DetailPage-MSS-KB

Microsoft small business knowledge base

Article ID: 2889130 - Last Review: September 24, 2013 - Revision: 7.0

Symptoms

Consider the following scenario:

  • DHCP NAP is enabled in the environment
  • There are two HSRP routers configured as DHCP relay agent (IP Helper-Address) for failover
  • Client connects to corporate network

In this scenario, clients get a restricted IP address instead full access IP.

Cause

This issue occurs because both DHCP Relay Agents responds to the DHCP requests and they don't work as fail over.

  • The client sends a DHCP request and both Relay Agent send the requests to the DHCP server
  • DHCP server responds to the request with restricted IP address
  • The client get the response from one of the DHCP relay agent and it then send a request with SOH       
  • Now the response from the second DHCP relay agent comes and the client assume the IP address with restricted IP and discard the   response  from the both relay agent with full access IP

 In the network trace, you will see:

16754  0.0000000 <Time><Date> 0.0.0.0             255.255.255.255  DHCP        DHCP:Request, MsgType = REQUEST, TransactionID = 0x401B504D
16784  0.0085968 <Time><Date> 192.168.100.3       DS-CLIENTDHCP    DHCP:Reply, MsgType = ACK, TransactionID = 0x401B504D
16793  0.0003589 <Time><Date> 0.0.0.0             255.255.255.255  DHCP        DHCP:Request, MsgType = REQUEST, TransactionID = 0x401B504D, SOH:Vendor = Microsoft, Version 2, Request
16804  0.0004382 <Time><Date> 192.168.100.2       DS-CLIENTDHCP    DHCP:Reply, MsgType = ACK, TransactionID = 0x401B504D  - The client gets response from second DHCP relay agent after it sends the request with SOH and the client assumes that IP address and discard the following acknowledge.
16985  0.0201080 <Time><Date> 192.168.100.2       DS-CLIENTDHCP    DHCP:Reply, MsgType = ACK, TransactionID = 0x401B504D, SOH:Vendor = Microsoft, Version 2, Response
16994  0.0153142 <Time><Date> 192.168.100.3       DS-CLIENTDHCP    DHCP:Reply, MsgType = ACK, TransactionID = 0x401B504D, SOH:Vendor = Microsoft, Version 2, Response.

Resolution

Cisco has fixed the issue of duplicate DHCP forwarding in a later release of IOS. Basically IP helpers were not “HSRP aware” till this fix.

See Cisco documentation at http://www.cisco.com/en/US/docs/ios-xml/ios/ipapp/configuration/12-4t/iap-udp-vrg.html#GUID-45267FAA-2D78-40AB-A010-30F5289C6D88

Benefits of the UDP Forwarding Support for Virtual Router Groups Feature

Forwarding is limited to the active router in the VRG instead of all routers within the VRG. Prior to the implementation of this feature, the only VRG support was HSRP. Within a VRG that is formed by HSRP, the forwarding of UDP-based broadcast and multicast packets is done by all the routers within the VRG. This process can cause some DHCP servers to operate incorrectly. The UDP Forwarding Support for VRGs feature limits forwarding to the active router in the VRG.

Note This is a "FAST PUBLISH" article created directly from within the Microsoft support organization. The information contained herein is provided as-is in response to emerging issues. As a result of the speed in making it available, the materials may include typographical errors and may be revised at any time without notice. See Terms of Use (http://go.microsoft.com/fwlink/?LinkId=151500) for other considerations.

Applies to
  • Microsoft Windows XP Professional
  • Windows Vista Enterprise
  • Windows Vista Ultimate
  • Windows 7 Enterprise
  • Windows 7 Professional
  • Windows 7 Ultimate
  • Microsoft Windows Server 2003, Enterprise Edition (32-bit x86)
  • Microsoft Windows Server 2003, Enterprise x64 Edition
  • Microsoft Windows Server 2003, Standard Edition (32-bit x86)
  • Microsoft Windows Server 2003, Standard x64 Edition
  • Windows Server 2008 Enterprise
  • Windows Server 2008 R2 Enterprise
  • Windows Server 2008 R2 Standard
  • Windows Server 2008 Standard
  • Windows 8
Keywords: 
KB2889130
Share
Additional support options
Ask The Microsoft Small Business Support Community
Contact Microsoft Small Business Support
Find Microsoft Small Business Support Certified Partner
Find a Microsoft Store For In-Person Small Business Support