[lug] Switching IP's

[John Hernandez]

This is completely transparent and automatic.  To illustrate what arp 
does and how it works, here's a little quote from Daryl's TCP/IP primer:

'In order for two Ethernet-attached stations to communicate with each 
other via IP, they must know the MAC address of each other. If station 
"A" knows the IP address of station "B", and knows station "B" is on 
the same subnet, station "A" will issue an Address Resolution Protocol 
(ARP) broadcast. An ARP broadcast is a message that says, "Who out 
there is 192.168.1.1?" The TCP/IP software running on the workstation 
or router at 192.168.1.1 is responsible for sending back an ARP 
response that says, "I am 192.168.1.1, and my MAC address is 
08:00:09:AF:24:33." All stations keep an ARP cache with the MAC and IP 
addresses of all the stations it recently communicated with directly. 
Try the command "arp -a" sometime on a UNIX or Windows workstation; on 
a Cisco router, the command is "show arp".'

As an exercise, try using the linux 'arp' command to identify (arp -a) 
and delete (arp -d) a specific entry from your linux box's arp cache. 
Then ping the deleted station, and dump the arp cache again.  Observe 
that it learns the address again by using broadcast to ask for it.  You 
can even do this while running a packet sniffer to see how it happens 
on the wire.

Greg Horne wrote:

> Will I need to reboot any of my computers after the command 'clear
> arp-cache' is used so that they will be discovered by the router?  Or
> will they be recognized automatically?
> 
> Thanks for the help,
> Greg
> 
> 
>>-----Original Message-----
>>From: lug-admin at lug.boulder.co.us [mailto:lug-admin at lug.boulder.co.us]
>>
> On
> 
>>Behalf Of John Hernandez
>>Sent: Tuesday, January 08, 2002 8:32 AM
>>To: lug at lug.boulder.co.us
>>Subject: Re: [lug] Switching IP's
>>
>>It will cause the router to rebuild it's arp table from scratch, but
>>that's nothing to be concerned about.  This should be practically
>>instantaneous and unnoticeable on a healthy network.  It's normal
>>behavior for a station that joins the network.
>>
>>Greg Horne wrote:
>>
>>
>>>Ah, that sounds juicy.  I'll look into that one.  Will it do
>>>
> anything
> 
>>>else that I should be aware of?  More importantly, if I do issue
>>>
> this
> 
>>>command how long will it take for my other servers (DNS, SMTP, ...)
>>>
> to
> 
>>>be recognized by the router?
>>>
>>>Thanks for the help,
>>>Greg
>>>
>>>
>>>
>>>>-----Original Message-----
>>>>From: lug-admin at lug.boulder.co.us
>>>>
> [mailto:lug-admin at lug.boulder.co.us]
> 
>>>On
>>>
>>>
>>>>Behalf Of John Hernandez
>>>>Sent: Monday, January 07, 2002 4:06 PM
>>>>To: lug at lug.boulder.co.us
>>>>Subject: Re: [lug] Switching IP's
>>>>
>>>>Get the router's enable password and issue:
>>>>
>>>>clear arp-cache
>>>>
>>>>-John
>>>>
>>>>Greg Horne wrote:
>>>>
>>>>
>>>>
>>>>>That's what I figured too, but I wasn't sure if there was a
>>>>>
>>>>>
>>>different
>>>
>>>
>>>>>solution.  Uptime at my company is very important though.  Would it
>>>>>
>>>>>
>>>be
>>>
>>>
>>>>>possible for me to make the necessary changes without rebooting?
>>>>>
>>>>>
>>>(I'm
>>>
>>>
>>>>>hoping :) )
>>>>>
>>>>>Thanks,
>>>>>Greg
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>>-----Original Message-----
>>>>>>From: lug-admin at lug.boulder.co.us
>>>>>>
>>>>>>
>>>[mailto:lug-admin at lug.boulder.co.us]
>>>
>>>
>>>>>On
>>>>>
>>>>>
>>>>>
>>>>>>Behalf Of D. Stimits
>>>>>>Sent: Monday, January 07, 2002 3:52 PM
>>>>>>To: lug at lug.boulder.co.us
>>>>>>Subject: Re: [lug] Switching IP's
>>>>>>
>>>>>>Greg Horne wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>>> Part 1.1    Type: Plain Text (text/plain)
>>>>>>>         Encoding: 7bit
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>Perhaps the cisco has the mac address memorized and does not
>>>>>>
> believe
> 
>>>>>>
>>>>>the
>>>>>
>>>>>
>>>>>
>>>>>>linux machine is correct. Try cycling the power on the cisco with
>>>>>>
>>>>>>
>>>the
>>>
>>>
>>>>>NT
>>>>>
>>>>>
>>>>>
>>>>>>box disconnected or off.
>>>>>>
>>>>>>D. Stimits, stimits at idcomm.com
>>>>>>_______________________________________________
>>>>>>Web Page:  http://lug.boulder.co.us
>>>>>>Mailing List: http://lists.lug.boulder.co.us/mailman/listinfo/lug
>>>>>>
>>>>>>
>>>>>>
>>>>>_______________________________________________
>>>>>Web Page:  http://lug.boulder.co.us
>>>>>Mailing List: http://lists.lug.boulder.co.us/mailman/listinfo/lug
>>>>>
>>>>>
>>>>>
>>>>--
>>>>
>>>>  - John Hernandez - Network Engineer - 303-497-6392 -
>>>> |  National Oceanic and Atmospheric Administration   |
>>>> |  Mailstop R/OM12. 325 Broadway, Boulder, CO 80305  |
>>>>  ----------------------------------------------------
>>>>
>>>>_______________________________________________
>>>>Web Page:  http://lug.boulder.co.us
>>>>Mailing List: http://lists.lug.boulder.co.us/mailman/listinfo/lug
>>>>
>>>>
>>>_______________________________________________
>>>Web Page:  http://lug.boulder.co.us
>>>Mailing List: http://lists.lug.boulder.co.us/mailman/listinfo/lug
>>>
>>>
>>
>>--
>>
>>   - John Hernandez - Network Engineer - 303-497-6392 -
>>  |  National Oceanic and Atmospheric Administration   |
>>  |  Mailstop R/OM12. 325 Broadway, Boulder, CO 80305  |
>>   ----------------------------------------------------
>>
>>_______________________________________________
>>Web Page:  http://lug.boulder.co.us
>>Mailing List: http://lists.lug.boulder.co.us/mailman/listinfo/lug
>>
> _______________________________________________
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> 


-- 

   - John Hernandez - Network Engineer - 303-497-6392 -
  |  National Oceanic and Atmospheric Administration   |
  |  Mailstop R/OM12. 325 Broadway, Boulder, CO 80305  |
   ----------------------------------------------------