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stupid arp question

I am trying to find information on a situation at work..

We have two device that communicate accross the wan via ethernet

one day they stopped communicating even though any other device can communicate with it.

I decided that we should remove the device which is a hardware device but not a pc without ability to ping. I than said that we should take a laptop configure it to the devices IP address and ping to see whats going on. I was told by the cisco guy that the arp cahce in the routers take 8 hours to update in a network like this

PC ---- Switch --- router ---- Cloud ---- router ---- switch ------ device

if we remove the device we have to wait 8 hours or restart all the equipment because the cisco devices will not be able to route until the arp cache is updated becuase the old MAC even though it is dynamic will not update

I think this is not realistic any instight would be appreciated

thanks
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Commented:
The arp cache could well be set to 8 hours.

However, just powercycling the router would clear the arp cache.  If it is Cisco the timeout is 4 hours normally.

Or you can log into the router and delete the arp entry.

Author

Commented:
This would cause the device being changed to a PC not to route though. we can't afford to power cycle because everything would come down. I just can not understand that cisco would block devices if the IP address is changed

Commented:
The arp cache is there to avoid the need for asking for the MAC address for an IP address all the time.  All devices do this.

So all you need to do is clear the cache (if Cisco):

    clear arp-cache

Or

    no arp <ip-address> <mac> arpa

I would imagine just pinging the router from the new PC would be enough for it to realise the arp entry it has for the IP address currently stored is incorrect.
Commented:
Your cisco guy is correct.

Who knows what's in that cloud in terms of hardware and routing protocols; perhaps dozens of devices whose routing information must be updated. Routers need to know of other routers routing table. There are time tickers involved in each routing exchange transaction and it best to leave it as it is because it's highly impractical to change those time tickers for just one PC. The router on the right hand side must know the device or PC's mac address fefore anything can get to the device or PC. The router uses that mac address to route packets that correspond to the device.

Both the IP address and the MAC address are important here. So you could use the same IP Address for the device and the PC you put in its place, but they will have different MAC addresses.

Author

Commented:
He wouldn't let me test this but if this is correct you are saying that if I took the device off the network and put the latop on the network that the ping request would fail becuase the arp cache would have the devices MAC address this doesn't sound to adequet how would we be able to make sure this is not an issue if he proves to be right

Commented:

Punctuation!

He is right about the arp cache.  Clearing the arp entry would fix the problem.   You can do this by power cycling (which you are unable to do) or by using the commands above (which you have not yet responded to).
Don JohnstonInstructor
Top Expert 2015

Commented:
>one day they  stopped communicating even though any other device can communicate with  it.

So you have one of these devices at each location and they can't talk to each other. But any PC at either location can still communicate with both of these devices?

>I was told by the  cisco guy that the arp cahce in the routers take 8 hours to update in a  network like this

Only if it was changed. By default, The ARP cache aging is set to a 4 hours. As previously stated, "clear arp" will flush the cache.



Author

Commented:
we  have cleared the arp cache once and it did not fix the problem but we did not have antone on site. I wanted to simply put the laptop online with the device IP my query is will the arp cache update with the new Mac address. I thought once a ping request is sent to these devices it would update the arp cache unless they are set to static which this device is not

Commented:
Use more punctuation.

Yes it will.  You can do "sh arp" to see what MAC address it has for the IP and compare against ipconfig /all

Why not tell us the underlying problem is?
Don JohnstonInstructor
Top Expert 2015
Commented:
I suspect that you've got something else going on. When traffic from an IP address that's in the ARP cache starts showing up with a different MAC address, the ARP cache is automatically updated.

For example. This is the ARP cache from a Cisco router. The second output is 30 seconds after the first. The only difference is I changed the MAC address of the 33.2 device.

So let's reassess the situation:

These two devices can't talk to each other but then can communicate with ANY other device? At either location?


Don JohnstonInstructor
Top Expert 2015

Commented:
Forgot the output.



P3R1#sh arp
Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet  192.168.33.1            -   0012.43ac.7020  ARPA   FastEthernet0/0
Internet  192.168.33.2            0   0012.7f80.fa80  ARPA   FastEthernet0/0
P3R1#
P3R1#
P3R1#sh arp
Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet  192.168.33.1            -   0012.43ac.7020  ARPA   FastEthernet0/0
Internet  192.168.33.2            0   0000.0000.1234  ARPA   FastEthernet0/0
P3R1#

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Author

Commented:
the devices where fine one is a server the other a meter with an ethernet connection. the one day they stop communicating we could not test ping from the meter but from the server we saw and verified with AT &T that the packet were getting to the second subnet but no replies were being returned. any other pc on any network could communicate with the meter pings would reply. the cisco guy came to the conclusion it was ARP related and will not let us test the change of machine with one that can acctually ping stating the arp cache will block the new mac from communicating. The junior cisco guys who is now offsite told me last week this is not true
Don JohnstonInstructor
Top Expert 2015

Commented:
Let's come up with some nomenclature to help communicate about this:

Device will the meter
Server will the server (duh)
The device is at location B
The server will be at location A

Please confirm the following:

PC at location B can successfully ping the device
PC at location A can successfully ping the device
PC at location A can successfully ping the server
 PC at location B can successfully ping the server
PC at location A can successfully ping PC at location B
Server can NOT ping device.

 
 
If the 2 devices are on either end of the WAN the arp should not matter that much. The routers LAN MAc address will be used. The MAC address is important when both devices are on the same LAN.

I would ping the device from a  couple of workstations in the same LAN and check the arp tables to see if everything is in order both in the workstation arp tables and switch arp tables.

Also check you don't have a static arp on ther server.
Don JohnstonInstructor
Top Expert 2015

Commented:
Excuse me...

What was the problem?

Author

Commented:
sorry it was acttually a device Moxa Switch on the lan we uplugged it an added a Herscman instead