ospf - neighbours and topology

Unanswered Question
Sep 15th, 2008

Hi all

Just a little question about ospf

fistly, when we talk about neighbours, does it only form neighbour relationships with directly connected routers, and will only directly connected neighbours show in the neighbour table?

and secondly when we talk about the topology table, is this a list of all the routers ihe area and how to get there? and is this the same on each router?

can anyone possibly explain this and show me an output from a router for both ?

I have this problem too.
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satish_zanjurne Mon, 09/15/2008 - 01:21


1.Yes it is directly connected neigbors only , in OSPF

2.It is list of all links in the area & their states , router ID's ,

3.See the output below

R4-4K(4)# show ip ospf database router

OSPF Router with ID ( (Process ID 20)

Router Link States (Area 0)

LS age: 202

Options: (No TOS-capability, DC)

LS Type: Router Links

Link State ID:

Advertising Router:

LS Seq Number: 80000002

Checksum: 0xC899

Length: 60

Number of Links: 3

Link connected to: another Router (point-to-point)

(Link ID) Neighboring Router ID:

(Link Data) Router Interface address:

Number of TOS metrics: 0

TOS 0 Metrics: 64

Link connected to: a Stub Network

(Link ID) Network/subnet number:

(Link Data) Network Mask:

Number of TOS metrics: 0

TOS 0 Metrics: 64

Link connected to: a Stub Network

(Link ID) Network/subnet number:

(Link Data) Network Mask:

Number of TOS metrics: 0

TOS 0 Metrics: 1


HTH...rate if helpful..

carl_townshend Mon, 09/15/2008 - 01:40

hi there, how come you have typed the ip of another router? can you not see all routers?

what is the benefit of know all this information ?

satish_zanjurne Mon, 09/15/2008 - 01:44


If you want see databse for all routers, then you don't have to type any IP Address.

This very useful to to the troubleshooting releated to LSAs.

carl_townshend Mon, 09/15/2008 - 02:43

why does each router need to know about the whole topology? know other distance vector use routing by rumour,but I would like to know the benefit of know about whats going on several hops away.


satish_zanjurne Mon, 09/15/2008 - 06:00

1.That's the difference between link state & distance vector protocol.Link state protocol keeps information of complete topology.

2.faster convergence & scalibility are the advantages of link state routing protocols.

HTH..rate if helpful..

carl_townshend Mon, 09/15/2008 - 07:39

Can you tell me how knowing about the whole network would make it converge quicker ?

carl_townshend Fri, 09/26/2008 - 00:19

Hi There

When we say ospf neighbour, how does this work when say multiple routers are plugged into a switch and they can all see each other on a broadcast lan? would all routers be a neighbour of eachother ?

Giuseppe Larosa Fri, 09/26/2008 - 07:29


yes to be neighbor two routers must share a network segment

ospf hellos have destination with TTL=1 and cannot be propagated by a L3 device in the middle.

Multiple routers that have an interface in the same vlan can become neighbor of each other if mandatory OSPF parameters match:

hello interval

dead interval

OSPF area id

OSPF stub flag = OSPF area type

OSPF authentication (if used)

and OSPF router-ids are not duplicated

Actually one router is elected DR, another is BDR.

Full adjancencies are built between all other routers and DR and BDR.

neighbor = see each other router-ids in hellos

adjacent = link state DB in sync state

Hope to help


carl_townshend Fri, 09/26/2008 - 13:37

hi there, so

will they all be neighbours on the same segment?

when you talk about being adjacent, are they only adjacent with the dr and bdr?

can show show me an outout of this ?

Giuseppe Larosa Fri, 09/26/2008 - 14:12

Hello Carl,

the answers are yes, yes

an example is

mclaren#sh ip ospf n

Neighbor ID Pri State Dead Time Address Interface 1 FULL/DR 00:00:32 Ethernet4/3 1 FULL/DR 00:00:32 FastEthernet0/0.147 1 FULL/DROTHER 00:00:39 FastEthernet0/0.147 1 FULL/DR 00:00:38 FastEthernet0/0.150 1 FULL/BDR 00:00:34 FastEthernet0/0.64


in vlan 147 there are three routers and router with RID is DRother the router on which the command is given is the BDR for the segment.

Hope to help


carl_townshend Mon, 01/12/2009 - 08:58

hi there, so what if we have point to point links? ie a e1 connecion? do we need to do any manual config, or will it set itself as point to point on these types of interface and have no dr/bdr? in which case it will be fully adjacent ?


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