Highly Available Networks

In Figure 13, routers are added into the star topology. The same rules apply for this configuration as
discussed in "Figure 11: Highly Available Bus Topology with Cross-Connected Routers". The cables
can be replaced with token ring or optical fiber and the hubs replaced with concentrators. Everything
would work the same as discussed for the twisted-pair bus topology.
Highly Available Ring Topologies
As indicated in the first section, DAS FDDI has a self-healing feature which added with the fact that
there is no concentrator in a ring topology means there is no SPOF for the LAN itself. However, the
DAS FDDI card on each system is a SPOF for that server.
One way to react to a failure of a DAS FDDI card is to implement the remote failover feature of
MC/ServiceGuard. If a DAS card fails, the server application and its IP address are moved to the second
server. The client must reconnect, but the outage duration will be minimal. For higher levels of
availability, dual DAS FDDI rings could be implemented. However, the overhead and cost of this
solution compared with the small increase in availability is a difficult tradeoff. Finally, a combination of
a SAS star topology as a primary LAN with a DAS ring as a alternate LAN is another solution.
Example HA Networks
Figure 14: Highly Available Network