OTV is mentioned in passing on the CCIE blueprint, so its worth knowing the basics.
These are my notes which barely scratch the surface, and there are whole courses on understanding it! …
- Layer 2 VPN encapsulated as IPv4 packets.
- Used as a Scalable Data Centre Interconnect (DCI)
- Thought to be needed for vMotion.
- Optimises control and data plane.
- Less flooding (inc. multicast forwarding)
- Loop prevention (Multiple Vlans)
- Redundancy supporting multiple edge routers
- Runs IS-IS (Mac in IP routing) Encap as CG-Multicast (control Group) (New TLV) 42 bytes
- OTV runs M/C over M/C and unicast over unicast, but with an adjacency server can run m/c over unicast (e.g. the internet or other WAN)
- Edge Device
- The edge device performs OTV functions: it receives the Layer 2 traffic for all VLANs that need to be extended to remote locations and dynamically encapsulates the Ethernet frames into IP packets that are then sent across the transport infrastructure.
- Authoritative Edge Device (AED)
The AED has two main tasks:
Forwarding Layer 2 traffic (unicast, multicast and broadcast) between the site and the overlay (and vice versa).
Advertising MAC reachability information to the remote edge devices.
- Site Vlan
- OTV use a VLAN called “Site VLAN” within a site to detect and establish adjacencies with other OTV edge devices. OTV used this site adjacencies as an input to determine Authoritative Edge devices for the VLANS being extended from the site.
- Overlay Interface
- Every time the OTV edge device receives a Layer 2 frame destined for a remote data center site, the frame is logically forwarded to the Overlay interface. This instructs the edge device to perform the dynamic OTV encapsulation on the Layer 2 packet and send it to the Join interface toward the routed domain.
- Join Interface
- The Join interface is a Layer 3 entity and with the current NX-OS release can only be defined as a physical interface (or subinterface) or as a logical one (i.e. Layer 3 port channel or Layer 3 port channel subinterface). A single Join interface can be defined and associated with a given OTV overlay. Multiple overlays can also share the same Join interface.
- Internal Interface
- To perform OTV functionality, the edge device must receive the Layer 2 traffic for all VLANs that need to be extended to remote locations. The Layer 2 interfaces, where the Layer 2 traffic is usually received, are named internal interfaces
Alternatives are dark fibre, L2TPv3, AToM, VPLS, Bridgign over Gre BUT they have disadvantages and less optimised for L2 transport as a DCI.