In enterprise data center design, selecting the appropriate High Availability (HA) mechanism at the aggregation/core layer depends directly on whether the downstream access tier operates at Layer 2 or Layer 3.
When designing a traditional two-tier data center topology where the Top-of-Rack (ToR) access switches operate strictly as Layer 2 forwarding entities, default gateway interfaces (SVIs) and inter-VLAN routing must terminate at the core layer. In this architecture, Aruba best practices recommend implementing Virtual Switching Extension (VSX) across the pair of core switches. Deploying VSX at the core allows the downstream Layer 2 access switches to connect to both core switches via multi-chassis link aggregation groups (VSX LAGs). This design creates a loop-free, active-active topology that eliminates blocked Spanning Tree Protocol (STP) links, maximizes link utilization, and provides sub-second failover using VSX active-gateway technology for downstream servers.
Conversely, when the access layer is deployed with EVPN-VXLAN or routed Layer 3 OSPF, equal-cost multi-pathing (ECMP) natively provides redundancy and load distribution, making core-level VSX multichassis clustering unnecessary.
Reference Topics: HPE Data Center Design – Two-Tier vs. Three-Tier Architectures; Applying VSX at the Layer 2/Layer 3 Core Boundary.
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