IP conflict/duplicate addresses in Cisco LAN

[Ayad]

Hello,

How is it possible to detect/locate/discover a duplicate IP address in a network (LAN)?

I have recently found in one of my LAN (remote site) that a switch has the same address as the virtual VRRP (adress of my two WAN devices). I had often many connectivity issues because of this conflict but when i connect to WAN devices i ping the shared address without any problem, thus for me no problem with the adress.

- How can I know if the adress match the VRRP or the switch ?

- Does someone know some cisco advanced command to see this in cisco routers or switchs ?

- is the show mac-address-table can give me the two mac if any conflict occurs ? so i can locate the port.

- from my switch, is there any command to have all ip address of devices connected in LAN ? or mac-addresses ?

Many thanks for help.

Regards,

[Sebastian Helmer]

Hi,

- To see if it is the VRRP address I would say you should use the vrrp show command to see the IP and the MAC

VRRP Show
Router# show vrrp Ethernet0/1 - Group 1 State is Master Virtual IP address is 10.21.0.10 Virtual MAC address is 0000.5e00.0101 ..............

-  "show arp" will show you if you have 2 MAC's or more with the same IP

- To find the direction, you can search trough the network with "show mac add add aaaa-bbbb-cccc" and you will the the port, channel, etc..

     "show ehterchannel summary" and "show cdp neigh gig" (For example)" helps to find the direction.

Hope that helps or give you some ideas.

best regards,

Sebastian

[paolo bevilacqua]

s.helmer wrote:
-  "show arp" will show you if you have 2 MAC's or more with the same IP

Incorrect, since per standard there cannot be more than one MAC per IP, °show arp° will never show that.

[Sebastian Helmer]

your're right Paolo, Sorry.

Maybe a syslog entry could help.

[jhosa_reddy]

Use the  show arp command

example,

Example:-

Switch#show arp

Protocol  Address          Age (min)  Hardware Addr   Type   Interface

Internet  10.1.1.1                -   00E0.F97A.7958  ARPA   Vlan1

Internet  10.1.1.20               0   0001.64EE.6301  ARPA   Vlan1

Note :- Red result is the duplicate address of two routers and "0" indicates duplicate address

OR

use "show cdp neighbors detail"

Example :-

Switch#show cdp neighbors detail

Device ID: Router
Entry address(es):
  IP address : 10.1.1.20
Platform: cisco C1841, Capabilities: Router
Interface: FastEthernet0/1, Port ID (outgoing port): FastEthernet0/0
Holdtime: 148

Version :
Cisco IOS Software, 1841 Software (C1841-ADVIPSERVICESK9-M), Version 12.4(15)T1, RELEASE SOFTWARE (fc2)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2007 by Cisco Systems, Inc.
Compiled Wed 18-Jul-07 04:52 by pt_team

advertisement version: 2
Duplex: full
---------------------------

Device ID: Router
Entry address(es):
  IP address : 10.1.1.20
Platform: cisco C1841, Capabilities: Router
Interface: FastEthernet0/2, Port ID (outgoing port): FastEthernet0/0
Holdtime: 128

Version :
Cisco IOS Software, 1841 Software (C1841-ADVIPSERVICESK9-M), Version 12.4(15)T1, RELEASE SOFTWARE (fc2)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2007 by Cisco Systems, Inc.
Compiled Wed 18-Jul-07 04:52 by pt_team

advertisement version: 2
Duplex: full

Switch#

[jhosa_reddy]

Switch# show ip arp

Protocol  Address          Age (min)  Hardware Addr   Type   Interface

Internet  10.1.1.1                -   00E0.F97A.7958  ARPA   Vlan1

Internet  10.1.1.20               21  0001.64EE.6301  ARPA   Vlan1

[Ayad]

Hi reddy,

Thanks for your help.

Using the show arp or show ip arp in my router or my catalyst 2960, i see just 3 or 4 mac-address whereas i have many others workstations in the LAN.

is there any possibility or others command to have MAC or IPs addresses of all devices conencted to my LAN ?

Thanks again.

Regards,

AA