Precautions for configuring dhcp and relay for cisco Routers

When configuring dhcp and relay on a cisco router, you must note that ip dhcp relay information trusted must be configured for the port connecting the router to the vswitch. Otherwise, the client cannot obtain the ip address. This is because the switch is configured with ipdhcp snooping information option (by default). In this case, the switch inserts option 82 in the DHCP request message sent by the client. On the other hand, because the DHCP server (Cisco ios dhcp server here) is in the same VLAN as the client, the request has not actually passed through the DHCP relay proxy. For the Cisco ios dhcp server, if the DHCP request it receives is inserted with option 82 information, it will think that this is a request message from the DHCP relay proxy, however, it checks the giaddr field of the packet and finds that it is 0.0.0.0, rather than a valid IP address (the giaddr field in the DHCP request packet is the IP address of the first DHCP relay agent that the packet passes through, for details, refer to the DHCP packet format). Therefore, the message is considered "invalid" and will be discarded. Refer to the debug process of dhcp on the vrodhcp. There is a command in Cisco IOS specifically used to handle such DHCP request packets: ip dhcp relay information trusted (interface command) or ip dhcp relay information trust-all (Global Command, valid for all vro interfaces). These two commands allow the option 82 to be inserted, but their giaddr field is 0.0.0.0. II. If option 82 is not inserted on the vswitch, that is, no ip dhcp relay information option is configured, no ip address cannot be obtained from the client, the vro does not need to configure the ip dhcp relay informationtrusted command. III. The Windows DHCP server should not check this type of DHCP request mechanism. Therefore, the client can always obtain an IP address in the previous instance regardless of whether the switch is inserted with option 82.