16 - The Cluster Alias Subsystem (CLUA)

Abstract

The alternative to having names for network locations is to always use 32 bit Internet Protocol (IP) addresses to refer to targets. All of the cluster members have network interfaces, and all of the interfaces have IP addresses associated with them. The cluster alias name will have to be associated with an IP address that is not tied to a single network interface on a single member of the cluster; otherwise, the bulk of the cluster network services will be executing on one cluster member. An IP address is usually associated with a 48-bit Media Access Control (MAC) address. The MAC addresses are “burned” onto a PROM chip on the network device to uniquely identify each device. When an IP address is assigned to the network device, the MAC address is associated with the IP address. Somehow, IP address is taken, that is not associated with any particular interface. To avoid the problem of overloading, the member on which the device using the MAC address physically resides, the selected cluster member is deemed as the “proxy ARP master” for the cluster. The proxy ARP master cluster member will be responsible for handling incoming requests for access to the cluster alias and routing the requests to other cluster members where appropriate. The cluster software tries to compensate for this by sending out a gratuitous ARP broadcast to inform all network nodes that they need to replace an entry in their ARP cache. This eliminates the potential havoc caused by the client ARP cache being behind the times. The goal of Virtual MAC is to allow client systems to reference an IP address that resolves to a MAC address that does not change as the proxy ARP mastering responsibilities move from one cluster member to another.