1.2.b Benefits and limitations of Catalyst SD-WAN solution

What this objective tests

SD-WAN is useful because it turns the WAN into a centrally managed overlay. It is limited because the overlay still runs on real circuits, real routing, and real operational skill.

Benefits

• Transport independence: MPLS, broadband, dedicated Internet access, and LTE all participate.
• Centralized policy: define intent once and push it to all sites.
• Application-aware routing: traffic prefers a path based on loss, latency, jitter, or policy.
• Segmentation: service VPNs keep routing domains separate across the WAN.
• Faster branch rollout: templates and zero-touch provisioning reduce repetitive configuration.
• Better visibility: tunnel, application, and path statistics are collected centrally.

Limitations

• The underlay still matters. Bad circuits give you a bad overlay.
• Controllers matter. Design for reachability, redundancy, certificates, and failure behavior.
• Brownfield migration is hard. Existing routing, firewalls, NAT, and QoS do not disappear.
• Encryption and tunnel overhead decrease MTU and throughput.
• Centralized policy permits centralized mistakes. One bad template can break many sites.
• Teams must learn a new troubleshooting order: underlay, control plane, overlay policy, application.

Troubleshooting order

Use this order when a branch reports "SD-WAN is down."

1. Is the transport interface up?
   show ip interface brief
   show ip route vrf 0

2. Are control connections up?
   show sdwan control connections
   show sdwan control connections-history

3. Is OMP exchanging routes?
   show sdwan omp peers
   show sdwan omp routes

4. Are data tunnels healthy?
   show sdwan bfd sessions
   show sdwan tunnel statistics

5. Is policy steering traffic as expected?
   show sdwan app-route stats

Lab: benefit versus limit

Draw two branches with two transports. Assign voice to MPLS, bulk data to Internet, and guest traffic to a separate VPN.

Do these steps:

1. Write what happens when MPLS fails.
2. Write what happens when the Internet circuit has high loss.
3. Write what happens when a controller is unreachable after the edge learned routes.
4. Attach one proof command to each failure case.

The lab passes when each failure case has an expected behavior and a command that proves it.

Exam traps

• SD-WAN does not guarantee cheaper, faster, or more secure. The design decides the result.
• A working overlay does not prove a clean underlay. Check the transports first.

Pass check

You are ready when you can name two benefits, two limits, and the five-step troubleshooting order from memory.