CCNA 2 v6 RSE Chapter 8 Exam Online

When a DHCP client receives DHCPOFFER messages, it will send a broadcast DHCPREQUEST message for two purposes. First, it indicates to the offering DHCP server that it would like to accept the offer and bind the IP address. Second, it notifies any other responding DHCP servers that their offers are declined.

When the lease of a dynamically assigned IPv4 address has expired, a workstation will send a DHCPDISCOVER message to start the process of obtaining a valid IP address. Because the workstation does not know the addresses of DHCP servers, it sends the message via broadcast, with destination addresses of FF-FF-FF-FF-FF-FF and 255.255.255.255.

Dynamic allocation is the most commonly implemented allocation mechanism. It leases the IP parameters for a predefined period of time.

When a DHCPv4 client does not have an IPv4 address, a DHCPv4 server will send a DHCPOFFER message back to the client hardware address of the requesting DHCPv4 client.

When a DHCP client lease is about to expire, the client sends a DHCPREQUEST message to the DHCPv4 server that originally provided the IPv4 address.​ This allows the client to request that the lease be extended.​

By default, the ip helper-address command forwards the following eight UDP services:Port 37: Time
Port 49: TACACS
Port 53: DNS
Port 67: DHCP/BOOTP client
Port 68: DHCP/BOOTP server
Port 69: TFTP
Port 137: NetBIOS name service
Port 138: NetBIOS datagram service

The ip dhcp excluded-address command is followed by the first and the last addresses to be excluded from being leased to DHCP clients.

The ip address dhcp command activates the DHCPv4 client on a given interface. By doing this, the router will obtain the IP parameters from a DHCPv4 server.

SOHO routers are frequently required by the ISP to be configured as DHCPv4 clients in order to be connected to the provider.

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.

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.

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.

Stateless DHCPv6 allows clients to use ICMPv6 Router Advertisement (RA) messages to automatically assign IPv6 addresses to themselves, but then allows these clients to contact a DHCPv6 server to obtain additional information such as the domain name and address of DNS servers. SLAAC does not allow the client to obtain additional information through DHCPv6, and stateful DHCPv6 requires that the client receive its interface address directly from a DHCPv6 server. RA messages, when combined with an EUI-64 interface identifier, are used to automatically create an interface IPv6 address, and are part of both SLAAC and stateless DHCPv6.

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.

The EUI-64 process uses the MAC address of an 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.

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

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.

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.

The router is configured for stateless DHCPv6 operation, which is shown by the last two lines of the show command output. Hosts will configure their IPv6 addresses by using the prefix information that is provided by RA messages. They will also obtain additional configuration information from a DHCPv6 server. The “No virtual link-local address” option and the “Duplicate Address Detection” option are irrelevant to DHCP configuration. Option “SLAAC configuration” is incorrect because by definition SLAAC will use only the information that is provided by RA messages to configure IPv6 settings.

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.