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DHCP relay doesn't work because of NAT between the ME60 and DHCP server

Publication Date:  2012-07-27 Views:  108 Downloads:  4

Issue Description

We had the ME60 with the software version V100R006C05SPC600 and wanted to implement the DHCP relay on it. But according to the trace information, the DHCP relay didn't respod to the DHCP client with DHCP offer (see the attachment trace_problem.txt). The configuration of the BRAS is as follows:
dhcp-server group test
 dhcp-server 10.99.99.248 weight 5
ip pool pppoe remote
 gateway 192.168.10.1 255.255.255.0
 dhcp-server group test
aaa
  domain ppp
 radius-server group test
 ip-pool pppoe
interface GigabitEthernet1/0/0.11
 user-vlan 11
 bas
  access-type layer2-subscriber  default-domain  authentication ppp
  authentication-method  bind
  default-user-name-template test

Alarm Information

No any alarms existed on the ME60

Handling Process

To handle the problems we asked the customer to disable NAT between the DHCP server and BRAS

Root Cause

      After investigating the problem we captured the packets between the BRAS and DHCP server (see the attachment capture.pcap). We found out that that the DHCP server sends the DHCP offer with the source ip different from its own. 
     After discussion with the customer it was found out that they implement NAT between the DHCP server and BRAS. 

Suggestions

If it is some problem with the user authentication on BRAS it it very usefull the function "trace" on BRAS

END