IP Helper Address 轉送後的封包, 來源位址應該是什麼?

我做了一個小小的驗證 Lab. 發現 "IP Helper Address 轉送後的封包, 來源位址應該就是啟用 "IP Helper Address" 的那張網卡的IP位址.

R0的畫面擷取說明如下:

1. 尚未啟動 "Helper" 前, 所有的 DHCP broadcasts (DISCOVERY) 都不被理會.
2. 啟動 "Helper". 請注意, 啟動的網卡IP位址就是 "2.1.1.10", 轉送的目的地IP位址就是 "1.1.1.1"
3. 啟動後, Debug訊息顯示, 該 DHCP 封包來源位址從 "0.0.0.0" 改成 "2.1.1.10", 目的地位址從"255.255.255.255" 改成 "1.1.1.1". 然後被送到 DHCP server (R1).
   
4. DHCP server 所回應的封包 (OFFER) 來源位址是 "1.1.1.1", 目的地位址是 "2.1.1.10".
5. 這個封包再被 "Helper" 轉送給 DHCP Client. 同時來源位址再次改成她自己的位址 "2.1.1.10", 目的地改成"255.255.255.255", 猶如這個 DHCP 回應就是她自己所產生的.

R0#show ip int brief
Interface                  IP-Address      OK? Method Status                Protocol
FastEthernet0/0            unassigned      YES unset  up                    up      
FastEthernet0/0.200        2.1.1.10        YES manual up                    up      
FastEthernet0/1            unassigned      YES unset  administratively down down    
Serial1/0                  1.1.1.10        YES manual up                    up      
Serial1/1                  unassigned      YES manual administratively down down    
Serial1/2                  unassigned      YES unset  administratively down down    
Serial1/3                  unassigned      YES unset  administratively down down    
R0#
*Mar  1 00:42:43.599: IP: s=0.0.0.0 (FastEthernet0/0.200), d=255.255.255.255, len 604, rcvd 2
*Mar  1 00:42:47.111: IP: s=0.0.0.0 (FastEthernet0/0.200), d=255.255.255.255, len 604, rcvd 2
*Mar  1 00:42:51.159: IP: s=0.0.0.0 (FastEthernet0/0.200), d=255.255.255.255, len 604, rcvd 2
R0#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R0(config)#int f0/0.200
R0(config-subif)#ip helper-address 1.1.1.1
R0(config-subif)#end
R0#
*Mar  1 00:43:07.607: %SYS-5-CONFIG_I: Configured from console by console
*Mar  1 00:43:09.239: IP: s=0.0.0.0 (FastEthernet0/0.200), d=255.255.255.255, len 604, rcvd 2
*Mar  1 00:43:09.255: IP: tableid=0, s=2.1.1.10 (local), d=1.1.1.1 (Serial1/0), routed via FIB
*Mar  1 00:43:09.259: IP: s=2.1.1.10 (local), d=1.1.1.1 (Serial1/0), len 604, sending
*Mar  1 00:43:09.439: IP: tableid=0, s=1.1.1.1 (Serial1/0), d=2.1.1.10 (FastEthernet0/0.200), routed via RIB
*Mar  1 00:43:09.443: IP: s=1.1.1.1 (Serial1/0), d=2.1.1.10, len 328, rcvd 4
*Mar  1 00:43:09.459: IP: s=2.1.1.10 (local), d=255.255.255.255 (FastEthernet0/0.200), len 328, sending broad/multicast
*Mar  1 00:43:09.567: IP: s=0.0.0.0 (FastEthernet0/0.200), d=255.255.255.255, len 604, rcvd 2
*Mar  1 00:43:09.583: IP: tableid=0, s=2.1.1.10 (local), d=1.1.1.1 (Serial1/0), routed via FIB
*Mar  1 00:43:09.587: IP: s=2.1.1.10 (local), d=1.1.1.1 (Serial1/0), len 604, sending
*Mar  1 00:43:09.619: IP: tableid=0, s=1.1.1.1 (Serial1/0), d=2.1.1.10 (FastEthernet0/0.200), routed via RIB
*Mar  1 00:43:09.623: IP: s=1.1.1.1 (Serial1/0), d=2.1.1.10, len 328, rcvd 4
*Mar  1 00:43:09.635: IP: s=2.1.1.10 (local), d=255.255.255.255 (FastEthernet0/0.200), len 328, sending broad/multicast
R0#
R0#

所以, 這本書有關 "ip helper address" 的描述是錯誤的: Wendell Odom, Rus Healy, Naren Mehta."CCIE Routing and Switching Exam Certification Guide, 3rd Edition", Page 128 和 Page 134!