Note A DHCP client may receive offers from multiple DHCP servers and can accept any one of the offers; however, the client usually accepts the first offer it receives. Additionally, the offer from the DHCP server is not a guarantee that the IP address will be allocated to the client; however, the server usually reserves the address until the client has had a chance to formally request the address.

Note The formal request for the offered IP address (the DHCPREQUEST message) that is sent by the client is broadcast so that all other DHCP servers that received the DHCPDISCOVER broadcast message from the client can reclaim the IP addresses that they offered to the client.

If the configuration parameters sent to the client in the DHCPOFFER unicast message by the DHCP server are invalid (a misconfiguration error exists), the client returns a DHCPDECLINE broadcast message to the DHCP server.

The DHCP server will send to the client a DHCPNAK denial broadcast message, which means the offered configuration parameters have not been assigned, if an error has occurred during the negotiation of the parameters or the client has been slow in responding to the DHCPOFFER message (the DHCP server assigned the parameters to another client) of the DHCP server.

 

二。参数分析

PC1(config-if)#
*Apr 19 21:16:39.803: IP: s=0.0.0.0 (local), d=255.255.255.255 (FastEthernet0/0), len 604, sending broad/multicast  //这是PC1向dhcp server获取IP address.采用广播
*Apr 19 21:16:40.119: IP: s=192.168.1.2 (FastEthernet0/0), d=255.255.255.255, len 328, rcvd 1 //回复用的单播
*Apr 19 21:16:40.123: IP: s=0.0.0.0 (local), d=255.255.255.255 (FastEthernet0/0), len 604, sending broad/multicast  //
*Apr 19 21:16:40.239: IP: s=192.168.1.2 (FastEthernet0/0), d=255.255.255.255, len 328, rcvd 1
*Apr 19 21:16:41.755: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up
*Apr 19 21:16:42.599: IP: s=172.16.1.1 (FastEthernet0/0), d=255.255.255.255, len 328, rcvd 1 //因为存在两个DHCP server,
*Apr 19 21:16:42.755: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to up
*Apr 19 21:16:43.295: %DHCP-6-ADDRESS_ASSIGN: Interface FastEthernet0/0 assigned DHCP address 192.168.1.9, mask 255.255.255.0, hostname PC1   //可以看到现在PC1得到192.168.1.9的地址

 

PC1(config-if)#
*Apr 19 21:34:23.967: IP: s=0.0.0.0 (local), d=255.255.255.255 (FastEthernet0/0), len 604, sending broad/multicast
*Apr 19 21:34:25.923: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up
*Apr 19 21:34:26.923: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to up
*Apr 19 21:34:26.951: IP: s=192.168.1.2 (FastEthernet0/0), d=255.255.255.255, len 328, rcvd 1  //因为PC1是最先收到192.168.1.2的地址。
*Apr 19 21:34:26.955: IP: s=0.0.0.0 (local), d=255.255.255.255 (FastEthernet0/0), len 604, sending broad/multicast
*Apr 19 21:34:27.411: IP: s=172.16.1.1 (FastEthernet0/0), d=255.255.255.255, len 328, rcvd 1
*Apr 19 21:34:27.447: IP: s=192.168.1.2 (FastEthernet0/0), d=255.255.255.255, len 328, rcvd 1  //从上面可以看到PC1收192.168.1.2与172.16.1.1收过来的地址，而172.16.1.1先到收到，但为什么用了192.168.1.2发过来的地址呢？
*Apr 19 21:34:30.499: %DHCP-6-ADDRESS_ASSIGN: Interface FastEthernet0/0 assigned DHCP address 192.168.1.10, mask 255.255.255.0, hostname PC1

 

PC1(config-if)#
*Apr 19 21:38:30.339: IP: s=0.0.0.0 (local), d=255.255.255.255 (FastEthernet0/0), len 604, sending broad/multicast
*Apr 19 21:38:30.699: IP: s=172.16.1.1 (FastEthernet0/0), d=255.255.255.255, len 328, rcvd 1
*Apr 19 21:38:30.703: IP: s=0.0.0.0 (local), d=255.255.255.255 (FastEthernet0/0), len 604, sending broad/multicast
*Apr 19 21:38:30.891: IP: s=172.16.1.1 (FastEthernet0/0), d=255.255.255.255, len 328, rcvd 1
*Apr 19 21:38:33.339: IP: s=192.168.1.2 (FastEthernet0/0), d=255.255.255.255, len 328, rcvd 1
*Apr 19 21:38:33.935: %DHCP-6-ADDRESS_ASSIGN: Interface FastEthernet0/0 assigned DHCP address 172.16.1.5, mask 255.255.255.0, hostname PC1   //得到的是172.16.1.0是地址

 

R3#sh ip dhcp pool

Pool xia :
 Utilization mark (high/low)    : 100 / 0
 Subnet size (first/next)       : 0 / 0
 Total addresses                : 254
 Leased addresses               : 1   //租借时间分一天
 Pending event                  : none
 1 subnet is currently in the pool :
 Current index        IP address range                    Leased addresses
 172.16.1.6           172.16.1.1       - 172.16.1.254      1  //当前分配的是172.16.1.6的地址