" Leaf & Spine L3 Design with VXLAN Overlay Networking"
With L3 design you don’t have the 2 switches limitation on the spines level. You can scale vertically and horizontally as many spines and Leafs switches as you want subject to limitation on the ECMP scalability and port availability on the switch’s hardware. ECMP uplinks from the TORS to the spines are for forwarding. Depending on the routing protocols used for your underlay networking determine the cost of reaching your destination Leaves.
Since it is L3 there is no STP involved one thing to take into consideration is that the rest of the networks are typically connected via dedicated Service Leaf Core or Boarder Leaf switches. In a L3 design, it required 2 routing protocols for routes exchange one for the underlay to provide reachability between all switches (Leaf & Spine) in the topology. The best practice is to use eBGP between the Leaf and Spine. Physical interface IPs are used for eBGP for the underlay peering to ensure route withdrawal in case of a link failure.
The second routing protocol is needed to exchange EVPN routes using MP-BGP for your overlay network. EVPN is the control plane function of the VXLAN data plane.
· Benefits allows L2 communication across L3 boundaries.
· High level of Horizontal and Vertical expansion as you grow and scale out
· No STP problems since it pure L3 DC design
· Interoperability in multi-vendor deployment since its open standard base on IEEE & IETF
· Allow ECMP load sharing and deterministic failover paths
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