Routed access-layer campus network using L3 switching and multi-area OSPF — replacing traditional L2 access–distribution trunks with routed inter-switch links for simplified convergence and fault isolation. Based on the Cisco Routed Access Design Guide.
| Layer | Components | Role |
|---|---|---|
| Core | L3 switches | OSPF Area 0 backbone, inter-area routing |
| Distribution | L3 switches | Area border routers, route summarization toward core |
| Access | L3 switches | SVIs per VLAN, local routing, DHCP relay, OSPF stub areas |
| Server | Infrastructure/App VLANs | HSRP for gateway redundancy (only tier requiring FHRP) |
| Traditional L2 Access | Routed Access |
|---|---|
| STP required between access and distribution | STP only used for host-facing segments |
| Blocked redundant links | All uplinks actively forward traffic |
| HSRP/VRRP needed at every distribution pair | FHRP only needed for server VLANs |
| Trunk management overhead | No inter-switch trunks (L3 point-to-point) |
| Broadcast storms can propagate across tiers | Faults contained to local subnet |
| Extended STP convergence times | Sub-second OSPF convergence |
Trade-offs: Higher upfront cost (L3-capable access switches), more complex initial design, and L2 adjacency not available across routed segments.
| Component | Addressing |
|---|---|
| Access VLANs | Unique /24 per VLAN (e.g., VLAN 10: 10.10.10.0/24, VLAN 20: 10.10.20.0/24) |
| Server VLANs | VLAN 100 (Infrastructure), VLAN 200 (Applications) — HSRP-protected |
| Loopbacks | Summarization anchors (10.10.0.0/16, 10.20.0.0/16) |
| DHCP | Relay via ip helper-address under access SVIs |
OSPF Area Design:
| Area | Assignment | Type |
|---|---|---|
| 0 | Core-facing interfaces | Backbone |
| 10 | Access-layer networks (site 1) | Stub |
| 20 | Access-layer networks (site 2) | Totally stubby |
Route summarization applied at distribution toward the core to reduce LSDB size and SPF computation overhead. Loopback interfaces configured as passive.
Routing — OSPF with per-area stub configuration. Inter-switch links are routed (no switchport); trunks retained only for server VLAN distribution where HSRP is required.
Redundancy — HSRP on VLAN 100 and 200 with priority and preemption. No FHRP needed at the access layer since the local L3 switch is the default gateway.
Spanning Tree — Rapid PVST+ on access switches for host-facing redundancy only. No STP dependency between network tiers.
SVIs — Configured on access switches (not distribution), enabling local routing decisions and faster convergence.
- Simulated link failures showed fault containment to the affected subnet with no propagation to adjacent areas.
- All uplinks remained active and forwarding during steady-state operation.
- OSPF reconvergence completed within expected timers on simulated topology changes.
- No broadcast storms or STP instability observed during failure scenarios.