OSPF Non-Broadcast Network Type over Frame-Relay

Hi Rene
I have my network configured just as you, but I cannot ping Spoke 3 from Spoke 2 , why is that ?

Hi Houari,

There’s a couple of things to check. First of all, do you have an entry in your routing tables of spoke 2+3 so they know how to reach each other?

If so, are they able to reach those next hop IP addresses? If not, you might have to add another frame-relay map on spoke 2 and 3.

Rene

Hi Rene

I don’t see DLCI configured on Hub and spoke routers,
the illustration show DLCI numbers.
please explain

Thank you

19 posts were merged into an existing topic: OSPF Non-Broadcast Network Type over Frame-Relay

Hi,
Is it possible to simulate this lab in gns3 ?

If R1-----R2-------R3 connected like this , Is it a non broadcast type

Thanks

Hello Sims!

The network topology that you are describing would not COMPLETELY simulate the frame relay situation in this lesson. What you are describing would require all communication between R1 and R3 to go through R2 just like in the lesson, however, this also means that the two interfaces on R2 that connect R1 and R3 would have to be on the same subnet, something that is not possible. So you would just be creating two point to point links on different subnets, however, in the frame relay example in the lesson, all three routers are in the same subnet.

What you require is to create a frame relay switch on GNS3. You can find some frame relay examples in GNS3 from Rene’s GNS3 Vault here: http://gns3vault.com/labs/frame-relay/

I hope this has been helpful!

Laz

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Hi,
“If you select the non-broadcast network type then OSPF will assume you are running a multi-access network. Couple of key things to remember here:”
can you provide a non multi-access ospf network ? And why it is called non multi access ?

The broadcast and non-broadcast network types require a DR/BDR
So which type does not require a DR/BDR ?

Thanks

Actually who cares about frame-relay. Am I not mistaken this is a dead subject.
These lessons should be tailored for DMVPN now. And yes I realize you can probably relate the two technologies but I have enough in my head to learn and I’m sure DMVPN does not behave exactly like frame-relay all the time. Don’t need frame-relay examples ENOUGH UPDATE THIS STUFF. PLEASE :slight_smile:

Hi,
The question was really not about frame-relay .
It was about multi access ,non broadcast and broadcast ospf .
Thanks

Hello Sims!

Although I’m sure you know most of this, let me start by stating the network types for OSPF and stating an example for each:

**Point to Point** - a serial connection between two routers
**Broadcast** - two or more routers on the same subnet connected to an Ethernet switch

The next two types of network are used to accommodate multi access technologies that don’t support broadcast. These technologies include Frame Relay and ATM.

**Non Broadcast Multi Access** - This is used when you have multiple routers connected on the same subnet over a non-broadcast technology such as frame relay. OSPF must emulate a broadcast by sending multiple unicast packets to all destinations. In this case, a DR must be elected and it must be the HUB router that is the DR. Otherwise OSPF will not function correctly.

**Point to Multipoint -** This is a network where the topology is the same as in the Non Broadcast Multi Access, however, the HUB router creates multiple point to point connections to each remote router, thus emulating multiple point to point connections. Each point to point connection has its own subnet. A DR/BDR is not necessary in this topology.

So, to specifically answer your questions:

**“can you provide a non multi-access ospf network ?”**
A non-multi access network is just a point to point network. Multi access just means that there are more than two routers involved in a specific topology.

**" And why it is called non multi access ?"**
There is no specific term “non multi access” used for OSPF network topologies. Non multi access just means point to point. If you are referring to Non Broadcast, then that just means that you are using a technology that doesn’t allow broadcasts to occur such as Frame Relay.

**“The broadcast and non-broadcast network types require a DR/BDR**
**So which type does not require a DR/BDR ?”**

Specifically, out of the four types of networks, the Broadcast network and the Non Broadcast Multi Access require DR/BDRs. The Point to Point and Point to Multipoint do not since the basis of their topologies are point to point connections. Note that in a Broadcast network the DR can be any router based on the DR elections, while in the Non Broadcast Multi Access, the DR must be the HUB router for OSPF to function.

I hope this has been helpful!

Laz

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Hi
Why did you choose frame-relay for this example . If we use ip ospf network non-broadcast command in an ethernet broadcast network and get the same reult .

