Does E-VPN Spell the End for OTV?

If you are considering how best to do Layer 2 stretch for virtual machine mobility, then you might be considering Overlay Transport Virtualization (OTV). OTV designed by Cisco to offer L2 stretch with what they said was an easy to deploy protocol. It was only available on the Nexus switching product line, which didn’t support VPLS/MPLS. Until recently MPLS/VPLS was Juniper’s recommended technology for network segmentation and Layer 2 stretch, which Cisco also offers on the ASR routers.  We’ve recently announced E-VPN, which is MPLS/VPLS based and brings all of the benefits of VPLS and then some. Cisco has announced E-VPN on the ASR router as well. Now that E-VPN is available, maybe it’s time to consider your best option. Let’s take a look at why OTV isn’t the best choice for VM mobility and why E-VPN is.

Why OTV was Invented
OTV has been in the Cisco’s news announcements, highlighted at Cisco Live and featured in several Cisco blogs. It’s something I’ve been meaning to cover along with my blogs on LISP and VXLAN as these all get discussed together as parts of a complete solution for live VM migration. Cisco first announced OTV on Feb. 8, 2010. Overlay Transport Virtualization is a Cisco proprietary protocol which provides Layer 2 extensions, over IP, to interconnect remote data centers.  Cisco claims that the OTV is a simpler technology than MPLS/VPLS, which is a standards-based and proven technology for network segmentation and Layer 2 Extension.  They said that OTV can be provisioned within minutes, using only four commands, and that it provides increased scalability (however without seeing the independent studies we don’t know if this is true).  It was only offered it only on the Nexus 7000, which didn’t offer MPLS/VPLS technology. With OTV, Cisco pushed yet another proprietary protocol that is not as well proven as standards-based MPLS/VPLS or the newer E-VPN. Cisco supports VPLS on the ASR router so it is curious that they did OTV on the Nexus which doesn’t sit at the right place in the network to do L2 Stretch. The Cisco ASR, like the Juniper MX, is meant to do L2 stretch at the data center edge, not in the data center core where the Nexus switches sit.

Enhancing VM Mobility with VxLAN, OVSDB and EVPN

Organizations are increasingly using virtual machine mobility to optimize server resources, ensure application performance and to aid in disaster avoidance. Typically VM live migration has relied on increasing the scale of the L2 broadcast domain to ensure that the VMs can be reached after migrations using their current addressing. This has resulted in the increasing use of VLANs and the need for L2 extension over the WAN.  As a result organizations are looking for ways overcome the limitations with VLAN scale and for methods to extend the L2 domain over the WAN that ensure the best performance. VxLAN has emerged as an alternative technology to VLANs, and EVPN has emerged at a better way to transport VMs over the WAN. Together these technologies can enable VM live migration over the WAN, or long distance vMotion in VMware parlance, but they need to all work together effectively and this is where OSVDB, VxLAN routing and a new technology from Juniper called ORE come in to play.

VxLAN Increases VLAN Scale
Organizations are increasingly looking to VxLAN as a solution. The primary goals behind this network architecture is to increase traditional VLAN limits from 4,094 and to enable VM mobility across Layer 3 subnets. VxLAN is a tunneling technology and is used to create an overlay network so that virtual machines can communicate with each other and to enable the migration of VMs both within a data center and between data centers. VxLAN enables multi-tenant networks at scale, as a component of these logical, software-based networks that can be created on-demand. VxLAN enables enterprises to leverage capacity wherever it’s available by supporting VM live migration. VxLAN implements a Layer 2 network isolation technology using MAC in IP encapsulation that uses a 24-bit segment identifier to scale beyond the 4K limitations of VLANs.

Optimizing EVPN for Virtual Machine Mobility over the WAN

Organizations need to insure that their applications are available and performing. Server virtualization helps by enabling virtual machine mobility.  If a server is overworked or will be unavailable vMotion can be used to migrate live workloads to another server in the current data center or in another data center. This requires that the addressing including the MAC, IP address and VLAN ID remain the same so that sessions are not dropped when the VM move happens. This is done by extending the L2 domain to the new location, know as Layer 2 stretch. Within a subnet this is easy to do. Across subnets in the data center it becomes more difficult. Doing live migration over the WAN introduces considerable challenges. Juniper has introduced a number of technologies to make virtual machine live migration possible.

The challenge with VM mobility is how to do the Layer 2 stretch in a way that ensures that the VM can be reached after it is moved. There are a number of issues that need to be dealt with. The MAC and IP address no longer pinned to a site or to an interface as they have moved with the VM. You need fast convergence of network paths as VM moves so that traffic will reach it quickly. You need ingress and egress traffic convergence and optimization to avoid having traffic go through the former default gateway after the VM has moved. You need learning of the effects of the live motion event and information distribution control so that the network isn’t impacted by signaling traffic. You need proper L2 & L3 interaction so that everything happens in a timely manner to ensure the best experience for the users of the applications that are affected by the VM move. VPLS has been the traditional methods of doing this, and now Juniper is supporting EVPN to provide enhancements to the solution.