Describing High Availability in Multilayer Switching
Benefits and Drawbacks of redundant network topology

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.