6-stateful-dhcpv6

So we can see DHCPv6 pool supports address prefix and domain search list, DNS servers.

Question 6


Note: A DHCPv6 relay agent is used to relay (forward) messages between the DHCPv6 client and server.

Servers and relay agents listen for DHCP messages on UDP port 547 so if a DHCPv6 relay agent cannot receive DHCP messages (because of port 547 is blocked) then the routers (clients) will not obtain DHCPv6 prefixes.

We are not sure about answer D but maybe it is related to the (absence of) “Reload Persistent Interface ID” in DHCPv6 Relay Options. This feature makes the interface ID option persistent. The interface ID is used by relay agents to decide which interface should be used to forward a RELAY-REPLY packet. A persistent interface-ID option will not change if the router acting as a relay agent goes offline during a reload or a power outage. When the router acting as a relay agent returns online, it is possible that changes to the internal interface index of the relay agent may have occurred in certain scenarios (such as, when the relay agent reboots and the number of interfaces in the interface index changes, or when the relay agents boot up and has more virtual interfaces than it did before the reboot). This feature prevents such scenarios from causing any problems.

Reference: http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipaddr_dhcp/configuration/15-e/dhcp-15-e-book/dhcp-15e-book_chapter_010.html

Question 7


In this topology DSW1 is the DHCPv6 Relay agent so it should relay (forward) the DHCPv6 Request packets (from the clients) out of its Gi1/2 interface to the DHCPv6 server. The command “ipv6 dhcp relay destination …” is used to complete this task.

Note: There is no “default-router” command for DHCPv6. The “ipv6 dhcp relay destination” is not required to configure on every router along the path between the client and server. It is ONLY required on the router functioning as the DHCPv6 relay agent.

Reference: http://www.cisco.com/c/en/us/products/collateral/ios-nx-os-software/enterprise-ipv6-solution/whitepaper_c11-689821.html

Question 8


The “ip helper-address” command is only configured in interface mode so it is not the correct answer.

Note: The Cisco IOS software provides the global configuration command “ip forward-protocol” to allow an administrator to forward any UDP port in addition to the eight default UDP Services. For example, to forward UDP on port 517, use the global configuration command “ip forward-protocol udp 517”. But the eight default UDP Services include DHCP services so it is not the suitable answer.

Reference and good resource: http://www.ciscopress.com/articles/article.asp?p=330807&seqNum=9

A DHCP relay agent may receive a message from another DHCP relay agent that already contains relay information. By default, the relay information from the previous relay agent is replaced. If this behavior is not suitable for your network, you can use the ip dhcp relay information policy {drop | keep | replace} global configuration command to change it -> Therefore this is the correct answer.

Reference: https://www.cisco.com/en/US/docs/ios/12_4t/ip_addr/configuration/guide/htdhcpre.html

Question 9


If the DHCP Server is not on the same subnet with the DHCP Client, we need to configure the router on the DHCP client side to act as a DHCP Relay Agent so that it can forward DHCP messages between the DHCP Client & DHCP Server. To make a router a DHCP Relay Agent, simply put the “ip helper-address <IP-address-of-DHCP-Server>” command under the interface that receives the DHCP messages from the DHCP Client.

Question 10

  1. Guacca
    August 13th, 2017


    Why would clients on ALS2 already be receiving v6 addresses before putting in the relay destination command?

  2. kofyi
    August 13th, 2017

    kindly help, i need current dumps for exam next week-route 300-101 to {email not allowed}

  3. kofyi
    August 13th, 2017

    can some body help me where i can get dump to download {email not allowed}

  4. cass
    August 26th, 2017

    I think that comand “ipv6 dhcp relay destination 2002:404:404::404:404 GigabitEthernet1/2” must be under interface fa0/0 configuration mode.

  5. Correction
    September 23rd, 2017

    Question 3

    After testing various dynamic IPv6 address assignment methods, an engineer decides that more control is needed when distributing addresses to clients. Which two advantages does DHCPv6 have over EUI-64 (Choose two)
    A. DHCPv6 requires less planning and configuration than EUI-64 requires.
    B. DHCPv6 allows for additional parameters to be sent to the client, such as the domain name and DNS server.
    C. DHCPv6 providers tighter control over the IPv6 addresses that are distributed to clients.
    D. DHCPv6 does not require the configuration of prefix pools.
    E. DHCPv6 does not require neighbor and router discovery on the network segment.

    A and B are correct not C

  6. aaki
    September 25th, 2017

    A packet capture indicates that the router is not forwarding the DHCP packets that it receives on interface FastEthernet0/0. Which command needs to be entered in global configuration mode to resolve this issue?
    A. ip helper-address
    B. ip DHCP relay
    C. service DHCP
    D. ip forward-protocol

    Please help me to best choice. A or B?

  7. cl
    September 27th, 2017

    correct answer is C: service DHCP
    Explanation in very simple: dhcp server and dhcp relay functions work only if dhcp service is enabled (which is enabled by default but can be disabled with “no service dhcp”).

