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.|