Gratuitous ARPs

[Frank_9]

For what purpose does MC/Service Guard use "gratuitous ARPs"?

Are there any special network equipment (router/switch) configurations required to support "gratuitous ARPs" from MC/Service Guard?

Should MC/Service Gaurd match network equipment settings or the other way around for gratuitous ARPs?

Thanks,

Frank

[Steven Sim Kok Leong]

Hi,

A Gratuitous ARP is an unsolicited ARP reply. Routers cache this information by default. There is no router reconfiguration needed for this to work.

Servers in the MC/ServiceGuard cluster use a floating IP and Ethernet/MAC address as well as their real Ethernet and IP address. When the administrators initially setup this floating IP and floating Ethernet address to be shared by the servers in the cluster the servers would send a gratuitous ARP to the router.

The Ethernet frame originating from the server is configured so that the source Ethernet/MAC address is that of the server?s real Ethernet address/MAC. However, the Ethernet frame payload has the server?s floating Ethernet and IP address.

The router will cache the addresses found in the payload (ARP reply). Every time the router sent a frame containing the floating Ethernet/MAC address, the switch would send the frame to the server that initially sent the floating address to the router. This would render the cluster useless.

However by making use of the information within the payload, the switch has no idea which switch port the floating Ethernet address belongs to. As a result, it will send the traffic to all switch ports, thus all the servers participating in the cluster gets to see and respond to the traffic using their real Ethernet address as the source IP address.

Hope this helps. Regards.

Steven Sim Kok Leong
Brainbench MVP for Unix Admin
http://www.brainbench.com

[melvyn burnard]

Just a correction to the previous reply, ServiceGuard does NOT use a floating MAC address, only a floating IP address.
SG also does an ARP rebroadcast whenever it changes the location of the floating ip address, i.e. it moves the ip addres to a different card/system. This is to ensure that the network devices out there know to update their cache with the new ip 2 MAC address combination, otherwise SG does not do "spurious" arps.

[Steven Sim Kok Leong]

Hi Melvyn,

You are right. Thanks for the correction. I got confused with other clustering technologies.

Did a check on my existing ServiceGuard package cntl and cluster ascii configuration files. Other than the floating and real IP addresses, in nowhere was MAC addresses used or stored.

Regards.

Steven Sim Kok Leong
Brainbench MVP for Unix Admin
http://www.brainbench.com