Which kind of message is sent by a DHCP client when its IP address lease has expired a Dhcpdiscover broadcast message?

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Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).

The Dynamic Host Configuration Protocol (DHCP) dynamically assigns IP addresses to hosts and centrally manages host configurations. DHCP uses the client/server model. A client applies to the server for configuration parameters, such as an IP address, subnet mask, and default gateway address; the server replies with the requested configuration parameters.

With the DHCP client function configured, a device uses DHCP to dynamically request an IP address from the DHCP server. This achieves appropriate assignment and centralized management of IP addresses.

To obtain a valid dynamic IP address, a DHCP client exchanges different information with the DHCP server at different stages. Generally, the DHCP client and server interact in the following modes:

• A DHCP client dynamically obtains an IP address.

  As shown in Figure 4-5, the DHCP client establishes a connection with the DHCP server through the following four stages:

  1. Discovering stage: At this stage, the DHCP client searches for a DHCP server. The client broadcasts a DHCP Discover message and only DHCP servers respond to the message.

  2. Offering stage: At this stage, each DHCP server offers an IP address to the DHCP client. After receiving the DHCP Discover message from the client, each DHCP server selects an unassigned IP address from the IP address pool and sends a DHCP Offer message with the leased IP address and other settings to the client.

  3. Selecting stage: At this stage, the DHCP client selects an IP address. If multiple DHCP servers send DHCP Offer messages to the client, the client accepts the first received DHCP Offer message and broadcasts a DHCP Request message carrying the selected IP address.

  4. Acknowledgement stage: indicates the stage at which the DHCP server acknowledges the IP address that is offered. When the selected DHCP server receives the DHCP Request message, it searches for a related lease record based on the MAC address or Option 61 field in the received message.

     • If the related lease record exists, the DHCP server sends the DHCP client a DHCP ACK message containing the DHCP client's IP address. After receiving the DHCP ACK message, the DHCP client broadcasts a gratuitous ARP message to check whether any host is using the IP address assigned by the DHCP server. If the DHCP client does not receive a response within a specified period, it uses the IP address.
     • If the related lease record does not exist or the DHCP server fails to properly assign IP addresses, the DHCP server sends a DHCP NAK message to inform the DHCP client that it cannot assign a proper IP address. In this case, the DHCP client has to send another DHCP Discover message for a new application.

  Figure 4-5 DHCP client dynamically obtaining an IP address
  
  

• The DHCP client updates the lease period.

  Some DHCP clients use a fixed IP address for a long time, and some DHCP clients use a temporary IP address. After a DHCP client's lease time is expired, the DHCP server reclaims the IP address of the DHCP client and allocates this IP address to another DHCP client. You can configure an expected lease time for a DHCP client as required. In this case, while assigning an address lease time, the DHCP server compares the expected lease time with the address lease time of the current address pool and provides the DHCP client an appropriate lease time based on address assignment rules.

  After the lease time configured for a DHCP client to obtain a dynamic IP address from the DHCP server is expired, if the DHCP client wants to continue using this IP address, the IP address lease needs to be renewed.

  Figure 4-6 shows how a DHCP client establishes a connection with the DHCP server to update the IP address lease.

  1. When the IP address lease reaches 50% (T1), the DHCP client automatically sends a DHCP Request message in unicast mode to the DHCP server to renew the IP address lease.
     • If a DHCP ACK message is received, the IP address lease is successfully renewed.
     • If a DHCP NAK message is received, the DHCP client re-initiates the renewal procedure.
  2. When the IP address lease reaches 87.5% (T2), if the DHCP client has not received a DHCP ACK message yet, it broadcasts a DHCP Request message to DHCP servers to renew its IP address lease.
     • If a DHCP ACK message is received, the IP address lease is successfully renewed.
     • If a DHCP NAK message is received, the DHCP client re-initiates the renewal procedure.
  3. If the DHCP client receives no response before the IP address lease expires, the DHCP client stops using the current IP address and sends a DHCP Discover message to request a new IP address.

  Figure 4-6 DHCP client updating the lease period
  
  

• The DHCP client proactively releases the IP address.

  When the DHCP client no longer uses the assigned IP address, it proactively sends a DHCP Release message to the DHCP server to instruct the server to release the IP address lease. The DHCP server retains the DHCP client's configuration for reuse in case that the client re-applies for an IP address.

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Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).

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112. Which kind of message is sent by a DHCP clientwhen its IP address lease has expired?a DHCPDISCOVER broadcast messagea DHCPREQUEST broadcast messagea DHCPREQUEST unicast messagea DHCPDISCOVER unicast message113. What is a disadvantage of NAT?There is no end-to-end addressing.The router does not need to alter the checksum of theIPv4 packets.The internal hosts have to use a single public IPv4address for external communication.The costs of readdressing hosts can be significant for apublicly addressed network.114. Refer to the exhibit. The Gigabit interfaces on bothrouters have been configured with subinterface numbersthat match the VLAN numbers connected to them. PCson VLAN 10 should be able to print to the P1 printer onVLAN 12. PCs on VLAN 20 should print to the printers onVLAN 22. What interface and in what direction shouldyou place a standard ACL that allows printing to P1 fromdata VLAN 10, but stops the PCs on VLAN 20 from usingthe P1 printer? (Choose two.) 9.Which kind of message is sent by a DHCP client when its IP address lease hasexpired?a DHCPDISCOVER broadcast messagea DHCPREQUEST broadcast messagea DHCPREQUEST unicast messagea DHCPDISCOVER unicast messageAnswers Explanation & Hints:When the IP address lease time of the DHCP client expires, it sends a DHCPREQUEST unicasoriginally offered the IPv4 address.

Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).

IP addresses that are dynamically allocated by a DHCP server have leases. However, a DHCP client may request a specific lease term by adding information to a DHCP Discover message. When allocating network parameters, the DHCP server compares the expected lease with the lease specified in the address pool and allocates an IP address with a smaller lease to the DHCP client. When the lease expires or a client logs out and releases its IP address, the server reclaims the IP address, which can then be allocated to other clients. To continue using this IP address, the client must renew its IP address lease.

Figure 4-8 shows how a DHCP client renews its IP address lease.

Figure 4-8  Renewing an IP address lease

1. When the lease reaches 50% (T1) of its validity period, the DHCP client unicasts a DHCP Request message to the DHCP server to request lease renewal. If the server renews the lease (counted from 0), it sends a DHCP Ack message to the client. If the server rejects the renewal request, it sends a DHCP Nak message to the client, which must then send a DHCP Discover message to apply for a new IP address.

2. If no response is received from the DHCP server when the lease reaches 87.5% (T2) of its validity period, the DHCP client broadcasts a DHCP Request message to request lease renewal. Similar to T1, if the client receives a DHCP Ack message, the server has renewed the lease; if the client receives a DHCP Nak message, the client must send a DHCP Discover message to apply for a new IP address.
3. If no response is received when the lease expires, the client stops using the IP address and sends a DHCP Discover message to apply for a new IP address.

When a DHCP client no longer needs to use its allocated IP address and the lease has not expired, the client sends a DHCP Release message to the DHCP server to request IP address release. The server saves the configuration of this client and records the IP address in the allocated IP address list. The IP address can then be allocated to this client or other clients. To request a configuration update, a client can send a DHCP Inform message to the server.

The renewal process is similar when a DHCP relay agent is used. Figure 4-9 shows how a DHCP client renews its IP address lease when a DHCP relay agent is deployed.

Figure 4-9  Renewing the IP address lease when a DHCP relay agent is deployed

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Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).

IP addresses that are dynamically allocated by a DHCP server have leases. A DHCP Discover message from a DHCP client can carry an expected lease. When allocating network parameters, the DHCP server compares the expected lease with the specified lease in the address pool and allocates an IP address with the shorter of the two leases to the DHCP client. When the lease expires or the client goes offline, the DHCP server reclaims the IP address, which can then be allocated to other clients. This mechanism improves IP address utilization.

To continue to use this IP address, the DHCP client must renew its IP address lease.

Figure 3-5 shows how a DHCP client renews its IP address lease.

Figure 3-5  Renewing an IP address lease

1. When the lease reaches 50% (T1) of its validity period, the DHCP client unicasts a DHCP Request message to the DHCP server to request lease renewal. If the DHCP client receives a DHCP ACK message, the IP address lease is successfully renewed (counted from 0). If the DHCP client receives a DHCP NAK message, the DHCP client must send a DHCP Discover message to apply for a new IP address.
2. If no response is received from the DHCP server when the lease reaches 87.5% (T2) of its validity period, the DHCP client broadcasts a DHCP Request message to request lease renewal. If the DHCP client receives a DHCP ACK message, the IP address lease is successfully renewed (counted from 0). If the DHCP client receives a DHCP NAK message, the DHCP client must send a DHCP Discover message to apply for a new IP address.

3. If no response is received when the lease expires, the DHCP client stops using the IP address and sends a DHCP Discover message to apply for a new IP address.

When a DHCP client no longer needs to use the allocated IP address and the lease has not expired, the DHCP client sends a DHCP Release message to the DHCP server to request IP address release. The DHCP server saves the configuration of this DHCP client and records the IP address in the allocated IP address list. The IP address can then be allocated to this DHCP client or other clients.

To request a configuration update, a DHCP client can send a DHCP Inform message to the DHCP server.

Figure 3-6 shows how a DHCP client renews its IP address lease with a DHCP relay agent deployed.

Figure 3-6  Renewing the IP address lease when a DHCP relay agent is deployed

1. When the lease reaches 50% (T1) of its validity period, the DHCP client unicasts a DHCP Request message to the DHCP server to request lease renewal. If the DHCP client receives a DHCP ACK message, the IP address lease is successfully renewed (counted from 0). If the DHCP client receives a DHCP NAK message, the DHCP client must send a DHCP Discover message to apply for a new IP address.
2. If no response is received from the DHCP server when the lease reaches 87.5% (T2) of its validity period, the DHCP client broadcasts a DHCP Request message to request lease renewal. The DHCP relay agent then unicasts a DHCP Request message to the DHCP server. For details about how the DHCP relay agent processes received messages, see DHCP Server Allocating Network Parameters to New DHCP Clients. If the DHCP client receives a DHCP ACK message, the IP address lease is successfully renewed (counted from 0). If the DHCP client receives a DHCP NAK message, the DHCP client must send a DHCP Discover message to apply for a new IP address.

3. If no response is received when the lease expires, the DHCP client stops using the IP address and sends a DHCP Discover message to apply for a new IP address.

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