VXLAN Study 2

  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

   provider might service, the 4094 VLAN limit is often inadequate.  In
   addition, there is often a need for multiple VLANs per tenant, which
   exacerbates the issue.

   A related use case is cross-pod expansion.  A pod typically consists
   of one or more racks of servers with associated network and storage
   connectivity.  Tenants may start off on a pod and, due to expansion,
   require servers/VMs on other pods, especially in the case when
   tenants on the other pods are not fully utilizing all their
   resources.  This use case requires a "stretched" Layer 2 environment
   connecting the individual servers/VMs.

   Layer 3 networks are not a comprehensive solution for multi-tenancy
   either.  Two tenants might use the same set of Layer 3 addresses
   within their networks, which requires the cloud provider to provide
   isolation in some other form.  Further, requiring all tenants to use
   IP excludes customers relying on direct Layer 2 or non-IP Layer 3
   protocols for inter VM communication.