ARP cache

[Gary Yu]

Hi all,

I have 2 MC/SG floating IPs to reach from my server, and have 2 entries of the floating IPs in the /etc/hosts file. When the first package failed and switched to the server where the 2nd package runs, the 2 floating IPs actually bind to the same MAC address, so when I tried to reach the first pkg, the ARP cache got updated.

My question is, when the first package swithed back to it's original server(and also the network card), when I connect to it's floating IP, will the ARP cache updated automatically again to the original one or it will still use the one in the cache(the one currently in the ARP cache is still reachable, because it's a valiad one for the other package) -- it's a production environment, I can't emulate the scenario.

thanks,
Gary

[Duncan Edmonstone · Accepted solution]

Yes, your ARP cache will get updated, this is a direct quote from the MCSG Manual:

/start quote/

ARP Messages after Switching

When a floating IP address is moved to a new interface, either locally or remotely, an ARP message is broadcast to indicate the new mapping between IP address and link layer address. An ARP message is sent for each IP address that has been moved. All systems receiving the broadcast should update the associated ARP cache entry to reflect the change. Currently, the ARP messages are sent at the time the IP address is added to the new system. An ARP message is sent in the form of an ARP request. The sender and receiver protocol address fields of the ARP request message are both set to the same floating IP address. This ensures that nodes receiving the message will not send replies.

/end quote/

So your systems ARP cache will get updated *unless* there is some piece of network hardware in the way that doesn't forward on ARP broadcasts (I've seen some wireless RF base stations for example that don't do this)

You can read more on how network switching works here:

http://docs.hp.com/hpux/onlinedocs/B3936-90065/B3936-90065.html

HTH

Duncan

[U.SivaKumar_2]

Hi,
Whenever a LAN switchover occurs , A arp broadcast with ip address and new MAC address is sent to all hosts in the network. This happens automatically even without a arp request. This arp broadcast will update the arp
cache of your client machines immediately of the new mac adddress and it's ip address.

regards.
U.SivaKumar