Describing High Availability in Multilayer Switching
High availability: distribution layer best practices

Adopting these best practice recommendations at the distribution layer will support the intent of a providing a highly available and deterministic network.

  • Connect distribution switches with a Layer 3 EtherChannel link.
  • Use equal-cost redundant connections between the distribution and core for fastest convergence and to avoid black holes.
  • Summarization is required to facilitate optimum EIGRP or OSPF convergence. If summarization is implemented at the distribution layer, the distribution nodes must be linked or routing black holes occur.
  • Utilize GLBP/HSRP millisecond timers. Convergence around a link or node failure in the L2/L3 distribution boundary model depends on default gateway redundancy and failover. Millisecond timers can reliably be implemented to achieve sub-second (800 ms) convergence based on HSRP/GLBP failover.
  • Tune GLBP/HSRP preempt delay to avoid black holes. HSRP/GLBP standby peer so that traffic is not dropped while connectivity to the core is established. The delay should be adjusted to ensure that the node is ready to forward traffic before it preempts.
  • The hierarchical campus model implements multiple L3 equal-cost paths and traffic should be load balanced across these paths from the access layer across the distribution and core. The CEF hashing algorithm should be tuned at the core and distribution layers to vary decision input and avoid CEF polarization which can result in under-utilization of redundant paths. Use the default L3 information for the core nodes and use L3 with L4 information for the distribution nodes.