Aggregation Switch vs Core Switch: 4 Key Differences You Must Know

Aggregation Switch vs Core Switch Short answer for searchers: in a standard three-tier campus LAN, the aggregation (distribution) layer concentrates access traffic and enforces policy, while the core layer is a fast, highly available, low-latency backbone. Keep policy at aggregation; keep the core clean and quick.

Three-Tier Campus at a Glance

Access connects endpoints and handles PoE/authentication. Aggregation is the inter-VLAN routing and policy hub. Core provides high-reliability, high-speed transit between aggregation blocks and to the data center/edge.

Difference 1: Roles and Responsibilities

Core: minimal features, maximum reliability and speed. Prioritize fast reroute/ECMP, modular redundancy, and deterministic paths. Avoid spanning-tree dynamics in the core whenever possible.

Aggregation: the policy workhorse. Terminate VLANs/SVIs, perform inter-VLAN routing, apply ACLs and QoS, and often run segmentation (VRF or VXLAN). Default gateways live here; uplinks go northbound to the core.

Difference 2: Performance & Sizing (How to Read the Specs)

• Port mix：
  Aggregation typically concentrates many 1/10/25G downlinks and uplifts with 40/100G. Core leans toward high-count 100/400G line-rate ports.
• Throughput / fabric capacity：
  Backplane (switching capacity) should be greater than or equal to the sum of port line-rates. Treat marketing numbers cautiously—look for per-ASIC details when available.
• Packets per second (pps)：
  For conservative sizing with 64-byte frames (including overhead), a handy rule is:
  pps ≈ bandwidth(Gbps) × 10^9 ÷ 672.
  Use it to sanity-check whether the box can genuinely push line rate.
• Buffers and latency：
  Core targets microsecond-level latency and stable micro-burst absorption. Aggregation values richer QoS, shaping, and flexible buffering to handle varied user/application traffic.
• Table capacities：
  Aggregation cares about ACL TCAM, QoS queues, and policy terms. Core cares about FIB size, routing scale, ECMP paths, and MAC/ARP stability.
• Oversubscription：
  Aggregation aggregates many access switches and often runs N:1 oversubscription (e.g., 240G down to 100G up ≈ 2.4:1). The core carries site-wide load—keep oversubscription low, ideally near 1:1.

Difference 3: Reliability & Architecture

Core: design out single points from day one—dual supervisors/PSUs/fans, ISSU/NSF/SSO for hitless upgrades, sub-second convergence with IGP + ECMP. Chassis or high-end fixed with virtualization is common.

Aggregation: redundant yet change-friendly. Stacking/MC-LAG/VSS-like designs eliminate L2 loops and enable cross-box LAG. VXLAN-EVPN often terminates or bridges here. Keep policy churn from bubbling up into the core.

Practical topology tip: prefer L3 point-to-point links between aggregation and core (IGP + ECMP). Collapse L2 domains below aggregation to speed up convergence and contain faults.

Difference 4: Features & Operations

Aggregation does: identity-based access (802.1X/MAB), inter-VLAN routing (SVIs), ACL/QoS for voice/video/VDI, segmentation (VRF/VXLAN), frequent policy changes.

Core does: fast, simple routing (summarization, ECMP) and minimal policy. It changes rarely and follows a strict baseline.

Aggregation vs Core — At-a-Glance Comparison

At the functional layer, check our differences between Layer 2 and Layer 3 switches

Design Checklist You Can Apply Today

1. Keep L2 below aggregation; run L3 point-to-point + IGP/ECMP northbound.
2. Place SVIs/gateways at aggregation; keep the core gateway-free unless policy requires otherwise.
3. Dual-home access to two aggregation boxes; dual-home aggregation to two core boxes; use MC-LAG/stacking appropriately.
4. In the campus IGP, summarize at the core and use ECMP; integrate BGP to DC/edge as needed.
5. QoS: classify/priority at aggregation; keep core touch-light to avoid bottlenecks.
6. Change management: core changes quarterly-level; aggregation changes more often—always stage and have rollback.

Sizing Math

• Fabric capacity ≈ sum of all port line-rates × 2 (full-duplex rule of thumb).
• pps ≈ bandwidth(Gbps) × 10^9 ÷ 672 (64-byte worst case).
• LAG capacity ≈ per-member bandwidth × member count (mind hashing/flows).
• Oversubscription ≈ total downlink (× concurrency factor) ÷ total uplink.

Common Pitfalls to Avoid

• Letting policy creep into the core—risk and complexity explode; push it back to aggregation.
• Relying on stacking at the core—software faults gain a huge blast radius; prefer chassis/dual-control designs.
• Allowing L2 to bleed into the core—loops/jitter can impact the entire campus.
• Skipping oversubscription math—expect congestion at peaks.
• Moving policy boundaries upward—aggregation gets lighter but the core becomes “dirty,” hurting future operations.

Collapsed-Core Note

Small/medium sites often adopt a collapsed core (aggregation + core in one platform). Balance high-speed backbone requirements with policy scale (port count, table sizes). Keep the same principles: policy where it belongs, transit kept clean.

FAQ

Conclusion

When comparing aggregation switch vs core switch, follow one rule: push policy down to aggregation and reserve the core for fast, resilient transit. That principle produces cleaner designs, faster convergence, and easier day-2 operations—and scales from small collapsed cores to large multi-building campuses.

For management-focused differences, see Aggregation vs Core Switch guide

Cisco Core Switch: (Catalyst 9400/ Catalyst 9500 / Catalyst 9600)

Aggregation Switch: (Catalyst 9300)