1. Implementing a Highly Available Network
Implementing High Availability
Implementing a Highly Available Network

2. Layer 2 Distributed VLANs on Access Switches
Not a recommended design — slow convergence
Use only if Layer 2 VLAN spanning flexibility is required
Requires STP convergence for uplink failure and recovery
More complex because the STP root and HSRP should match
May be required for WLAN (standalone APs)

3. Layer 2 Local VLANs on Access Switches
Recommended design, tried and true
VLANs present on one access switch only
Does not require STP convergence for uplink failure recovery
Requires a distribution-to-distribution link for route summarization
Can map Layer 2 VLAN numbers to Layer 3 subnets for ease of use and management

4. Layer 3 Access-to-Distribution Interconnection
Best option for fast convergence, and easy to implement
Uses equal-cost Layer 3 load balancing on all links
Does not require STP for convergence
Does not require HSRP (FHRP) configuration
Does not support VLAN spanning distribution switches

5. Daisy-Chaining Access Layer Switches
Primary and secondary HSRP is active after failure.
Outbound traffic is sent from both HSRP instances.

6. Daisy-Chaining Access Switch Issues
Potential for black holes if no alternative path is provided

7. StackWise Technology Access Switches
StackWise Technology eliminates the daisy-chain issue:
Loopback links are not required.
A Layer 2 link in the distribution is not required.
StackWise switch provides redundancy.
Uplinks can be on different switches within stack.
Modular chassis–based switches can also eliminate the daisy-chain issue.

8. Avoiding Too Little Redundancy
Looped figure-8 topology for VLANs spanning access switches
Blocking on uplink from Access-b.
Initially forwarding traffic from both access switches

9. Impact of Uplink Failure
Traffic from Access A is dropped until HSRP goes active on Distribution B.
Blocking link on Access B takes 50 seconds to move to forwarding.
After STP converges, an HSRP preempt causes another transition. Access B is used as transit for Access A traffic.

10. Summary
Achieving redundancy can be achieved at Layer 2 and at Layer 3, by providing additional redundant paths between devices.
StackWise Technology can be used to join several physical switches into one virtual switch.
Redundancy is a balance between too much redundancy, which adds complexity to the network structure, and too little redundancy, which creates single points of failure.
When uplinks fail, convergence path as well as convergence time must be taken into account to evaluate the impact of the failure on the network infrastructure.