    Check this link https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipapp/command/iap-cr-book/iap-i1.html#wp1413119578 section ip helper-address, right before Examples:
    “The service dhcp command must be configured on the router to enable IP helper statements to work with DHCP. If the command is not configured, the DHCP packets will not be relayed through the IP helper statements. The service dhcp command is configured by default. ”

    Answer A is INCORRECT since it is a interface command (not global config mode as required).
    Answer B is INCORRECT because the command is used only for the (optional) DHCP option 82 which is mainly used for DHCP snooping security feature, which is not specified in our case.
    Answer D has some appeal to it as with “ip forwarding protocol udp portnumber” one can set which udp ports to be forwarded (but DHCP udp port 67,68 are forwarded by default). Answer D is less desirable than C.

    Hope this helps!

  8. learner
    October 13th, 2017

    Question 4
    Answer: A

    In Interface Configuration Mode
    In DHCPv6 server Facing Interface:

    ipv6 address autoconfig
    ipv6 enable

    In Clients Facing Interface

    ipv6 address
    ipv6 dhcp relay destination


  9. learner
    October 13th, 2017

    Question 3

    After testing various dynamic IPv6 address assignment methods, an engineer decides that more control is needed when distributing addresses to clients. Which two advantages does DHCPv6 have over EUI-64 (Choose two)
    A. DHCPv6 requires less planning and configuration than EUI-64 requires.
    B. DHCPv6 allows for additional parameters to be sent to the client, such as the domain name and DNS server.
    C. DHCPv6 providers tighter control over the IPv6 addresses that are distributed to clients.
    D. DHCPv6 does not require the configuration of prefix pools.
    E. DHCPv6 does not require neighbor and router discovery on the network segment.

    Answers: B, C

    The IPv6 Auto configuration versus DHCPv6 is a hotly debatable contemporary issue in the networking domain since both the standards are being simultaneously used in conjunction with each other. While DHCPv6 offers dedicated configuration mechanism catering to all the information needs in the form of required parameters to the network devices, IPv6 auto-configuration simplifies the configuration process in a streamlined manner.

    While DHCPv6 offers a more comprehensible solution to the configuration needs of a device over an IPv6 network, the auto-configuration feature makes the whole process much more simpler, streamlined and future-proof.

    At present, the auto-configuration feature doesn’t offer much beyond IP addressing but the feature is hardwired into the IPv6 protocol and does away with the need of using any other standard leading to streamlining of the configuration process thereby removing any scope for future compatibility issues among different protocols.

    DHCPv6 is an excellent short-term solution while IPv6 auto-configuration, in an evolved form, is in for long haul. While at present we see a majority of network administrators swearing by the benefits of DHCPv6, the auto-configuration feature ingrained in IPv6 feature will soon outweigh the advantages offered by DHCPv6 to become the de facto standard for the configuration of devices over an IPv6


  10. nickname
    October 17th, 2017

    @digitaltut Is the scenario on Question 6 correct? It mentions that TCP traffic on port 547 is blocked but DHCPv6 runs over UDP, not TCP. Blocking TCP shouldn’t have any negative impact to the DHCPv6 process.

  11. unreal
    October 25th, 2017


    Port(s) Protocol Service Details Source
    547 tcp,udp DHCP DHCP(v6) Server SG
    547 tcp,udp DHCPv6 server (official) Wikipedia
    547 tcp,udp dhcpv6-server DHCPv6 Server IANA
    547 udp dhcpv6-server DHCPv6 Server Nmap

  12. Swampy
    November 1st, 2017

    @Guacca I thought the same as you.

    Does anyone know the answer to Guacca’s question:

    Why would clients on ALS2 already be receiving v6 addresses before putting in the relay destination command?

  13. Paco
    November 2nd, 2017

    Q8 @cl is 100% right, right answer is “service DHCP”

  14. buttmunch
    December 19th, 2017

    Q8, answers a and b are interface commands so they are wrong.
    answers C and D will both work. how is it possible to decide between them?
    C assumes that ip helper-address was already entered (in interface mode) but perhaps dhcp service was disabled.
    D assumes nothing but forces you to specify the specific udp ports udp/67 and udp/68.

    How does this question separate a skilled router technician from an unskilled one?

  15. me
    February 28th, 2018

    Consider this scenario. TCP traffic is blocked on port 547 between a DHCPv6 relay agent and a DHCPv6 server that is configured for prefix delegation. Which two outcomes will result when the relay agent is rebooted? (Choose two)
    A. Routers will not obtain DHCPv6 prefixes.
    B. DHCPv6 clients will be unreachable.
    C. Hosts will not obtain DHCPv6 addresses.
    D. The DHCPv6 relay agent will resume distributing addresses.
    E. DHCPv6 address conflicts will occur on downstream clients

    I would think A and C?
    It says WHEN the server is rebooted, not AFTER the server is rebooted. Also the relay agent is not responsible for distributing ip addressess – a dhcp server is. But if the relay agent is restarted then C, hosts in a different subnet, would not be able to reach the DHCP server to get an IP

  16. Marcus
    April 11th, 2018

    I totally don’t understand Q6. DHCPv6 uses UDP/547, not TCP. The blocking of TCP/547 shouldn’t have any aftermaths for DHCPv6.

