Cisco Nexus 7000 Series Configuration Manual page 48

Introducing IP Fabric Overlays (VXLAN)
VXLAN as the overlay technology
VXLAN is a MAC in IP/UDP overlay that allows layer 2 segments to be stretched across an IP core. All the
benefits of layer 3 topologies are thereby available with VXLAN including the popular layer-3 ECMP feature
for efficient traffic spread across multiple available paths. The encapsulation and decapsulation of VXLAN
headers is handled by a functionality embedded in VXLAN Tunnel End Points (VTEPs). VTEPs themselves
could be implemented in software or a hardware form-factor.
VXLAN natively operates on a flood and learn mechanism where BU (Broadcast, Unknown Unicast) traffic
in a given VXLAN network is sent over the IP core to every VTEP that has membership in that network.
There are two ways to send such traffic: (1) Using IP multicast (2) Using Ingress Replication or Head-end
Replication. The receiving VTEPs will decapsulate the packet, and based on the inner frame perform layer-2
MAC learning. The inner SMAC is learnt against the outer Source IP Address (SIP) corresponding to the
source VTEP. In this way, reverse traffic can be unicasted toward the previously learnt end host.
Other motivations include:
1 Scalability — VXLAN provides Layer-2 connectivity that allows the infrastructure that can scale to 16
million tenant networks. It overcomes the 4094-segment limitation of VLANs. This is necessary to address
today's multi-tenant cloud requirements.
2 Flexibility— VXLAN allows workloads to be placed anywhere, along with the traffic separation required
in a multi-tenant environment. The traffic separation is done using network segmentation (segment IDs
or virtual network identifiers [VNIs]).
Workloads for a tenant can be distributed across different physical devices (since workloads are added as
the need arises, into available server space) but the workloads are identified by the same layer 2 or layer
3 VNI as the case may be.
3 Mobility— You can move VMs from one data center location to another without updating spine switch
tables. This is because entities within the same tenant network in a VXLAN/EVPN fabric setup retain the
same segment ID, regardless of their location.
Overlay example:
The example below shows why spine switch table sizes are not increased due to VXLAN fabric overlay,
making them lean.
VM A sends a message to VM B (they both belong to the same tenant network and have the same segment
VNI). ToR1 recognizes that the source end host corresponds to segment x, searches and identifies that the
target end host (VM B) belongs to segment x too, and that VM B is attached to ToR2. Note that typically the
communication between VM A and VM B belonging to the same subnet would first entail ARP resolution.
ToR1 encapsulates the frame in a VXLAN packet, and sends it in the direction of ToR2.
The devices in the path between ToR1 to ToR2 are not aware of the original frame and route/switch the packet
to ToR2.
Cisco Nexus 7000 Series NX-OS VXLAN Configuration Guide
40
Configuring VXLAN BGP EVPN