A short & sharp comparison of Layer 3/ECMP Leaf & Spine vs. Layer 2 Multi-chassis Link Aggregation, e.g. Dell EMC VLT (Virtual Link Trunking) Topologies.
| Spine & Leaf (Layer 3) | VLT/vPC/MCLAG (Layer 2) | |
| Interoperability | Built on Open/standards based Routing Protocols.
In theory, each layer can be multivendor as long as it supports ECMP/Routing Protocol of choice. |
Proprietary. Lock-in to the respective vendor. |
| Scalability in the Core | Very scalable. Total limited only by number of spine ports, oversubscription desired, & number of interconnections between them. | Limited to 2. |
| Traffic Pattern | East West optimized | With SVIs in Core, Flatter/bigger broadcast domains. Could be identical to SL |
| Core Congestion | Less impact due to flatter design | More impact as tiered/hierarchy.
ARP response and L2/L3 protocol Scale handled by Core. |
| Latency | All devices at equal hops from each other, so predictable latency | Depends. Can be identical to SL. |
| Routing/Switching | ECMP. Predictable. Load balancing.
Additional IP Address complexity for P2P links as these are L3 Protocol choice could increase Core overhead with scale (OSPF/Link State) |
Layer 2 Port Channel LAG groups. Active Active uplinks
No ip address complexity for P2P links as these are L2. |
| FHRP/Gateway Redundancy | In Layer 3 Spine/Leaf, FHRP in Access Layer/ Leaf ToR | FHRP in Core/Agg Typically.
(still possible to use a model with routed access layer) When using of FHRP/GW redundancy, ensure L2/L3 roots are aligned. |
| L2 Fault Domain (VLAN/STP) | In Layer 3 Spine/Leaf, Local to Rack. Stops at Rack ToR pair. | When VLAN/Broadcast domain SVIs in the Core, it Spans the Network. |
| Host Mobility + Multitenancy | No, unless Overlay/Encapsulation like VXLAN | Yes |
| Upgrade path | Pay as you grow
|
Forklift – higher capacity/capability switches |
| Cabling Complexity | Significant, and increases with the number of units | Consistent, minimal. |
| Maintenance | No downtime | No downtime |
| Failure Scenarios | Resilient layers. Can sustain multiple unit outages. | Split Brain possible for geographically dispersed Pairs. If manifested, would severely impact routing/forwarding. |
| Ideal Use | Scale out for both bandwidth and edge ports. (spine & Leaf layers), Layer 3 Multi-path | Simplicity, Layer 2 Multi-pathing with Active Active LAG group & Dual Active Control planes. |
Hasan Mansur 