Module 8 Pre-Assessment

A company uses the method SLAAC to configure IPv6 addresses for the workstations of the employees. A network administrator configured the IPv6 address on the LAN interface of the router. The interface status is UP. However, the workstations on the LAN segment did not obtain the correct prefix and prefix length. What else should be configured on the router that is attached to the LAN segment for the workstations to obtain the information?​

• R1(config-if)# ipv6 enable

• R1(config)# ipv6 unicast-routing

• R1(config-if)# ipv6 nd other-config-flag

• R1(config)# ipv6 dhcp pool < name of the pool >

Answer explanation

A PC that is configured to use the SLAAC method obtains the IPv6 prefix and prefix length from a router. When the PC boots, it sends an RS message to inform the routers that it needs the information. A router sends an RA message that includes the required information. For a router to be able to send RA messages, it must be enabled as an IPv6 router by the unicast ipv6-routing command in global configuration mode. The other options are not used to enable IPv6 routing on a router.

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 use the information that is contained in the RA message exclusively.

• It should use the information that is contained in the RA message and contact a DHCPv6 server for additional information.

• It should contact a DHCPv6 server for the prefix, the prefix-length information, and an interface ID that is both random and unique.

Answer explanation

ICMPv6 RA messages contain two flags to indicate whether a workstation should use SLAAC, a DHCPv6 server, or a combination to configure its IPv6 address. These two flags are M flag and O flag. When both flags are 0 (by default), a client must only use the information in the RA message. When M flag is 0 and O flag is 1, a client should use the information in the RA message and look for the other configuration parameters (such as DNS server addresses) on DHCPv6 servers.

Refer to the exhibit. What should be done to allow PC-A to receive an IPv6 address from the DHCPv6 server?

• Add the ipv6 dhcp relay command to interface Fa0/0.

• Configure the ipv6 nd managed-config-flag command on interface Fa0/1.

• Change the ipv6 nd managed-config-flag command to ipv6 nd other-config-flag .

• Add the IPv6 address 2001:DB8:1234:5678::10/64 to the interface configuration of the DHCPv6 server.

Answer explanation

Client DHCPv6 messages are sent to a multicast address with link-local scope, which means that the messages will not be forwarded by routers. Because the client and server are on different subnets on different interfaces, the message will not reach the server. The router can be configured to relay the DHCPv6 messages from the client to the server by configuring the ipv6 dhcp relay command on the interface that is connected to the client.

Refer to the exhibit. PC-A is unable to receive an IPv6 address from the stateful DHCPv6 server. What is the problem?

• The ipv6 dhcp relay command should be applied to interface Gig0/0.

• The ipv6 nd managed-config-flag should be applied to interface Gig0/1.

• The ipv6 nd managed-config-flag command should be ipv6 nd other-config-flag .

• The ipv6 dhcp relay command should use the link-local address of the DHCP server.

Answer explanation

The ipv6 dhcp relay command must be applied to the interface where the clients are located. The ipv6 dhcp relay command can use either the link-local or global unicast address of the DHCPv6 server, or even a multicast address. The ipv6 nd managed-config-flag indicates to the clients that they should use stateful DHCPv6 and is also applied to the interface where the clients are located.

Refer to the exhibit. A network administrator is implementing the stateless DHCPv6 operation for the company. Clients are configuring IPv6 addresses as expected. However, the clients are not getting the DNS server address and the domain name information configured in the DHCP pool. What could be the cause of the problem?

• The GigabitEthernet interface is not activated.

• The router is configured for SLAAC operation.

• The DNS server address is not on the same network as the clients are on.

• The clients cannot communicate with the DHCPv6 server, evidenced by the number of active clients being 0.

Answer explanation

The router is configured for SLAAC operation because there is no configuration command to change the RA M and O flag value. By default, both M and O flags are set to 0. In order to permint stateless DHCPv6 operation, the interface command ipv6 nd other-config-flag should be issued. The GigabitEthernet interface is in working condition because clients can get RA messages and configure their IPv6 addresses as expected. Also, the fact that R1 is the DHCPv6 server and clients are getting RA messages indicates that clients can communicate with the DHCP server. The number of active clients is 0 because the DHCPv6 server does not maintain the state of clients IPv6 addresses (it is not configured for stateful DHCPv6 operation). The DNS server address issue is not relevant to the problem.

Refer to the exhibit. A network administrator is configuring a router as a DHCPv6 server. The administrator issues a show ipv6 dhcp pool command to verify the configuration. Which statement explains the reason that the number of active clients is 0?

• The default gateway address is not provided in the pool.

• No clients have communicated with the DHCPv6 server yet.

• The IPv6 DHCP pool configuration has no IPv6 address range specified.

• The state is not maintained by the DHCPv6 server under stateless DHCPv6 operation.

Answer explanation

Under the stateless DHCPv6 configuration, indicated by the command ipv6 nd other-config-flag , the DHCPv6 server does not maintain the state information, because client IPv6 addresses are not managed by the DHCP server. Because the clients will configure their IPv6 addresses by combining the prefix/prefix-length and a self-generated interface ID, the ipv6 dhcp pool configuration does not need to specify the valid IPv6 address range. And because clients will use the link-local address of the router interface as the default gateway address, the default gateway address is not necessary .

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 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

• the global unicast address of the router interface that is attached to the network

Answer 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.

Refer to the exhibit. A network administrator is configuring a router for DHCPv6 operation. Which conclusion can be drawn based on the commands?

• The DHCPv6 server name is ACAD_CLASS.

• The router is configured for stateless DHCPv6 operation.

• Clients would configure the interface IDs above 0010.

• The router is configured for stateful DHCPv6 operation, but the DHCP pool configuration is incomplete.

Answer explanation

The DHCPv6 is for the stateless DHCPv6 operation that is indicated by changing the O flag to 1 and leaving the M flag as default, which is 0. Therefore, it is not configured for stateful DHCPv6 operation. Although the DNS server has the interface ID 0010, clients in stateless DHCPv6 operation will configure their interface IDs either by EUI-64 or a random number. The ACAD_CLASS is the name of the DHCP pool, not the DHCP server name.

Refer to the exhibit. Based on the output that is shown, what kind of IPv6 addressing is being configured?

• SLAAC

• stateful DHCPv6

• stateless DHCPv6

• static link-local

Answer explanation

Stateful DHCPv6 pools are configured with address prefixes for hosts via the address command, whereas stateless DHCPv6 pools typically only contain information such as DNS server addresses and the domain name. RA messages that are sent from routers that are configured as stateful DHCPv6 servers have the M flag set to 1 with the command ipv6 nd managed-config-flag , whereas stateless DHCPv6 servers are indicated by setting the O flag to 1 with the ipv6 nd other-config-flag command.

After a host has generated an IPv6 address by using the DHCPv6 or SLAAC process, how does the host verify that the address is unique and therefore usable?

• The host sends an ICMPv6 echo request message to the DHCPv6 or SLAAC-learned address and if no reply is returned, the address is considered unique.

• The host sends an ICMPv6 neighbor solicitation message to the DHCP or SLAAC-learned address and if no neighbor advertisement is returned, the address is considered unique.

• The host checks the local neighbor cache for the learned address and if the address is not cached, it it considered unique.

• The host sends an ARP broadcast to the local link and if no hosts send a reply, the address is considered unique.

Answer explanation

Before a host can actually configure and use an IPv6 address learned through SLAAC or DHCP, the host must verify that no other host is already using that address. To verify that the address is indeed unique, the host sends an ICMPv6 neighbor solicitation to the address. If no neighbor advertisement is returned, the host considers the address to be unique and configures it on the interface.