8.5.3 Module Quiz - SLAAC and DHCPv6 (Answers)

1. How does an IPv6 client ensure that it has a unique address after it configures its IPv6 address using the SLAAC allocation method?

It contacts the DHCPv6 server via a special formed ICMPv6 message.
It checks with the IPv6 address database that is hosted by the SLAAC server.
It sends an ARP message with the IPv6 address as the destination IPv6 address.
It sends an ICMPv6 Neighbor Solicitation message with the IPv6 address as the target IPv6 address.

Explanation: SLAAC is a stateless allocation method and does not use a DHCP server to manage the IPv6 addresses. When a host generates an IPv6 address, it must verify that it is unique. The host will send an ICMPv6 Neighbor Solicitation message with its own IPv6 address as the target. As long as no other device responds with a Neighbor Advertisement message, then the address is unique.

2. Which method would an IPv6-enabled host using SLAAC employ to learn the address of the default gateway?

neighbor advertisements that are eceived from link neighbors
router advertisements that are received from the link router
reply messages that are received from the DHCPv6 server
advertise messages that are received from the DHCPv6 server

Explanation: When using SLAAC, a host will learn from the router advertisement that is sent by the link router the address to use as a default gateway.

3. What two methods can be used to generate an interface ID by an IPv6 host that is using SLAAC? (Choose two.)

random generation
DAD
stateful DHCPv6
EUI-64
ARP

Explanation: A host that is using SLAAC has two means to configure an interface ID: EUI-64 and random generation by the host operating system.

4. A client is using SLAAC to obtain an IPv6 address for its interface. After an address has been generated and applied to the interface, what must the client do before it can begin to use this IPv6 address?

It must send a DHCPv6 INFORMATION-REQUEST message to request the address of the DNS server.
It must send a DHCPv6 REQUEST message to the DHCPv6 server to request permission to use this address.
It must send an ICMPv6 Router Solicitation message to determine what default gateway it should use.
It must send an ICMPv6 Neighbor Solicitation message to ensure that the address is not already in use on the network.

Explanation: Stateless DHCPv6 or stateful DHCPv6 uses a DHCP server, but Stateless Address Autoconfiguration (SLAAC) does not. A SLAAC client can automatically generate an address that is based on information from local routers via Router Advertisement (RA) messages. Once an address has been assigned to an interface via SLAAC, the client must ensure via Duplicate Address Detection (DAD) that the address is not already in use. It does this by sending out an ICMPv6 Neighbor Solicitation message and listening for a response. If a response is received, then it means that another device is already using this address.

5. Which command should be configured on a router interface to set the router as a stateful DHCPv6 client?

ipv6 enable
ipv6 address dhcp
ipv6 dhcp server stateful
ipv6 address autoconfigure

Explanation: When the ipv6 address dhcp command is configured on a router interface, it enables the router as a DHCPv6 client on this interface. The ipv6 enable command enables IPv6 on an interface and allows the router to configure its link-local address. The ipv6 address autoconfigure command tells the router to use either SLAAC or stateless DHCPv6 to configure its global unicast address. The ipv6 dhcp server command is used on a router that is running a DHCPv6 server to indicate what address information should be served to clients.

6. What message informs IPv6 enabled interfaces to use stateful DHCPv6 for obtaining an IPv6 address?

the ICMPv6 Router Advertisement
the DHCPv6 Reply message
the ICMPv6 Router Solicitation
the DHCPv6 Advertise message

Explanation: Before an IPv6 enabled interface will use stateful DHCPv6 to obtain an IPv6 address, the interface must receive an ICMPv6 Router Advertisement with the managed configuration flag (M flag) set to 1.

7. Which destination IP address is used when an IPv6 host sends a DHCPv6 SOLICIT message to locate a DHCPv6 server?

FE80::1
FF02::2
FF02::1:2
FF02::1

Explanation: DHCPv6 hosts will send a DHCP SOLICIT message to the all DHCP routers multicast address of FF02::1:2.

8. In which alternative to DHCPv6 does a router dynamically provide IPv6 configuration information to hosts?

SLAAC
EUI-64
ICMPv6
ARP

Explanation: Stateless Address Autoconfiguration (SLAAC) can be used as an alternative to DHCPv6. In this approach, a router provides global routing prefix, prefix length, default gateway, and DNS server information to a host. The host is not provided with a global unicast address by SLAAC. Instead, SLAAC suggests that the host create its own global unicast address based on the supplied global routing prefix. ARP is not used in IPv6. ICMPv6 messages are used by SLAAC to provide addressing and other configuration information. EUI-64 is a process in which a host will create an Interface ID from its 48-bit MAC address.

