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.