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DHCP & DHCPv6 Questions

July 22nd, 2017 in ROUTE 300-101 Go to comments

Question 1

Explanation

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

Explanation

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

Explanation

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

Question 5

Explanation

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

Explanation

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

Explanation

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

Explanation

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

Explanation

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

Comments
  1. Guacca
    August 13th, 2017

    Q7.

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

  2. kofyi
    August 13th, 2017

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    August 13th, 2017

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  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

    Q8
    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
    exit

    In Clients Facing Interface

    ipv6 address
    ipv6 dhcp relay destination

    https://supportforums.cisco.com/t5/network-infrastructure-documents/stateful-dhcpv6-relay-configuration-example/ta-p/3149338

  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

    https://www.ipv6.com/general/ipv6-auto-configuration-vs-dhcpv6/network.

  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

    @Nickname

    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
    Source:www*speedguide*net/port.php?port=547

  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”

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