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.