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firefox111_2000
Beginner

3750 has IP Conflict with another device

Hello.  I just found out that one of our 10 3750's has been assigned with an IP Address that is in conflict with a PC that has been in production long before this 3750 was deployed in our environment.  Good thing it is servicing  devices on a separate vlan.  The conflict IP Address is assigned to the default vlan of the switch so we can connect to it from the core.

If I change the IP Address of the switch, will it reboot and momentarily disconnect the devices connected to it?

Thanks in advance.

2 ACCEPTED SOLUTIONS

Accepted Solutions
John Blakley
Advisor

Carlos,

Changing the address of the switch won't cause a reboot. The problem you might see is if any of your clients is using the address as a default gateway. In that case, you could have several clients lose access to remote networks.

HTH,

John

HTH, John *** Please rate all useful posts ***

View solution in original post

Carlos,

Question: if there is a user from VLAN 975 that is accessing a VLAN 8   device connected to this switch, witll they be disconnected when I   change the IP Address of the VLAN 975 of the switch?

They'll be disconnected if they're using it for their default gateway.

If I ping the switch's VLAN 975 IP Address, success rate is zero.  Is this normal if there is an IP conflict?

Usually, no. What happens if you do a "show arp | inc "? Does it show up in the arp table at all? Otherwise, you should be getting a response from the local address.

HTH,

John

HTH, John *** Please rate all useful posts ***

View solution in original post

14 REPLIES 14
John Blakley
Advisor

Carlos,

Changing the address of the switch won't cause a reboot. The problem you might see is if any of your clients is using the address as a default gateway. In that case, you could have several clients lose access to remote networks.

HTH,

John

HTH, John *** Please rate all useful posts ***

View solution in original post

John,

Thank you.  All the devices attached to this switch are on VLAN 8.  The conflicted IP Address is on VLAN 975 (my apologies - I mentioned default VLAN).  The core, where most of the users (and servers) are connected, may access VLAN 8 devices from VLAN 975 from a core port.  Question: if there is a user from VLAN 975 that is accessing a VLAN 8 device connected to this switch, witll they be disconnected when I change the IP Address of the VLAN 975 of the switch?

Thanks again.

P.S.  I am able to telnet to the switch using the VLAN 8 IP address of it.  If I ping the switch's VLAN 975 IP Address, success rate is zero.  Is this normal if there is an IP conflict?  The switch will not even reply, even though I'm telneted to it via the other VLAN?

Again, thanks for your input.

Carlos,

Question: if there is a user from VLAN 975 that is accessing a VLAN 8   device connected to this switch, witll they be disconnected when I   change the IP Address of the VLAN 975 of the switch?

They'll be disconnected if they're using it for their default gateway.

If I ping the switch's VLAN 975 IP Address, success rate is zero.  Is this normal if there is an IP conflict?

Usually, no. What happens if you do a "show arp | inc "? Does it show up in the arp table at all? Otherwise, you should be getting a response from the local address.

HTH,

John

HTH, John *** Please rate all useful posts ***

View solution in original post

John,

When I do a "show arp", it gives me a list of all devices from Vlan8 under the "Interface" column, and one from Vlan975, which has mac address of the switch.  The MAC address of the conflicting PC does not show on the arp table.

Note that when I do a "show ip route", the routing table is empty.  And the person who configured this switch did not even set up a default gateway.

So, the switch is configured as L2? Can you do a "sh run int vlan975" and post that? Take addresses out if you need to. If it doesn't come back with anything at all, then the conflict isn't going to be with this switch if you don't have an address configured on it.

HTH, John *** Please rate all useful posts ***

John, this is the detailed vlan975 interface info:

MY-3750#show run int vlan975

Building configuration...

