IP Addressing Services

Document ID: 12233

Contents

Introduction
Prerequisites
      Requirements
      Components Used
      Conventions
Background Information
Configure
      Problem
      Solution
      Example
      BOOTP Trace
Verify
Troubleshoot
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Related Information

Introduction

The IBM RS6000 BOOTP server does not follow RFC 1542 . IBM addresses this issue with an authorized program analysis report (APAR).

Prerequisites

Requirements

There are no specific requirements for this document.

Components Used

This document is not restricted to specific software and hardware versions.

Conventions

Refer to Cisco Technical Tips Conventions for more information on document conventions.

Background Information

If a customer implements ip helper and HSRP (HSRP on the client segment), the Cisco router drops the RS6000 BOOTP server response. It can appear that HSRP caused BOOTP to "break."

Configure

Problem

In this section, you are presented with the information to configure the features described in this document.

Note: Use the Command Lookup Tool ( registered customers only) to find more information on the commands used in this document.

The IBM RS6000 modifies the "giaddr" field (BOOTP relay agent) in BOOTP to specify a default gateway for the BOOTP client in addition to a default gateway in Vendor Information Extension (see the trace included at the end of this Tech Tip).

The "giaddr" field is only for the IP address of the BOOTP relay agent. As per RFC 1542, "In no case does it (giaddr) represent a general router to be used by the client."

This "breaks" a Cisco router only with HSRP configured. The router sources the BOOTP request from its actual IP address, which is in the "giaddr" field of the request. The RS6000 replies to this packet with the HSRP virtual IP address in the "giaddr" field. Because this practice does not follow RFC 1542, the Cisco router drops the packet.

The RS6000 behavior is not an issue if HSRP is not configured because the IP address of the BOOTP relay agent and the default gateway of the clients are usually the same.

Another scenario when BOOTP can "break" does not involve HSRP (see the diagram). In the diagram, Router1 is the BOOTP relay agent, and Router2 is the default gateway.

Solution

The correct way to specify a default gateway for the client is in the Gateway subfield of the Vendor Information Extensions, and to not change the "giaddr" field.

Example

In this diagram, the actual IP address of the router on the client ring is 167.8.150.1. It also runs HSRP on the client interface. The standby IP address is 167.8.150.5.

BOOTP Trace

A BOOTP trace is shown here.

- - - - - - - - - - - - - - - - Frame 1 - - - - - - - - - - - - - - - - -
  BOOTP Request as relayed by the Cisco router
  
  DLC:  ----- DLC Header -----
  DLC:  Destination = Station 10005AB1D9F9
  DLC:  Source      = Station 0006C13E3990
  
  IP:   ----- IP Header -----
  IP:   Source address      = [167.8.150.1]     router's real ip
  IP:   Destination address = [167.8.14.4]
  UDP:  ----- UDP Header -----
  UDP:  Source port      = 68 (Bootp client)
  UDP:  Destination port = 67
  BOOTP:  ----- BOOTP Header -----
  BOOTP:  Boot record type        = 1 (Request)
  BOOTP:  Hardware address type   = 6 IEEE 802 Network
  BOOTP:  Hardware address length = 6 bytes
  BOOTP:  ^M
  BOOTP:  Hops                    = 0
  BOOTP:  Transaction id          = 5B8EBEAB
  BOOTP:  Elapsed boot time       = 2 seconds
  BOOTP:  Flags                   = 0000
  BOOTP:  Client self-assigned IP address   = [0.0.0.0] (Unknown)
  BOOTP:  Gateway IP address                = [167.8.150.1]  router's actual IP
  BOOTP:  Client hardware address = 10005A8CBE82
  BOOTP:  Host name      = ""
  BOOTP:  Boot file name = ""
  BOOTP:  Vendor Information tag = 63825363
  
  BOOTP Reply from RS6000
  DLC:  ----- DLC Header -----
  DLC:  Destination = Station 0006C13E3990
  DLC:  Source      = Station 10005AB1D9F9
  IP:   ----- IP Header -----
  IP:   Source address      = [167.8.14.4]
  IP:   Destination address = [167.8.150.5]   router's standby IP
  UDP:  ----- UDP Header -----
  UDP:  Source port      = 67 (Bootp server)
  UDP:  Destination port = 67
  BOOTP:  ----- BOOTP Header -----
  BOOTP:  Boot record type        = 2
  BOOTP:  Client self-assigned IP address   = [0.0.0.0] (Unknown)
  BOOTP:  Client server-assigned IP address = [167.8.150.10]
  BOOTP:  Server IP address                 = [167.8.14.4]
  BOOTP:  Gateway IP address                = [167.8.150.5]
  BOOTP:  Client hardware address = 10005A8CBE82
  BOOTP:  Host name      = ""
  BOOTP:  Boot file name = ""
  BOOTP:  Vendor Information tag = 63825363
  BOOTP:  Subnet mask= [255.255.255.128]
  BOOTP:  Gateway address            = [167.8.150.5]  router's standby IP

Verify

There is currently no verification procedure available for this configuration.

The Output Interpreter Tool ( registered customers only) (OIT) supports certain show commands. Use the OIT to view an analysis of show command output.

Troubleshoot

There is currently no specific troubleshooting information available for this configuration.

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