Thanks

Hello Sims

Yes, that may be the case, however, the very purpose of the different types of networks that you can configure for OSPF is to accomodate technologies that function differently such as Frame Relay. So the point is to show how these commands accomodate real technologies rather than just showing the fact that you can make OSPF function differently.

Also, we are trying to approximate real-world applications, and you would never EVER implement non-broadcast OSPF on on an Ethernet network.

I hope this has been helpful!

Laz

Hi Rene,
why there is no concept of DR/BDR in EIGRP, as it is in OSPF ? So in case of shared segments,there wont be any triggered update storm in EIGRP as in OSPF ?

Regards,
Nanu

Hello Nanu

It all has to do with the way OSPF as a link-state protocol functions as compared with EIGRP which is a distance vector protocol.

OSPF advertises a large amount of topological information about the network. Each router must learn what every metric is for every link in the OSPF area. When LSAs are exchanged, each router shares with other routers their perspective of the WHOLE network. EIGRP routers on the other hand share information about a portion of the network. So, first off, LSAs are much larger than the data exchanged by EIGRP, so a traffic regulator like the DR/BDR is necessary when there are multiple OSPF routers in the same subnet.

Secondly, and probably most importantly, OSPF creates neighbor relationships with ALL OSPF routers in its subnet whereas EIGRP creates neighbor relationships only with DIRECTLY CONNECTED EIGRP routers. So information exchange between EIGRP routers is drastically reduced as compared to OSPF.

I hope this has been helpful!

Laz

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Hello Laz,
Thank you so much for the explanation, but I still have following doubts:

  1. As you said " OSPF creates neighbor relationships with ALL OSPF routers in its subnet whereas EIGRP creates neighbor relationships only with DIRECTLY CONNECTED EIGRP routers. " so if there are 5 routers connected with Ethernet switch and are on same subnet and all 5 running eigrp and have advertised this common subnet in network commnad, in this case all 5 routers will be EIGRP neighbors with each other.
    And now if there is any network update, then wont there be the same problem as all routers will send the update to other router except from which the update has been received, and there is a chance of triggered update storm to happen in this case with EIGRP also? And to the best of my knowledge, in OSPF , to avoid this situation in case of shared segment, DR/BDR election happens, then isn’t the same threat is possible in EIGRP also?

Regards,
Nanu

Hello Nanu

Yes, I understand your concern. Keep in mind that EIRGP also has a split horizon rule, which states that any information that is learned via an interface is never advertised via that interface. This rule will prevent any “flooding” of EIGRP packets within a segment.

I hope this has been helpful!

Laz

Hello Laz,

Thank you so much for your guidance, this absolutely cleared my doubt.

Regards,
Nanu

2 Likes

Hello!

Make sure you have a frame-relay map statement with the broadcast keyword or you won’t be able to send multicast on your frame-relay network.

I can’t clearly understand the purpose of broadcast key word in dlci mapping (if inverse arp is disabled), because we are talking about NON-BROADCAST.
Is it right that this key word allows hub router to duplicate multicast ospf frames and send them to unicast addresses (that were configured by neighbor command)?

Hello Artem

Yes you are correct, that frame relay is a non-broadcast network type. This means that any broadcast OSPF packets that arrive at the hub must be propagated in the network. This can be done in one of two ways:

  1. Using the broadcast keyword on the frame relay maps. This essentially forwards any broadcast packets that have been received by the hub router to the IP address specified in the frame relay map as unicast packets. If the broadcast keyword is not there, no forwarding of broadcast packets will be performed, thus OSPF will not be able to send updates.

Yes that is correct.

  1. Inverse ARP. Inverse ARP when enabled will ask the “other end” what it’s IP address is. This allows dynamic mapping rather than static frame relay maps. If you have a static mapping with broadcast, you can safely turn off inverse arp.

I hope this has been helpful!

Laz

Thanks for the answer!
But I have one more question, it’s about BROADCAST network type and Frame Relay encap.
So it is clear for me now that in NON-BROADCAST network hub router will send ospf unicast packets to neighbors and hence it will use a certain DLCI of a certain neighbor on layer 2.
I’ve just tried the same topology as in the lesson(broadcast network type) in gns3 to figure out what dlci number will be in ospf multicast packet. But for some reason gns indicates that packets are encapsulated in HDLC (???),
So the question is: what DLCI number will the router put in frame relay header?