9. A company implements the stateless DHCPv6 method for configuring IPv6 addresses on employee workstations. After a workstation receives messages from multiple DHCPv6 servers to indicate their availability for DHCPv6 service, which message does it send to a server for configuration information?

DHCPv6 REQUEST
DHCPv6 INFORMATION-REQUEST
DHCPv6 ADVERTISE
DHCPv6 SOLICIT

Explanation: In stateless DHCPv6 configuration, a client configures its IPv6 address by using the prefix and prefix length in the RA message, combined with a self-generated interface ID. It then contacts a DHCPv6 server for additional configuration information via an INFORMATION-REQUEST message. The DHCPv6 SOLICIT message is used by a client to locate a DHCPv6 server. The DHCPv6 ADVERTISE message is used by DHCPv6 servers to indicate their availability for DHCPv6 service. The DHCPv6 REQUEST message is used by a client, in the stateful DHCPv6 configuration, to request ALL configuration information from a DHCPv6 server.

10. What process is used in ICMPv6 for a host to verify that an IPv6 address is unique before configuring it on an interface?

EUI-64
SLAAC
ARP
DAD

Explanation: Before an IPv6 host can enable and use an assigned IPv6 address, the host must verify that the address is unique on the network. To verify that no other hosts are using the IPv6 address, the host performs the duplicate address detection (DAD) process by sending a Neighbor Solicitation (NS) message to the IPv6 address.

11. What are two characteristics of the SLAAC method for IPv6 address configuration? (Choose two.)

Router solicitation messages are sent by the router to offer IPv6 addressing to clients.
The default gateway of an IPv6 client on a LAN will be the link-local address of the router interface attached to the LAN.
This stateful method of acquiring an IPv6 address requires at least one DHCPv6 server.
Clients send router advertisement messages to routers to request IPv6 addressing.
IPv6 addressing is dynamically assigned to clients through the use of ICMPv6.

Explanation: With SLAAC, the default gateway for IPv6 clients will be the link-local address of the router interface that is attached to the client LAN. The IPv6 addressing is dynamically assigned via the ICMPv6 protocol. SLAAC is a stateless method of acquiring an IPv6 address, a method that requires no servers. When a client is configured to obtain its addressing information automatically via SLAAC, the client sends a router solicitation message to the IPv6 all-routers multicast address FF02::2. The router advertisement messages are sent by routers to provide addressing information to clients.

12. After booting, a client receives an ICMPv6 RA message with the M flag set to 0 and the O flag set to 1. What does this indicate?

The client should request an IPv6 address directly from a DHCPv6 server.
The client should automatically configure an IPv6 address and then contact a DHCPv6 server for more information.
The client should automatically configure an IPv6 address without contacting a DHCPv6 server.
The client should be statically configured with an IPv6 address because the local router does not support autoconfiguration.

Explanation: The Managed Address Configuration (M) flag and the Other Configuration (O) flag in ICMPv6 RA messages are used to indicate to an IPv6 client how it should configure its IPv6 addresses. If the M flag is set to 0 it means that the host should automatically configure its own IPv6 interface address rather than asking for one from a DHCPv6 server. If the O flag is set to 1, it means that the client can find additional addressing information, such as a DNS server address, by contacting a DHCPv6 server after it has automatically configured its own address.

13. A network administrator is entering the command ipv6 unicast-routing to start configuring DHCPv6 operation on a router. Which statement describes the function of this command?

It is required for enabling DNS service in DHCPv6 configurations.
It is required for sending ICMPv6 RA messages.
It is required to configure stateless DHCPv6 server on the router.
It is required to configure stateful DHCPv6 server on the router.

Explanation: The ipv6 unicast-routing command is required to enable IPv6 routing on a router. This command is not necessary for the router to be a stateless or stateful DHCPv6 server, but is required for sending ICMPv6 RA messages.

14. A company uses the SLAAC method to configure IPv6 addresses for the employee workstations. Which address will a client use as its default gateway?

The all-routers multicast address
The global unicast address of the router interface that is attached to the network
The link-local address of the router interface that is attached to the network
The unique local address of the router interface that is attached to the network

Explanation: When a PC is configured to use the SLAAC method for configuring IPv6 addresses, it will use the prefix and prefix-length information that is contained in the RA message, combined with a 64-bit interface ID (obtained by using the EUI-64 process or by using a random number that is generated by the client operating system), to form an IPv6 address. It uses the link-local address of the router interface that is attached to the LAN segment as its IPv6 default gateway address.

15. A network administrator configures a router to send RA messages with the A flag and O flag set to 1. The M flag is set to 0. Which statement describes the effect of this configuration when a PC tries to configure its IPv6 address?

