An IP fabric with a leaf-spine architecture provides full forwarding and scalability through a non-blocking design. Leaf devices connect to access spines which aggregate traffic. Overlays create virtual tunnels for tenant communication using technologies like VXLAN that encapsulate Layer 2 frames in UDP packets, enabling Layer 2 adjacency across IP networks. VXLAN segments traffic using VXLAN Network Identifiers (VNIs) associated with VLANs to extend Layer 2 networks across hosts.

1. Leaf Spine Network and VXLAN
IP Fabric serving as the underlay to interconnect all devices, with an open standard design, provides Full
Way forwarding, scalable and non-blocking spine/ leaf architecture. Leaf spine roles assigned to fabric
network devices transport the overlay tunneled traffic. Leaf devices provide connectivity to access spine
devices which act as an aggregation point.
Being a Open standard provides / allows for cross vendor support.
Popular choices of routing protocols to provide for the control plane of an IP Fabric include:
IGP (OSPF etc.)
iBGP with RR Design can provide better performance and increased scale.
eBGP inherent loop avoidance with AS_PATH and multipath capabilities. Each Leaf/ Spine can be in a
separate AS resulting in distributed routing domains. Better engineering capabilities make the migration of
traffic easier through BGP attribute manipulation, to reroute the traffic to other Spine than under
maintenance. There are configuration complexities involved with policies and peers, however automation
tools can facilitate provisioning for the same.
Overlay networks create the communication channels between tenant applications / services through
creation of the logical / virtual tunnels.
These can support L2 – L2 overlays – extend L2 network so VMs can share the same IP subnet and L3
overlays – share L3 networks.
VXLAN is a networking technology that encapsulates MAC-based Layer 2 Ethernet frames within layer 3
UDP packets to aggregate and tunnel multiple layer 2 networks across a layer 3 infrastructure. VXLAN
enables layer 2 adjacency across IP networks.
Unlike Classical Data Centre architecture with Tenant Design, we used to segment using VRFs,VLANand
downstream Dot1Q tagged trunks with IP Forwarding (Providing North South Isolation/ Segregation)
network until the payload Aggregation switches, which are either running a Layer 2 Overlay, like TRILL
or Fabric Path providing upto 16 Way full way IP Forwarding for the East West Traffic to be distributed to
TOR Switches (from End of the Row distribution) and finally to the Virtualized workload VMs.
With VXLAN Architecture, you extend a Layer2 network by connecting VLANs from multiple hosts
through UDP Tunnels called VXLAN segments. VXLAN segments are identified by a VNI. Within a host,
each VLAN whose network is extended to other hosts is associated with a VNI.

2. VXLAN network assignments:
 VTEPs with IP Addresses 10.10.1.1, 10.20.2.2, 10.30.3.3 UDP port 4789
 VTIs that supports VXLAN Segments (UDP Tunnels): VNI 200, VNI 2000, VNI 20000
 Vlans Three VLANs can communicate with remote devices over layer 2.
Traffic flows in The Evolved IP Fabric based DC:

3. East-West (traffic across PODs within the DCs):
 Layer 2 (Intra-VRF Intra-Subnet) traffic between hosts in the same subnet for given tenant.
 Layer 3 (Intra-VRF-Inter-subnet):Traffic between hosts in different subnets for a given tenant.
 Layer 3 Security Insertion (Inter-VRF-Inter-Subnet): Traffic between hosts in different subnets
for different tenants and is manually service chained through a firewall NGFW (SRX Service
block).
North-South (Traffic between hosts across the WAN and within the DC):
Access:
 Two leaf devices are acting as pair of redundant Top of Rack switches.