Highly Available Networks

Figure 8: Highly Available Bus Topology
To eliminate the bus and interface cards as SPOFs, redundant
busses are used. These two separate busses are linked together by
a single hub.
Since clients are usually a PC or workstation and
MC/ServiceGuard does not run on these types of systems, we
must use a segmentation scheme to isolate faults to the smallest
possible area of impact.
In Figure 8 we can see the three servers have local LAN card
switching functionality and will continue to communicate after a
cable break. However, because each client is only connected to
one cable, half the clients would lose communications with the
servers with a cable break.
We can increase segmentation of the cables and decrease the area of impact caused by a cable break, by
adding a second HUB and moving the clients onto multiple cable segments, as shown in Figure 9.
Figure 9: Highly Available Bus Topology using HUBS
In this example, there is still one virtual LAN with separate segments connected by hubs. A cable break
from the PC to the LAN segment could cause a 25% loss of communications between the clients and the
servers. If a HUB or the cable interconnecting one of the HUBs failed, 50% of the PCs would loss their
connection. The highly user induced fault, client interconnect cable, has been decreased from causing
100% loss of communication with a single cable (as in Figure 1) to a 25% loss communication. The only
way to remove the HUBs and interconnecting cable as a large percentage of communication failures, is
to replace them with routers and use the "GateD" protocol. This will be discussed in the next section.
Figure 10: Highly Available Bus Topology with isolated clients using Routers