Highly Available Networks

Server and client are required to have MC/ServiceGuard installed to provide local switch
functionality if both on same LAN.
Requires two HUBs cross connected for no SPOF
Transparent to Applications
Fast Failover
Requires idle LAN host adapter
As will be discussed in the next section on LAN Topologies, the MC/ServiceGuard approach uses two
hubs connected together to create one LAN with alternate paths. After a failure, the 15.0.0.1 address is
transparently moved to the lan2 controller card. If the LAN is Ethernet, FDDI or TokenRing,
MC/ServiceGuard issues a re-arp to broadcast the new IP to MAC level mapping out to all the clients.
This causes the client to immediately know the new mapping and continue its connection. If the LAN is
IEEE 802.3, this protocol uses proxy instead of arp protocol and has no method of distributing a change
in IP to MAC level mapping; thus, requiring a 90 second timeout prior to the connection being
disconnected. The client will then need to reconnect to the server.
Figure 7: MC/ServiceGuard Remote Failover Example
Another feature of MC/ServiceGuard is subnet failure detection with IP movement to an alternate
system. This is useful if there are no alternate LAN cards available on the server, but there is an alternate
server. As shown in Figure 7, the alternate server’s LAN card becomes the redundant controller, but all
connections are broken. Although the connections are broken, the clients continue to use the same IP
address. They simply reconnect.
Eliminating SPOFs in LAN Topologies
In order for client reconnection and transparent failover to work, the client must have another hardware
path to the server. This requires redundant equipment manage the hardware in the event of failure. This
section will complete the picture by discussing the hardware necessary to create redundant LANs.
Highly Available Bus Topologies