SD-WAN vs. MPLS for a Retail Chain
A 120-store retail chain runs MPLS at each store (20 Mbps) back to two data centers. Stores also have a 100/20 Mbps cable circuit used only for guest Wi-Fi. POS transactions are latency-sensitive (target under 80 ms to the primary data center), and nightly backups push 15 GB per store. MPLS costs $900/store/month; cable is $120. The CIO wants to cut WAN costs by 40% while keeping POS reliable and improving backup speed. Stores are spread across three regions with varying cable quality.
1. Evaluate replacing MPLS with an internet-only SD-WAN design versus a hybrid approach.
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Internet-only SD-WAN saves the most (cable at $120 vs. $900, roughly 87% per store) but puts POS on a single best-effort path. With 20 Mbps upstream on cable and variable regional quality, latency to the data center may exceed 80 ms and jitter can break POS sessions. One circuit is also a single point of failure.
Hybrid (recommended): keep a small MPLS or business-fiber circuit as the primary POS path and use cable as secondary/active-active for backups and guest. SD-WAN policy routes POS over the lower-latency path with sub-second failover; backups use the cable path with rate limiting so they do not starve POS.
Cost check: MPLS at $900 plus cable at $120 is $1,020; a hybrid with a cheaper 10–20 Mbps business circuit (say $400) plus cable is $520 - about 49% savings, meeting the 40% target while preserving POS quality. Pure internet-only is only justified in regions where measured latency and jitter meet the 80 ms target and a second circuit exists.
2. How would you handle the 15 GB nightly backups across 120 stores?
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15 GB per store × 120 = 1.8 TB per night. At 20 Mbps upstream, one store's backup takes about 100 minutes - acceptable overnight if staggered, but it will contend with POS on a shared circuit.
Design:
- Stagger backup windows across stores (e.g., 10 stores per hour) so WAN aggregation links and data-center ingest are not overwhelmed.
- Dedicate the cable circuit to backup traffic via SD-WAN policy, with QoS capping backup at ~80% of upstream so POS retains headroom.
- Enable deduplication/compression at the store appliance; retail backup sets compress well, often 3–5:1.
- Verify data-center ingest capacity: 1.8 TB over an 8-hour window needs about 500 Mbps sustained - confirm the backup target can absorb it.
- Consider local caching with changed-block backup to cut nightly volume dramatically after the first full backup.