  17. Anonymous
    April 27th, 2018
  18. KA
    May 5th, 2018

    I agree … so blocking TCP547 would have no affect… so what does anyone suggest the answer should be

  19. pacman
    May 8th, 2018

    Question 6.
    I disagree with the answer being D because if the Relay agent is not working then it won’t be able to distribute addresses.
    I believe the correct answer is C because the Relay agent won’t be able to send IPv6 addresses to the hosts.

    I therefore think the correct answer is:

    A. Routers will not obtain DHCPv6 prefixes.
    C. Hosts will not obtain DHCPv6 addresses.

  20. kingkong46566
    May 30th, 2018

    Latest 100% valid CCNP Exam questions


  21. Anonymous3000
    June 8th, 2018

    C is not correct. The DHCPv6 server is set up for PREFIX DELEGATION, it’s serving prefixes to routers, not addresses to hosts. The hosts will keep receiving adresses from the routers that had already got their prefixes before the traffic was blocked.

  22. ShutNoShut
    July 31st, 2018

    Hi Guys in Q6 A & C are correct and the only valid answers. The explanation above is inaccurate.

    dhcpv6-client 546/tcp DHCPv6 Client
    dhcpv6-client 546/udp DHCPv6 Client
    dhcpv6-server 547/tcp DHCPv6 Server
    dhcpv6-server 547/udp DHCPv6 Server

    Check the question again:
    “blocked on port 547 between a DHCPv6 relay agent and a DHCPv6 server that is configured for prefix delegation”

    so it’s pertaining to prefix delegation only and port 547 is where Server-Relay Agent listens and since its blocked, A is correct.

    And port 546 is being used by clients so Relay Agent still resumes distributing addresses.


  23. mad_nerdy
    August 25th, 2018

    hi guys,

    Q8 answer is C >>> Service DHCP

    Question is asking a command to be entered in “Global Configuration Mode”. So the only answer is “Service DHCP” as per Cisco article.

    1. When configuring DHCP relay on a router, verify that the ip helper-address command is located on the correct interface. The ip helper-address command must be present on the inbound interface of the DHCP client workstations and must be directed to the correct DHCP server.

    2. Verify that the global configuration command no service dhcp is not present. This configuration parameter will disable all DHCP server and relay functionality on the router. The default configuration, service dhcp, will not appear in the configuration, and is the default configuration command. If the service dhcp is not enabled, the clients do not receive the IP addresses from the DHCP server.


  24. _saiko
    October 29th, 2018

    Some new questions

    how do routers distribute prefixes obtained from ISPs using DHCPv6?
    A. PPPv6
    B. DHCP
    C. Stateful Natv6
    D. NPTv6
    E. IPv4

    Wth is stateful natv6?

  25. Sorlags
    November 14th, 2018


    I find information on this link https://serverfault.com/questions/880440/ipv6-how-does-my-isp-know-how-to-route-traffic-to-my-delegated-subnet

    But I don’t see the answer in your list. The answer can be DHCPv6-PD

  26. Simeli
    December 1st, 2018

    Q6, only A + C are valid combinations, everything else makes no sense. For sure, B & E is wrong, so only D would maybe possible. But if you take D, then Answer A or C makes no sense in combination because Router can not distribute ipv6 adresses if he doesn’t know the prefixes and it’s not possible that hosts don’t get a IP if relay agent resumse distributing. So if you have take to answers, only A + C makes sense.

  27. jose
    December 23rd, 2018

    Q8, Answer c.
    To enable the Cisco IOS Dynamic Host Configuration Protocol (DHCP) server and relay agent features on your router, use the “service dhcp” global configuration command.

  28. USS_Defiant
    June 19th, 2019

    What is the answer to this question:
    Which functions are included in the two-message rapid exchange that a DHCPv6 client can
    receive from a server?

  29. dynamik
    September 18th, 2019

    Q8: Answer is C.
    Seen this in the real world many times. The dhcp service must be enabled on the router, so ensure it is by using that global config command “service dhcp”

  30. tar
    December 13th, 2019

    Q:6 Correct Answer: AD
    The DHCPv6 use UDP protocol for distribution IPv6 addresses and prefixes. The routers dont need in the DHCPv6 prefixes from DHCPv6 server, its work for network administrator. DHCPv6 messages are exchanged over UDP port 546 and 547. Clients listen for DHCP messages on UDP port 546 while servers and relay agents listen for DHCP messages on UDP port 547.

  31. bluetailfly
    June 23rd, 2020

    why can’t ya speak proper English?

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