IP OSPF Interface non-broadcast

Unanswered Question
Jul 12th, 2010

Hi,

If the theory explain that when I have a "IP OSPF INTERFACE NETWORK TYPE" = NON-BROADCAST is necessary the NEIGHBOR command, to established adjacency.

Who can explain to me the following:

I have Two Routers (RouterA, RouterB) both are configure with

!

interface FastEthernet0/0

ip address 183.1.45.X 255.255.255.0   (RouterA = .5 and RouterB= .4)

ip ospf network non-broadcast

end

!

router ospf 1

log-adjacency-changes

network 183.1.45.X 0.0.0.0 area 45 (RouterA = .5 and RouterB= .4)

!

At this time everything is fine, nobody send hello messages.

When a configure the NEIGHBOR command over ONE of the router the OSPF ADJACENCY is established with FULL STATE; without the neccesary of configure it (NEIGHBOR COMMAND) on both routers.

Why is not necessary the NEIGBORG command on both routers?

I think that one router respond a OSPF hello always without verified if the source of the hello message is a "neighbor" configure under OSPF process;  this may sound logical; but I need theorical information about that

I appreciate if anybody can help me.

I have this problem too.
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Reza Sharifi Mon, 07/12/2010 - 20:07

David,

You do not need neighbor command for OSPF, ISIS, EIGRP, RIP and etc... For only protocol you need the neighbor command is when you configure BGP.

HTH

Reza

mazhar mahadik Tue, 07/13/2010 - 01:03

Hi Reza,

David is correct, we need neighbor command in ospf if interface is configured for ospf type Non-broadcast. since its auto discovery via multicast address is disabled, so u hv to explictly define the neighbors with whom u wish to build neighborship.

David : - this type of config is done even in NBMA frame relay networks, where only on hub router u define the neighbors and not on spoke routers, but i dont know the explanation behind it. let me browse some docs on this, n get back to this post, meanwhile if u get the answer plz update it on the post.

regards,

Mazhar

shirangp3 Tue, 07/13/2010 - 04:57

The simple explanation is that when you specify that you work in an NBMA mode, you simply say to your router or in your case to your interface that you want to work in unicast and when working in unicast you must specify who you want to be neighbor with for hello, and LSA exchange.

Only one side is needed to be configured with the neighbor as once the configured side will start unicast the router that receive the hello will also reply to that unicast (if all parameters are sync that include security if applied)

Example: if you have Frame Relay hub and spoke network it is better that you set the neighbor command on the FR hub and not the spoke...

David Salazar Tue, 07/13/2010 - 06:49

Hi,

The FR HUB-and-Spoke is not the only scenario where can find it the IP OSPF NETWORK TYPE = NON-BROADCAST; I am doing a lab for the CCIE exam and the OSPF section say:

2. IPv4

2.1 OSPF

* Ensure that R5 is always elected the designated Router for the segment between R3, R4 and R5 (R5 = HUB, R3 and R4 = Spoke / Frame-Relay Hub and Spoke topology)

* Ensure that host devices running OSPF on the segment between R4 and R5 cannot intercept the OSPF communication between R4 and R5. (between R4 and R5 there is an ethernet segment, apart from the serial connection)

This is the point of my question.

Because I use the "IP OSPF NETWORK NON-BROADCAST" command is neccesary use the neighbor command.

The book = CCIE Routing and Switching Certification Guide (Fourth Edition), in Chapter 8 "OSPF" Page 264 Section "Caveats Regarding OSPF Network Types over NBMA Networks" say:


"If one router requires a static neighbor command, typically the other router on the other end oh the PVC does not require a neighbor command. For clarity, however, it is better to configure neighbor command on both routers"


what's the theorical basement for it?

Thanks a lot

shirangp3 Tue, 07/13/2010 - 07:27

To prevent miss use of the NDA of the exam I hope you mean that the question is not a real lab question but only a preparation example

If you read what I have written then you would see that I mentioned the FR Hub and Spoke as an example.

"If one router requires a static neighbor command, typically the other router on the other end oh the PVC does not require a neighbor command. For clarity, however, it is better to configure neighbor command on both routers


what's the theorical basement for it?"

I think I have explained it in my last reply, when you set "ip ospf network non-broadcast" that mean that you set it to work in unicast mode mean multicast to 224.0.0.5 or 224.0.0.6 (DR/BDR) will not be sent only unicast destination and for unicast destination to be sent the router need to know the destination ip address of his neighbor.

Good Luck on preperation

Shiran Guez

CCIE #20572

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