Traditionally, VLANs / Virtual LANs have been used to segment broadcast domains.
Such segmentation has been beneficial due to numerous benefits:
a. Smaller broadcast domains means fewer hosts that will process broadcasts from other hosts.
b. This also saves CPU and memory of all involved devices in a broadcast domain.
These VLANs are nowadys being replaced by VXLAN in data centers due to some major benefits.
Why
VXLAN based Data Centers?
1. Multitenancy
Many
VXLAN segments as compared to VLANs.
Virtual Network Identifiers VNIs are used to identify the overlay segments; these overlays are created over existing physical networks.
Virtual Network Identifiers VNIs are used to identify the overlay segments; these overlays are created over existing physical networks.
Virtual
Network Identifier VNI is 24 bit thereby enabling many VXLAN based overlays as
compared to VLANs (Max 4094)
This means that you are able to provision more customers in your
VXLAN based Data Center as compared to traditional data centers.
2. Layer 2 adjacency across different
sites means extending layer 2 but at the same time extending Spanning Tree
Protocol - STP domains across data centers.
Such
layer 2 extensions with inherent STP have been known to cause instabilities in
network.
Instability
of Layer 2 only WAN link will cause a state change and Spanning Tree
re-convergence.
VXLAN based logical switches do not extend STP domains, there is
no STP in a VXLAN based network.
Traffic flow in a VXLAN based
network.
This topic is very fundamental
to the understanding of VXLAN encapsulation & it is worthwhile having a
thorough understanding of this topic.
Just like the understanding of IP routing process, a good understanding of this
topic will enable network engineers to design, implement and operate a VXLAN
based network.
In the above figure, VXLAN
encapsulation and de capsulation is done by VTEPs.
In case of VMWare NSX, hypervisor ESXi is the VTEP and
VXLAN based tunnels/overlays are between these VTEPs.
For traffic that is coming into
VTEP A, VXLAN header is introduced before the traffic gets into IPv4 network.
IPv4 network provides necessary transport for VXLAN encapsulated frames to move
between VTEPs.
Vxlan
frame format sheds a lot of light upon the different fields in VXLAN
header.
For traffic sent out from
virtual machine A to virtual machine B in the other site, these headers will
be:
a. VNI
This field identifies the overlay virtual network
where virtual machine resides.
b. Outer
source IP
IP address of VTEP A
c. Outer
destination IP
This IP will be either destination IP of VTEP B
or this destination IP will be a
multicast IP address mapped to a particular VNI ( This VNI corresponds to
overlay virtual network where virtual machine A is connected)
d. Outer
Source MAC address
This will MAC address of VTEP A
e. Outer
Destination MAC address
As per the topology above, this will be MAC
address of intermediate router / next hop router.
