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A DHCP User Failed to Obtain an IP Address from the DHCP Server Because the DHCP Discover Packets Did Not Carry Option82

Publication Date:  2013-09-22 Views:  36 Downloads:  0
Issue Description

Version:
V600R003C05SPC300

Networking:
STB ------ MSAN ------ (METRO-E) ------ ME60(DHCP Relay) ------ DHCP Server

Symptom:
A STB connected to a site failed to obtain an IP address from the DHCP server.
Handling Process

Possible causes for such a problem are as follows:
1. The DHCP server does not respond to user's request.

2. The DHCP relay device does not forward the response packets from the DHCP server.

3. The DHCP relay device does not forward user's requests.

To address the issue, Huawei performed the following operations and observed the following information:

1. Checked the packet statistics on the DHCP relay device.
<BSRTRM01>display dhcp relay statistics
    Bad Packets recieved:                    0
    DHCP packets received from clients:      35
        DHCP DISCOVER packets received:      35   ---
(The DHCP relay device received DHCP DISCOVER packets from the STB.)
        DHCP REQUEST packets received:       0    
        DHCP INFORM packets received:        0
        DHCP DECLINE packets received:       0
    DHCP packets received from servers:      0
        DHCP OFFER packets received:         0
        DHCP ACK packets received:           0
        DHCP NAK packets received:           0
    DHCP packets sent to servers:            0
  --- (The DHCP relay device did not forward the DHCP discover packets.)
 
   DHCP packets sent to clients:            0
        Unicast packets sent to clients:     0
        Broadcast packets sent to clients:   0

Therefore, the problem was caused because the DHCP relay device did not forward DHCP discover packets.

2. Debugged the DHCP relay.
<BSRTRM01>debugging dhcp relay all
<BSRTRM01>t d
<BSRTRM01>t m
Nov 23 2012 15:05:21.810.4+08:00 BSRTRM01 DHCPR/7/dhcpr_debug_paket:
Hardware Type = 1, Hardware Address Length = 6       
Hops = 0, Transaction ID = 1846971967       
Seconds = 0, Broadcast Flag = 32768       
Client IP Address = 0.0.0.0, Your IP Address = 0.0.0.0       
Server IP Address = 0.0.0.0, Gateway IP Address = 0.0.0.0       
Client Hardware Address = 9094-e428-fd7e       
Server Host Name = Not Configured, Boot File Name = Not Configured       
Option82 = Not Configured          
Dhcp message type = DISCOVER  
--- Discover packets received

Nov 23 2012 15:05:21.810.5+08:00 BSRTRM01 DHCPR/7/dhcpr debug:
The main IP address of the interface is selected as giaddr(10.31.31.254).

Nov 23 2012 15:05:21.810.6+08:00 BSRTRM01 DHCPR/7/dhcpr_debug_error:
Failed to select IP address of DHCP server.

The DHCP relay device discarded the DHCP discover packets because it failed to find the IP address of the DHCP server.

3. Checked configurations of the relay interface.
interface GigabitEthernet11/0/0.9
 vlan-type dot1q 966
 description "Connect to METRO E NPEHWTLG22 Gi3/0/0  CCS-RELAY"
 ip binding vpn-instance TMCCS
 ip address 10.31.31.254 255.255.224.0  
 ip relay address 10.254.253.65 dhcp-option 82
 dhcp select relay
 trust upstream default
 trust 8021p
 statistic enable

The server IP address configured on the relay interface was ip relay address 10.254.253.65 dhcp-option 82, which meant that the interface considered that only packets carrying DHCP option 82 could be forwarded to the DHCP server 10.254.253.65.

It was suspected that the problem was caused because the DHCP discover packets did not carry DHCP option82.

4. Instructed the frontline personnel to enable DHCP option82 on the downstream MSAN.
The STB
obtained an IP address.
Root Cause
The server IP address configured on the relay interface was ip relay address 10.254.253.65 dhcp-option 82, which meant that the interface considered that only packets carrying DHCP option 82 could be forwarded to the DHCP server 10.254.253.65. However, the downstream MSAN was not enabled with DHCP option 82.
Solution
DHCP option 82 was enabled on the MSAN.
Suggestions
If a DHCP user cannot obtain an IP address through a DHCP relay device, diagnose the problem by analyzing the packet statistics on the relay device, debugging information, and downstream device information.

END