It should contact a DHCPv6 server for all the information that it needs.
It should contact a DHCPv6 server for the prefix, the prefix-length information, and an interface ID that is both random and unique.
It should use the information that is contained in the RA message and contact a DHCPv6 server for additional information.
It should use the information that is contained in the RA message exclusively.

Explanation: ICMPv6 RA messages contain flags to indicate whether a workstation should use SLAAC, a DHCPv6 server, or a combination to configure its IPv6 address. The A flag determines whether to use SLAAC. The O flag indicates whether to use a stateless DHCPv6 server. The M flag indicates whether to use stateful DHCPv6. The M and O flags are independent of SLAAC.

16. An administrator wants to configure hosts to automatically assign IPv6 addresses to themselves by the use of Router Advertisement messages, but also to obtain the DNS server address from a DHCPv6 server. Which address assignment method should be configured?

RA and EUI-64
SLAAC
Stateful DHCPv6
SLAAC and stateless DHCPv6

Explanation: SLAAC and stateless DHCPv6 enable clients to use ICMPv6 Router Advertisement (RA) messages to automatically assign IPv6 addresses to themselves, and also allow these clients to contact a stateless DHCPv6 server to obtain additional information, such as the domain name and address of DNS servers. Because the M flag is 0 by default, stateful DHCPv6 will not be used. RA messages are used to automatically create an interface IPv6 address.

17. What is used in the EUI-64 process to create an IPv6 interface ID on an IPv6 enabled interface?

A randomly generated 64-bit hexadecimal address
An IPv4 address that is configured on the interface
An IPv6 address that is provided by a DHCPv6 server
The MAC address of an Ethernet interface

Explanation: The EUI-64 process uses the MAC address of an Ethernet interface to construct an interface ID (IID). Because the MAC address is only 48 bits in length, 16 additional bits (FF:FE) must be added to the MAC address to create the full 64-bit interface ID. The 7th bit is flipped, which modifies the second hex digit of the interface id.

18. A network administrator is implementing DHCPv6 for the company. The administrator configures a router to send RA messages with M flag as 1 by using the ipv6 nd managed-config-flag interface command, and the A flag is set to 0 using the ipv6 nd prefix default no-autoconfig command. What effect will this configuration have on the operation of the clients?

Clients must use all configuration information that is provided by a DHCPv6 server.
Clients must use the information that is contained in RA messages.
Clients must use the prefix and prefix length that are provided by a DHCPv6 server and generate a random interface ID.
Clients must use the prefix and prefix length that are provided by RA messages and obtain additional information from a DHCPv6 server.

Explanation: Under stateful DHCPv6 configuration, which is indicated by setting M flag as 1 (through the ipv6 nd managed-config-flag interface command), the dynamic IPv6 address assignments are managed by the DHCPv6 server. Clients must obtain all configuration information from a DHCPv6 server. The A flag determines whether to use SLAAC.

19. An organization requires that LAN clients generate their IPv6 configuration using SLAAC. You have configured the IPv6 GUA on the router LAN interface and verified that the interface is UP. However, hosts are not generating an IPv6 GUA. Which other command should be configured to enable SLAAC?

R1(config)# ipv6 dhcp pool pool-name
R1(config)# ipv6 unicast-routing
R1(config-if)# ipv6 enable
R1(config-if)# ipv6 nd other-config-flag

Explanation: For a router to be able to send RA messages, it must be enabled as an IPv6 router using the ipv6 unicast-routing global config command.

20. A network administrator configures a router to send RA messages with M flag as 0 and O flag as 1. Which statement describes the effect of this configuration when a PC tries to configure its IPv6 address?

It should contact a DHCPv6 server for all the information that it needs.
It should contact a DHCPv6 server for the prefix, the prefix-length information, and an interface ID that is both random and unique.
It should use the information that is contained in the RA message and contact a DHCPv6 server for additional information.
It should use the information that is contained in the RA message exclusively.

Explanation: When the A flag is set to 1 (default) the client will use SLAAC to configure its GUA address. When M flag is 0 and O flag is 1, a client will look for other configuration parameters (such as DNS server addresses) from a stateless DHCPv6 server.

21. When SLAAC is used, which address will a client use as its default gateway?

The connecting router interface GUA
The connecting router link-local address
The IPv6 all-nodes group multicast IPv6 address FF02::1
The IPv6 all-routers group multicast IPv6 address FF02::2

Explanation: Unless a device has been configured statically with a default gateway address, the device can only obtain its default gateway dynamically from the Router Advertisement message. The device will use the link-local address of the router interface, the source IPv6 address of the RA, that is attached to the LAN segment as its IPv6 default gateway address.