What OSPF network types exist, and what changes between them?
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Broadcast (DR, 10/40 timers, multicast), point-to-point (no DR, 10/40), non-broadcast or NBMA (DR, 30/120, neighbors configured by hand with unicast hellos), point-to-multipoint (no DR, 30/120, host routes for each endpoint) and loopback. The type decides whether there is a DR, the timers and how the link appears in LSAs.
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IOS defaults and behavior
| Type | Default on | DR/BDR | Hello/Dead | Neighbors |
|---|---|---|---|---|
| broadcast | Ethernet | Yes | 10/40 | Multicast discovery |
| point-to-point | Serial HDLC/PPP, GRE tunnels, p2p subinterfaces | No | 10/40 | Multicast discovery |
| non-broadcast | Frame Relay physical and multipoint | Yes | 30/120 | neighbor statements, unicast |
| point-to-multipoint | Never by default | No | 30/120 | Multicast discovery |
| point-to-multipoint non-broadcast | Never by default | No | 30/120 | neighbor statements |
| loopback | Loopbacks | n/a | n/a | none |
Mixing them
Two routers with different types can be fine or broken:
- Broadcast against non-broadcast fails, because 10/40 does not match 30/120.
- Broadcast against point-to-point has matching timers and can reach Full, yet routes are missing. One side describes a transit network with a DR, the other a point-to-point link, so SPF fails the two-way link check.
- Broadcast against point-to-multipoint: timers differ, so no neighbor unless you change them.
Loopbacks
The loopback type always advertises a /32 host route whatever mask you configure. Set
ip ospf network point-to-point on the loopback to advertise the configured mask.
They will ask next
Why does NBMA need neighbor statements?
The cloud, classically Frame Relay, cannot carry multicast hellos, so each router must know its neighbors and send unicast hellos. The neighbor sits in ATTEMPT while it waits for a reply.
In a hub-and-spoke NBMA network, where must the DR be?
On the hub. Spokes cannot reach each other directly, so a spoke DR could not flood to the other spokes. Set
ip ospf priority 0on every spoke.
Traps
- Saying a GRE tunnel defaults to broadcast. On IOS it is point-to-point.
- Forgetting the loopback /32 behavior when someone asks why a /24 loopback shows up as /32.