Current configuration : 65 bytes

!

interface Vlan975

ip address 192.168.0.208 255.255.255.0

end

MY-3750#show interface vlan975

Vlan975 is up, line protocol is down

  Hardware is EtherSVI, address is 0027.0ccd.0cc1 (bia 0027.0ccd.0cc1)

  Internet address is 192.168.0.208/24

  MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec,

     reliability 255/255, txload 1/255, rxload 1/255

  Encapsulation ARPA, loopback not set

  ARP type: ARPA, ARP Timeout 04:00:00

  Last input never, output 2y14w, output hang never

  Last clearing of "show interface" counters never

  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0

  Queueing strategy: fifo

  Output queue: 0/40 (size/max)

  5 minute input rate 0 bits/sec, 0 packets/sec

  5 minute output rate 0 bits/sec, 0 packets/sec

     0 packets input, 0 bytes, 0 no buffer

     Received 0 broadcasts (0 IP multicasts)

     0 runts, 0 giants, 0 throttles

     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored

     31 packets output, 1984 bytes, 0 underruns

     0 output errors, 0 interface resets

     0 output buffer failures, 0 output buffers swapped out

MY-3750#

Carlos,

It says that the line protocol is down. Do you have any hosts in that vlan at the moment? And can you do a "sh vlan"?

HTH, John *** Please rate all useful posts ***

John,

There is no host in vlan 975 in this switch.  Below is a snippet of the "sh vlan":

10   VLAN0010                         active

11   VLAN0011                         active

15   VLAN0015                         active

16   VLAN0016                         active

20   VLAN0020                         active

30   VLAN0030                         active

32   VLAN0032                         active

33   VLAN0033                         active

100  VLAN0100                         active

975  VLAN0975                         active

1002 fddi-default                     act/unsup

1003 token-ring-default               act/unsup

1004 fddinet-default                  act/unsup

VLAN Name                             Status    Ports

---- -------------------------------- --------- -------------

1005 trnet-default                    act/unsup

MY-3750#

Carlos,

If the vlan 975 is the problem address and no one is in that vlan, it's safe to change that ip anytime without worrying if someone will get disconnected. The line protocol is down which generally indicates no ports are associated with the vlan that are in the UP state.

Sent from Cisco Technical Support iPhone App

HTH, John *** Please rate all useful posts ***

Thank you. I'm changing it now from home.

---

Sent from my wifi-only iPhone.

Thank you. I'm changing it now from home.

---

Sent from my wifi-only iPhone.

I changed the IP Address of Int Vlan975 to 192.168.0.206 but status still shows "Vlan975 is up, line protocol is down".  I still cannot telnet to the Vlan 975 of that switch.  What am I missing? 

Carlos,

If there's nothing in vlan975 that's connected and up, the switch won't show UP/UP. It's going to show Up/Down until a host that's assigned to the vlan that comes up. This is also the reason you can't ping the address when you're telnetted into vlan 8. I'm woondering if vlan 975 may have been used in the past, but since then someone has migrated all hosts to another vlan or was using this vlan as a test vlan. The vlan itself won't be active without a host in it. Here's an example:

Router#sh vlan-swit

VLAN Name                             Status    Ports

---- -------------------------------- --------- -------------------------------

1    default                          active    Fa1/0, Fa1/1, Fa1/2, Fa1/3

                                                Fa1/4, Fa1/5, Fa1/6, Fa1/7

                                                Fa1/8, Fa1/9, Fa1/10, Fa1/11

                                                Fa1/12, Fa1/13, Fa1/14, Fa1/15

975  VLAN0975                         active  

Router#sh int vlan 975

Vlan975 is up, line protocol is down

  Hardware is EtherSVI, address is c418.7763.0000 (bia c418.7763.0000)

  Internet address is 192.168.50.1/24

I have a router connected to f1/1, so I'll add that port to vlan 975:

Router(config-if)#switchport access vlan 975

Router(config-if)#

*Mar  1 00:01:51.995: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan975, changed state to up

Router(config-if)#do sh int vlan 975

Vlan975 is up, line protocol is up

  Hardware is EtherSVI, address is c418.7763.0000 (bia c418.7763.0000)

  Internet address is 192.168.50.1/24

Since you don't have any hosts on the vlan, it makes sense as to why you couldn't ping the address for vlan 975 while connected to the switch:

Router(config-if)#do ping 192.168.50.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 192.168.50.1, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms

Above I could ping while the host was in the vlan. Below I'll remove it, and notice that the line protocol goes back down:

Router(config-if)#no switchport access vlan 975

Router(config-if)#

*Mar  1 00:03:07.555: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan975, changed state to down

Now I can't ping the local address for vlan 975:

Router(config-if)#do ping 192.168.50.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 192.168.50.1, timeout is 2 seconds:

.....

You have to have at least one member in the vlan up in order for the vlan itself to be up and functional.

HTH,

John

HTH, John *** Please rate all useful posts ***