OSPF non-broadcast network without neighbor-command

From: Radoslav Vasilev (deckland@gmail.com)
Date: Fri Aug 11 2006 - 09:55:48 ART

Hi Group,

Can anyone shed some light of why the following setup actually works?

The setup is from IEWBv3 lab 17, task 4.1 - basic ospf setup

the underlying topology is frame-relay hub-and-spoke with R5 being the hub
and R1 and R2 the spokes.
The spokes use physical interfaces with manual FR mapping to R5. The hub
uses In-ARP to go to the spokes.

R5:

interface Serial0/0.125 multipoint
 ip address 173.1.125.5 255.255.255.0
 ip ospf network non-broadcast
 ip ospf priority 255
 frame-relay interface-dlci 501
 frame-relay interface-dlci 502

router ospf 100
 router-id 150.1.5.5
 log-adjacency-changes
 network 173.1.125.5 0.0.0.0 area 0

R1:
interface Serial0/0
 ip address 173.1.125.1 255.255.255.0
 no ip directed-broadcast
 encapsulation frame-relay
 no ip mroute-cache
 frame-relay map ip 173.1.125.2 105
 frame-relay map ip 173.1.125.5 105 broadcast
 no frame-relay inverse-arp

router ospf 100
 network 173.1.125.1 0.0.0.0 area 0

R2:

interface Serial0/0
 ip address 173.1.125.2 255.255.255.0
 encapsulation frame-relay
 clock rate 64000
 frame-relay map ip 173.1.125.1 205
 frame-relay map ip 173.1.125.5 205 broadcast
 no frame-relay inverse-arp

router ospf 100
 router-id 150.1.2.2
 log-adjacency-changes
 network 173.1.125.2 0.0.0.0 area 0

Now the problem: area 0 has to be configured ove that FR segment - R1 and R2
are not allowed any interface level commands to achieve that task.
As R1 and R2 use physical interfaces, their ospf interface type is by
default non-broadcast, so is the multipoint sub-iface of R5.
We're not allowed neighbor commands anywhere as well.

The IEsolution, as visible above includes configuring `ip ospf network
non-broadcast` on R5.
No neighbor commands, nothing else.

150.1.1.1 1 FULL/DR 00:01:47 173.1.125.1
Serial0/0.125
150.1.2.2 1 FULL/DROTHER 00:01:54 173.1.125.2
Serial0/0.125

Any explenation what exactly happens so the non-broadcast neighbors actually
become adjacent?

Thanks in advance!
Rado

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