Vendor Tech Talk

Nov 30 2012   4:35PM GMT

Troubleshooting Network Problems – Part 4: IP Address Conflicts

Posted by: SolarWinds
Network Management

This is the final installment of a four part blog series on troubleshooting network problems.  The other parts of the series address:

The System Has Detected an IP Address Conflict!  We’ve all seen this message before.  But what does it really mean and how do we go about identifying and resolving the problem?

IP address conflicts occur when two devices on a network are assigned the same IP address resulting in one or both being disabled and losing connectivity until the conflict is resolved.  IP address conflicts are almost always the result of configuration errors including:  assignment of the same static IP address by a network administrator; assignment of a static IP address within the DHCP range (dynamic range) resulting in the same address being automatically assigned by the local DHCP server; an error in the DHCP server; or a system coming back online after an extended period in stand-by or hibernate mode with an IP address that has been re-assigned and is in use on the network.

Here are a number of steps that you can take to troubleshoot this pesky problem.

Step 1 – Look For Overlapping IP Address Ranges on Your DHCP Server

If you are using multiple DHCP servers, you will first want to verify that no two servers have overlapping IP address ranges.  This can be as simple as comparing the IP address ranges and looking for overlaps when the servers are using dynamic or automatic allocation of IP addresses.  If they are using static allocation, then you will need to review each hard coded IP address assignment.

Step 2 – Look for Duplicate Static IP Addresses

Look for devices on the network segment that have been statically configured with the duplicate IP address.  Once found, you can either reconfigure the device to use DHCP or you can configure the DHCP server to stop assigning the duplicated IP address.

Step 3 – Find the Conflicting MAC Addresses

If steps 1 and 2 do not produce results, you will need to find the MAC addresses of the conflicting devices.  Since the MAC address is unique for each device on the network, you can look for devices that contain the same IP address but with different MAC addresses. You can use the Address Resolution Protocol (ARP) to establish a correspondence between the IP address and the MAC address.  Start at your core router and use the show ip arp command:

Router# show ip arp

Protocol  Address               Age(min)  Hardware Addr      Type     Interface
Internet         -            0000.0c59.f892    ARPA   Ethernet0/0
Internet         -            0000.0c07.ac00    ARPA   Ethernet0/0
Internet            -            0000.0c63.1300   ARPA   Ethernet0/0
Internet         -            0000.0c36.6965   ARPA   Ethernet0/0
Internet             -            0000.0c63.1300   ARPA   Ethernet0/0
Internet         9            0000.0c36.6965   ARPA   Ethernet0/0

If you were to see two IP addresses with differing hardware addresses then you have located your problem devices.

Step 4 – Trace the Location of the Device

Perhaps you want to know the physical location or at least the switch port that the offending devices are connected to.  One way is to go to the switch and use the show mac-address table command.  This will show you the MAC address for each port.

switch# show mac-address-table

           Mac Address Table
Vlan    Mac Address          Type                Ports
—–     —————          ——–             ——
1           0007.e9e2.2d7d    DYNAMIC     Fa0/5
1           0009.0f30.07e9    DYNAMIC     Fa0/48
1           0009.5bbc.af04    DYNAMIC     Fa0/28
1           00e0.bb2c.30d1    DYNAMIC     Gi0/1
1           00e0.bb2c.3e5f     DYNAMIC     Gi0/1
Total Mac Addresses for this criterion: 5


Unfortunately, you need to run this command from each switch and, if the network is down, you will have to go to the console of each switch.  This can be very tedious and time consuming not to mention logistically challenged in the case of geographically distributed networks.

Another alternative is the use of commercially available network monitoring tools

SolarWinds User Device Tracker Endpoint Details

that will trace the location of a device on a network automatically.  SolarWinds User Device Tracker is a device tracking and switch port management software solution that quickly locates a device on the network by searching on the IP address, Hostname or MAC address.

Preventing Conflicts in the Future

Once you have identified and corrected IP address conflicts, here are some tips to prevent future conflicts:

  • Use DHCP to reduce the chances of manually assigning duplicate addresses.
  • Set your DHCP server to detect IP address conflicts.
  • Modify the DHCP lease duration to something less than the default lease time of 8 days.
  • Use multiple DHCP servers, each having it own specific scope.
  • Reserve IP addresses instead of assigning static IP addresses.
  • Use automated DHCP, DNS, and IP address management and monitoring tools

Even by following these tips, there still remains the possibility that IP conflicts will occur.  I would encourage you to evaluate a commercially available IP address management product that allows you to centrally manage, monitor, alert, and report on your IP infrastructure.  By proactively managing and monitoring your IP address space you can significantly reduce the chances of IP address conflicts.

SolarWinds IP Address Manager

SolarWinds IP Address Manager

SolarWinds IP Address Manager(IPAM) enables you and your team to ditch your spreadsheets for an easy-to-use, centralized IP address monitoring and management solution. With IPAM you can manage Microsoft® DHCP services, monitor Microsoft DNS and Cisco® DHCP servers, and manage your IP address space; all from an intuitive, centralized Web console.

  • Centrally manage, alert, & report on your IP address space
  • Manage & monitor Microsoft DHCP/DNS services & monitor Cisco DHCP servers
  • Delivers role-based access & control from an intuitive web based interface
  • Alert notifications help prevent your subnets & DHCP scopes from filling up
  • Automatically discovers used & unused addresses & typically deploys in less than an hour

By Brad Hale, Product Marketing Principal for SolarWinds. SolarWinds (NYSE: SWI) provides powerful and affordable IT management software to customers worldwide.

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