To provide the LAN extension in any site, we can use two OTV Edge device. As due to redundant OTV edge device and STP BPDU not flowing across OTV Overlay, may lead to creation of End to End loop.
To prevent above situation, AED was introduced which has two main task:
- Forwards Layer 2 traffic between Sites and overlay
- Advertise MAC reachability information among remote sites
In AED, Concepts AED role is selected per VLAN basis between all OTV Edge devices at same site. OTV uses Site-Vlan configured at each site to detect and establish adjacency with other OTV edge device as shown in below figure:
Now there is possibility that OTV edge device on same site may get failed to detect each other and ends with Active-Active mode and can also result in creation of LOOP.
To avoid this Each OTV edge device maintains the dual adjacencies with other OTV edge device in site and this type of adjacency is also called as Site Adjacency. In addition to this, OTV edge device also forms a second type of adjacency called Overlay adjacency which is established via Join interface over Layer 3 domain and to achieve this each site must be configured with unique site identifier. All OTV edge device who are at same site must be configured with same Site-ID. This Site identifier is advertised in IS-IS hello packet which are sent over overlay as well as on Site VLAN at same site.
To identify neighbor Edge device at same site: Site identifier + IS-IS system ID.
Dual Site adjacency state is used to determine AED role for each VALN which is to be extended across Overlay. Each OTV Edge device inform their neighbor about its capability to become AED and its forwarding readiness at local site.
The AED capability change received from a neighboring edge device in the same site influences the AED assignment, and hence will trigger an AED election. If a neighbor indicates that it is not AED capable, it will not be considered as active in the site. An explicit AED capability down notification received over either the site or the overlay adjacency will bring down the neighbor’s dual site adjacency state into inactive state and the resulting AED election will not assign any VLANs to that neighbor.
AED can be used for Odd and Even VLANs between two OTV edge devices by a use of deterministic algorithm. If we talk more specifically, OTV Edge device who has lower System-ID will become AED for all Even VLANs and the OTV edge device which has higher System-ID will be AED for ODD extended VLANs.
Use of AED also prevent the End to End STP loops. Let’s suppose that a Broadcast is generated at Site-1 DC and frame reaches to both AED OTV device bit only one OTV edge device that is AED is allowed to encapsulate the frame and send it over OTV overlay. Now once the Frame reaches to other site Site-2, all other AED on Site-2 will receive that frame and only AED will decapsulate the frame and send it in to site.