A complementary way to build highly available networks is to provide
redundancy in the links between devices in the network topology. In the campus
network design shown in the figure, there is a backup for every link and for
every network device in the path between the client and server. Using network
links to supplement devices fault tolerance has these advantages:
- The network elements providing redundancy can be geographically disparate.
This reduces the probability that problems with the physical environment will
interrupt service.
- Software errors and changes can be dealt with separately in the primary and
secondary forwarding paths without completely interrupting service.
- Device-level fault tolerance can be concentrated in the Building Core and
Building Distribution layers of the network where a hardware failure would
affect a larger number of users. By partially relaxing the requirement for
device-level fault tolerance, the cost per network device is reduced. To some
degree, this offsets the requirement for more devices.
- Redundant links provided for fault tolerance can be used to balance the
traffic load between the respective layers of the network topology (that is,
the Building Access to the Building Distribution also Distribution to Core)
during times of normal operation. Therefore, network-level redundancy can also
provide increased aggregate performance and capacity.
- Redundant resources can be configured to fail over from primary to
secondary facilities automatically. Failover times can be as low as sub-second,
depending on the failure mode.
- Fast EtherChannel and Gigabit EtherChannel provide both fault-tolerance and
high-speed links between switches with minimal convergence times in the event
of link loss.