what triggers pc's to arp for new gateway mac address

carl_townshend · ‎08-06-2010

Hi all

I was wondering, if I change the router for a new one, which will obviously have a new mac address, on a windows 2k server etc, what will make the server arp for the new address when I swap out the router ?

cheers

Tharak Abraham · ‎08-06-2010

Carl,

The first packet in the stream inbound to the server makes the router "arp" for the mac-add of the server..(for eg:- a tcp syn)

i.e you try to access the server which is behind the new router, the router would send an arp out of its Gig/Fa/Eth interface connected to the subnet.

The server would respond to this and both the parties would update their tables.

carl_townshend · ‎08-06-2010

but what about when the client first tries to connect to the gateway, if the client has the old mac address of the router in its table how does the client update its local table ?

Tharak Abraham · ‎08-06-2010

Carl,

Yes thats right, the client would never know the Mac changed.

1.The arp cache in the client has to be timed out and it will happen if no one replies to the frames forwarded with the old Mac.

So the answer is arp cache getting timed out or manually delete the entry eg: arp -d

Dynamic Arp cache entries have a time-out value associated with them to remove entries in the cache after a specified period of time.

For Windows 2000 its given a maximum time of 10 minutes before being removed.

2.Gratuitous Arp if deployed at the gateway device would help in the other way round.