stupid arp question

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ASKER

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

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.

SOLUTION

atech1

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ASKER

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

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).

>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.

ASKER

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

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?

SOLUTION

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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#

Open in new window

ASKER

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

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.

SOLUTION

JeffSchaper

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Excuse me...

What was the problem?