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The DHCP Server Fails to Obtain the IP Address Because the ME60 Changes the Server IP Address Carried in the Option54

Publication Date:  2013-09-30 Views:  10 Downloads:  0
Issue Description
Version:
V600R002C02SPC700

Networking:
ap(dhcp-client)----sw---ME60(dhcp-relay)-----(221.181.x.x)R(NAT)----(172.16.x.x)AC(dhcp-server) 
Note:
In this network, ap is the DHCP client, ME60 is the DHCP relay, AC is the DHCP server. The source IP address of AC is 172.16.x.x. NAT is performed on R and the source IP address is converted to 221.181.x.x. That is, only the IP address 221.181.x.x is visible to ME60.

Symptom:
In a WLAN service deployment, the attached AP fails to obtain the IP address from the AC.
Handling Process

1. Problems in the DHCP interaction process.
2. Problems in the DHCP interaction packets.
3. Field errors in the DHCP switch packets.
The problem in this case belongs to the third condition.
First, on the ME60, check the reason why the AP fails to go online. The returned messages are as follows:
<SHSJ-PC-CMNET-BRAS01-ME60>display aaa online-fail-record mac-address 001f-1625-31xx
  -------------------------------------------------------------------
  User name          :
SHSJ-PC-CMNET-BRA-0910010240xxxx@xxxx-iptv
  Domain name        : xxxx-iptv
  User MAC           : 001f-1625-31xx
  User access type   : IPoE
  User interface     : GigabitEthernetx/x/0.99999999
  User access PeVlan/CeVlan    : 1024/512
  User IP address    : -
  User IPv6 address  : -
  User ID            : 492
  User authen state  : Authened
  User acct state    : AcctIdle
  User author state  : AuthorIdle
  User login time    : 2012-07-31 15:57:37
  Online fail reason : DHCP with server no response


Then, trace the MAC address. It is found that the ME60 sends the request to the DHCP server but the server does not return the ack message. The returned messages are as follows:
  Dhcpacc send a packet to server.
 DHCPACC proc a REQUEST pkt
。。。。。。。

[trace info:
 Dhcpacc send msg SRV_MSG_DHCPACC_CM_CUT_REQ.
 msgcode:209, DhcpaccIndex:251, cib/DhcpaIndex:494
 DhcpaccIndex:251, DhcpaIndex:4294967295, cib:494, state:6( DHCPACC_WAIT_SERVER_ACK), DownReason:16]
 User leave: DHCP with server no response]


The AC manufacturer reckons that the DHCP server does not handle the request because the server IP address in the option54 of the DHCPREQUEST packet sent from the ME60 is inconsistent with the server IP address in the option54 of the DHCPOFFER packet sent from the AC to the ME60. As the manufacture reckoned, because NAT is performed on the server IP address, the server IP address configured in the DHCP server group of ME60 is 221.181.x.x whereas the server IP address in the offer packets from the DHCP server is 172.16.x.x. 
This problem can be resolved by running the DHCP check-server-pkt loose command on the ME60 system view.
Note:
After this command is run, the ME60 does not modify option54 carried server IP address when it receives request packets sent from the client. (Before this command is run, the server IP address is changed to the one configured in the ME60 dhcp-server group). In this way, the server IP address in the option54 of the request packets is the same as that in the offer packets.
Root Cause
The IP address of the server is changed because an NAT device is deployed between the ME60 and the DHCP server. In addition, the ME60 checks the source IP address by default.
Solution
Run the DHCP check-server-pkt loose command on the ME60 system view.
Suggestions
For problems that the DHCP user fails to obtain the server address, checks on the AC side are required sometimes.

END