Description of the GIADDR field in DHCP packets

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For S series switches, the giaddr field in DHCP packets has the following function:
The giaddr field (gateway IP address) indicates the IP address of the first DHCP relay agent that DHCP packets pass through. If the DHCP server and client are located on different network segments, the first DHCP relay agent fills its IP address into this field of the DHCP Request packet sent by the client and forwards the packet to the DHCP server. The DHCP server determines the network segment where the client resides based on this field, and allocates an IP address on this network segment from an address pool.

Other related questions:
Functions of the GIADDR field of DHCP messages on S series switches
Functions of the GIADDR field of DHCP messages on S series switches: he GIADDR field (gateway IP address) in DHCP messages records the IP address of the first DHCP relay agent that DHCP messages pass through. After a client sends a DHCP Request message, the first DHCP relay agent fills its own IP address in this field when forwarding this DHCP Request message to the DHCP server if the server and the client are located on different network segments. The DHCP server uses the field to determine the network segment on which a client resides so that the server can assign an IP address on this network segment to the client.

Priority mapping
Packets transmitted over different networks carry different QoS precedence fields, for example, EXP field on an MPLS network, 802.1p field on a virtual local area network (VLAN), and DSCP field on an IP network. The mapping between priority fields must be configured on the gateway to retain priorities of packets when the packets traverse different networks. The priority mapping mechanism provides the mapping from precedence fields of packets to internal priorities (local precedences) or the mapping from internal priorities to precedence fields of packets. This mechanism uses the DiffServ domain to manage and record the mapping of precedence fields and internal priorities. When packets reach the AR, the AR maps priorities in packets or the default 802.1p priorities of inbound interfaces to local priorities. The AR then determines the queues that packets enter based on the mapping between internal priorities and queues and performs traffic shaping, congestion avoidance, and queue scheduling. In addition, the AR can re-mark priorities in outgoing packets so that the downstream device can provide different QoS levels based on packet priorities.

Description of OSPF route fields on the AR router
Destination: indicates the destination route. Cost: indicates the cost to the route. Type: specifies the type of the destination route, which can be stub or transit. AdvRouter: specifies the device that advertises the route, that is, router ID of the router that generates the LSA. Area: specifies the area in which the route is learned. Priority: specifies the priority of the route.

Method used to configure the DHCP Option field on the AR
Option fields in a DHCP message carry control information and parameters that are not defined in protocols. There are known options and customized options. For details about known options, see RFC 2132. Customized options are defined by vendors. For example, to set Option 64 to the hexadecimal string 0x11 and configure known Option 121 to assign classless static route to clients (destination IP address 10.10.10.10/24 and gateway address 10.11.11.11), perform the following configurations: - Interface address pool [Huawei] interface gigabitethernet1/0/1 [Huawei-GigabitEthernet1/0/1] dhcp server option 64 hex 11 [Huawei-GigabitEthernet1/0/1] dhcp server option121 ip-address 10.10.10.10 24 10.11.11.11 - Global address pool [Huawei] ip pool global1 [Huawei-ip-pool-global1] option 64 hex 11 [Huawei-ip-pool-global1] option121 ip-address 10.10.10.10 24 10.11.11.11 - DHCP option template (applicable to only the scenario where user-defined network parameters are allocated to static clients) [Huawei] dhcp option template template1 [Huawei-dhcp-option-template-template1] option 64 hex 11 [Huawei-dhcp-option-template-template1] option121 ip-address 10.10.10.10 24 10.11.11.11

Configuring the DHCP Option 43 field on an AC
Perform the following operations to configure the DHCP Option 43 field on an AC that functions as a DHCP server: 1. Run the system-view command to enter the system view. 2. Run the dhcp enable command to enable the DHCP function. 3. Run the ip pool ip-pool-name command to enter the global address pool view. 4. Run the option 43 sub-option 2 ip-address ip-address &<1-8> command to set the DHCP Option 43 field to the IP addresses of the active and standby ACs.

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