200-301 practice labs

200-301 practice labs.Hosts that use DHCP leased IP addresses (and know other set values) rely on the network to send DHCP messages. In particular, interconnected networks that use a centralized DHCP server have multiple remote LAN subnets that use the DHCP server, then the router has to enable a feature called DHCP Relay (DHCP Relay) for DHCP to work properly. Without DHCPRelay, a host's DHCP requests will never leave the local LAN subnet.Figure 5-9 shows how DHCPRelay works in general. In this example, the DHCP client (Host A) is on the left and the DHCP server (172.16.2.11) is on the right. The client sends a DHCP Discover message to start the DHCP leasing process. This message can only be transmitted on the local LAN without configuring DHCP Relay on router R1, and to forward the Discover message, R1 needs to use the ip helper-address172.16.2.11 command configured on its G0/0 interface to enable DHCPRelay.The steps in the figure point out the requirements for DHCPRelay. In step 1, Host A sends a message with a destination IP and broadcast address of 255.255.255.255 and packets sent to this IP address (the "local subnet broadcast address") should not be forwarded through the router. All devices on the subnet will receive and process the frame. In addition, because the ip helper-address command is configured on R1, router R1 will continue to unblock the frame and packet in order to recognize it as a DHCP request and act accordingly. Step shows the result of the DHCPRelay, in which R1 changes the source and destination IP addresses to route the packet to the address given in the command: 172.16.2.11.Figure 5-9 Role of IP Helper Address Now we return to troubleshooting. A message sent by a DHCP client can be delivered to a DHCP server if the following conditions are true. , . /. Speech i. The server is on the same subnet as the client and the connection between the two is working properly. I . The server is on another subnet with a router on the same subnet as the client, DHCPRelay is implemented correctly, and there is an IP connection between this router and the DHCP server. two common errors in DHCP Relay are fairly obvious. If the router ignores the ip helper-address command on the LAN interface (or subinterface, if using ROAS, or VLAN interface and multilayer switching [MLS] configuration), DHCP will fail for these clients. If the configuration includes the ip helper-address command, but lists the wrong DHCP server IP address, then DHCP fails completely again. In both cases, the symptom is that the client is unable to obtain any information using DHCP. For example, Example 5-2 gives an upgraded configuration of ROAS on Router R3 based on the same scenario in Figure 5-8. The router configuration is working properly, supporting IPv4 and making the router reachable. However, only one subinterface is listed with the ip helper-address command.4.2 Basic Content 93Example 5-2 Forgetting to Support DHCP Relay on the ROAS SubinterfaceinterfaceGigabitEthernet 0/1ip address 10.1.3.3255.255.255.192ip helper-address 10.1.2.130!