Overlay Networks

Overlay Networks

Network Overlays

These overlays have been in use to extend physical network (underlay) subnets/segments across physical boundaries. The routers and switches work as end points. Some examples are:- 

1. OTV
2. LISP
3. Traditional VPNs
4. FabricPath (Source Learning) - All Links Active Active

Host Overlays

The Hypervisor Vswitches or Physical switches like N9k act as end points, usually a single domain admin using a single controller like APIC can deploy and administer this kind of network,

VXLAN (Multicast - Flooding - UDP)
NVGRE (Unicast)
STT (Stateless Transport Tunneling)

Hybrid Overlays

The traffic can move between virtual and physical node, both working as end points to have a seamless extension of L2 physical boundary.

VXLAN - A tunneling protocol which encapsulates L2 Ethernet frames in Layer 3 UDP packets on port 4789, this allows L2 subnets spanning across physical L3 networks.

VXLAN has 24 bit = 16 million identifiers whereas VLAN as only 4096

1. L2 VNI - VXLAN Network Identifier carried in VXLAN packets bridged across VTEPs.
2. L3 VNI - VXLAN Network Identifier carried in VXLAN packets routed across VTEPs. This VNI is linked per tenant VRFs.
3. VNI - VXLAN Network Identifier.
4. VTEP - VXLAN Tunnel Endpoint.
5. VXLAN L2 Gateway - VTEP capable of switching VLAN-VXLAN, VXLAN-VLAN packets within same VNI.
6. VXLAN L3 Gateway - VTEP capable of routing VXLAN across different VNIs.

VXLAN packets received from the undelay networks with L4-L3-L2 headers are encapsulated in a new IP + MAC header (overlay) before being multicasted to multicast IP such as 239.1.1.2

Reference :- https://www.youtube.com/watch?v=kAoa7djX3Ew