Highly Available Networks

Figure 4: Ring Topology: DAS FDDI without concentrators
The two most common ring topologies are Token Ring and
DAS FDDI. This paper will focus on DAS FDDI.
Unlike the SAS configuration, there is no need for a
concentrator except to connect to SAS based systems. In
addition, DAS FDDI has a self-healing capability. If there is a
segment failure, DAS can wrap the failure and send the data in
the other direction. Therefore, as shown in Figure 4, there is no
SPOF that would bring down the entire LAN. However, like
SAS, the DAS controller card on each system is a SPOF for
that system to the LAN. DAS FDDI is most commonly used
for server to server LAN connections.
Eliminating Single Points of Failure in a Local Area Network
Now that we have discussed some of the SPOFs in a network, the question is "how can we protect
against these types of failures?". First let us list the two requirements we want to meet:
The ability for the client and server to react then recover from a LAN fault.
Provide redundant equipment and control software to manage the hardware to meet the first
requirement.
The key is to ensure all SPOFs are protected and failover/recovery is automatic.
Client Recovery
Client recovery during a network outage can involve a procedure as simple as the user reconnecting to a
different address to something as advanced as automatic LAN recovery. First, manual and programmatic
techniques will be discussed. Then, the benefits of automatic LAN recovery with MC/ServiceGuard will
be discussed.
Multiple subnets and gateway protection
The client can react and recover from a network outage in a number of ways.
Two separate subnets can be configured between the server and clients. Server and client
applications can be programmed to switch communications to an alternate subnet IP address in the
event the first IP address connection times out.
Another method is to configure the server and clients as gateways between the two subnets. This
automatically protects against cable faults that occur between systems, but not against controller
card faults.