RFC 7348 VXLAN August 2014
3. VXLAN Problem Statement
This section provides further details on the areas that VXLAN is
intended to address. The focus is on the networking infrastructure
within the data center and the issues related to them.
3.1. Limitations Imposed by Spanning Tree and VLAN Ranges
Current Layer 2 networks use the IEEE 802.1D Spanning Tree Protocol
(STP) [802.1D] to avoid loops in the network due to duplicate paths.
STP blocks the use of links to avoid the replication and looping of
frames. Some data center operators see this as a problem with Layer
2 networks in general, since with STP they are effectively paying for
more ports and links than they can really use. In addition,
resiliency due to multipathing is not available with the STP model.
Newer initiatives, such as TRILL [RFC6325] and SPB [802.1aq], have
been proposed to help with multipathing and surmount some of the
problems with STP. STP limitations may also be avoided by
configuring servers within a rack to be on the same Layer 3 network,
with switching happening at Layer 3 both within the rack and between
racks. However, this is incompatible with a Layer 2 model for inter-
VM communication.
A key characteristic of Layer 2 data center networks is their use of
Virtual LANs (VLANs) to provide broadcast isolation. A 12-bit VLAN
ID is used in the Ethernet data frames to divide the larger Layer 2
network into multiple broadcast domains. This has served well for
many data centers that require fewer than 4094 VLANs. With the
growing adoption of virtualization, this upper limit is seeing
pressure. Moreover, due to STP, several data centers limit the
number of VLANs that could be used. In addition, requirements for
multi-tenant environments accelerate the need for larger VLAN limits,
as discussed in Section 3.3.
3.2. Multi-tenant Environments
Cloud computing involves on-demand elastic provisioning of resources
for multi-tenant environments. The most common example of cloud
computing is the public cloud, where a cloud service provider offers
these elastic services to multiple customers/tenants over the same
physical infrastructure.
Isolation of network traffic by a tenant could be done via Layer 2 or
Layer 3 networks. For Layer 2 networks, VLANs are often used to
segregate traffic -- so a tenant could be identified by its own VLAN,
for example. Due to the large number of tenants that a cloud
Mahalingam, et al. Informational [Page 5]
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