Highly Available Networks

Figure 5: Separate Subnet Example
In Figure 5, both systems have been configured with two LAN controller
cards and with two separate cables between Server1 and Client1 to create
two subnets. Each LAN card has a different IP address. If the cable
connecting to lan1 fails, this would bring down access to the server’s IP
address 15.0.0.1. To recover, the client could connect to Server1 via the
16.0.0.1 address through lan2.
The client which normally does a telnet 15.0.0.1 would simply do a telnet
16.0.0.1 after a failure on the "15" network. A smart client application
program could automatically try reconnecting via the second IP address.
The tradeoff to the smart application approach is either making the subnet aware or using non-start
applications where user connections are lost. Limitations to the gateway approach include no protection
for LAN card failures and the configuration of multiple gateways can be extremely complex.
Local Failover Functionality of MC/ServiceGuard
Products such as Hewlett-Packard’s MC/ServiceGuard provide a different mechanism for automatically
recovering from LAN failures. MC/ServiceGuard will transparently move the IP address from one LAN
card on Server1 to the other standby LAN card. If the client is using a connection based protocol such as
TCP, or even a connectionless protocol like UDP with a standard data transmission verification
algorithm, the client applications and users will not know a LAN failure had occurred.
MC/ServiceGuard can accomplish a local Ethernet recovery in less than 5 seconds and FDDI in less than
1 second.
Figure 6: MC/ServiceGuard Local Failover Example
Key Points