OSPF Smart-discover

Definition

Hello packets are periodically sent on OSPF interfaces of routers. By exchanging Hello packets, the routers establish and maintain the neighbor relationship, and elect the DR and the Backup Designated Router (BDR) on the multiple-access network (broadcast or NBMA network). OSPF uses a Hello timer to control the interval at which Hello packets are sent. A router can send Hello packets again only after the Hello timer expires. Neighbors keep waiting to receive Hello packets until the Hello timer expires. This process delays the establishment of OSPF neighbor relationships or election of the DR and the BDR.

Enabling Smart-discover can solve the preceding problem.

**Table 1** Processing differences with and without Smart-discover
With or Without Smart-discover	Processing
Without Smart-discover	Hello packets are sent only when the Hello timer expires. Hello packets are sent at the Hello interval. Neighbors keep waiting to receive Hello packets within the Dead interval.
With Smart-discover	Hello packets are sent directly regardless of whether the Hello timer expires. Neighbors can receive packets and change the state immediately.

Principles

In the following situations, Smart-discover-enabled interfaces can send Hello packets to neighbors regardless of whether the Hello timer expires:

On broadcast or NBMA networks, neighbor relationships can be established and a DR and a BDR can be elected rapidly.
- The neighbor status becomes 2-way for the first time or returns to Init from 2-way or a higher state.
- The interface status of the DR or BDR on a multiple-access network changes.
On P2P or P2MP networks, neighbor relationships can be established rapidly. The establishment of neighbor relationships on a P2P or P2MP network is the same as that on a broadcast or NBMA network.