OSPF over Frame-Relay: Non-Broadcast

When OSPF is run on a network, two important events occur before routing information is exchanged.


OSPF neighbors are discovered using multicast Hello packets.

For a multi-access network (like Ethernet and Frame-relay), a DR (Designated Router) and a BDR (Backup Designated Router) are elected. All routers on that segment form adjacency with the DR and BDR only.

Hello packets are multicast to 224.0.0.5 address. In order for the Hello packets to reach remote routers, the underlying Layer 2 technology must allow broadcast/multicast packets to be forwarded. Frame-relay, inherently, does not forward broadcast or multicast packets to be sent. Hence, in NBMA network topology, the neighbors are not discovered automatically. Neighbors must be configured manually. OSPF elects DR and BDR for every multi-access network. In a frame-relay hub-and-spoke environment, ideally, the Hub router must be made the DR.

The below network is a simple hub-and-spoke topology. Each Spoke router has a single DLCI towards the Hub router. The Hub has multiple DLCIs towards Spoke routers.

Frame-Relay Configuration


The frame-relay configuration on the Hub and Spoke routers is as below.

Hub FR Configuration


interface serial 0/0
encapsulation frame-relay
ip address 10.1.1.1 255.255.255.0
frame-relay interface-dlci 102
frame-relay interface-dlci 103
!

Spoke1 FR Configuration

interface serial 0/0
encapsulation frame-relay
ip address 10.1.1.2 255.255.255.0
frame-relay interface-dlci 201
!

Spoke2 FR Configuration

interface serial 0/0
encapsulation frame-relay
ip address 10.1.1.3 255.255.255.0
frame-relay interface-dlci 301
!
This provides reachability from Hub to Spoke routers but not between Spoke routers.