OSPF Non-Broadcast Network Type over Frame-Relay


(Rene Molenaar) #1

This topic is to discuss the following lesson:


(system) #2

Hi Rene

Its excellent explanation about OSPF Network Type… Really appreciate your effort & time. Keep up good work…

I am following your post from last couple of days… Nice experience through your lessons & explanation is awesome…


(Rene Molenaar) #3

Thank you Shahin, glad to hear you enjoy my tutorials!


(system) #4

Great job with your website, really appreciate the time and effort you put into providing this very helpful content!!


(system) #5

Hey Rene,

Great explanations!! Im confused about a couple things. In this example you set "ip ospf network non-broadcast " so does this mean, that the neighbors will not automatically discover each other, and why would you do this. So if you set "ip ospf network broadcast " would this mean that it would send a psuedo broadcast to be able to discover the neighbor automatically so you dont have to staticaly configure them?

thank you for all your help


(Rene Molenaar) #6

Hi David,

Glad you like it! By default OSPF will try to discover neighbors on the same segment. Since frame-relay is NBMA this is impossible so we have to tell OSPF that it can’t use multicast (if you can’t use broadcast, multicast also doesn’t work). If you use “ip ospf network broadcast” then OSPF will use multicast packets and tries to discover neighbors on its own.

Rene


(system) #7

good job, but i have a question
i have 2 routers connected via serail interface in my area 0 Network , i need to configure these 2 routers as nonbrocast network, how can i do that.

thanks


(Rene Molenaar) #8

If it’s a serial link running PPP or HDLC it’s best to use the point-to-point network type. Use “ip ospf network point-to-point” on the serial interfaces.


(system) #9

Hello,

I had read your lesson, I thought about examples. You said, network is non-broadcast and you had configured neighbor command on hub router but why you didn’t configure it on spoke routers? I though, after you configure non-broadcast command on router any broadcast and multicast is sent so spoke router from yours example didn’t sent any multicast data and I think, this is reason why neighbors command on spoke router must be configure too. Am I wrong?
Regards,
Arek


(Rene Molenaar) #10

Hi Arek,

Good question, it sounds logical to configure the neighbor command on the spoke routers as well but it’s not required.

It probably works like this:

When you enable the neighbor command on the hub router, it won’t use multicast packets anymore but sends unicast packets to the spoke router. The spoke receives these unicast packets and will respond with unicast packets as well, and they become neighbors.

I haven’t debugged this so it might be a good exercise (try “debug ip ospf adj”) to see this in action.

Rene


(system) #11

Ok, I try to check, I add comment if I find an answer.
A.


(system) #12

This appears to still leave an issue where spoke1 and spoke2 are not be able to communicate with each other. For example, I added a loopback to spoke1 and the routing table on spoke2 was pointing to spoke1 as the next hop, which is unreachable. I was able to make it work one of two ways:

  1. Adding a frame-relay map statement to both spoke1 and spoke2 to force traversal through the hub to get to each other. Such as:

    Spoke1(config-if)#frame-relay map ip 192.168.123.3 201
    Spoke2(config-if)#frame-relay map ip 192.168.123.2 301

  2. Switching the hub to “point-to-multipoint non-broadcast” and then the two spokes configure a sub interface as a frame-relay “point-to-point”. This removes the DR/BDR, but then the frame-relay maps are no longer needed.

I’m learning this for my next certification test, so I want to make sure I fully understand it. There’s probably a way to get the config you have above working, but I’m missing something.


(system) #13

AHA! I figured it out. You configured all the routers with “ip ospf network non-broadcast”. Which I thought was odd, because that’s the default. But instead, if you configure the routers with “ip ospf network point-to-multipoint non-broadcast” then it all works! I see how OFPS automatically adds a route for each spoke to the other spokes so they can get to each other through the hub. That’s pretty cool.


(system) #14

If we are using the non-broadcast mode, no DR/BDR will be elect, why should we use the command ip ospf priority 0 on the spoke routers?


(system) #15

I already find the answer: (For a multi-access network (like Ethernet and Frame-relay), a DR (Designated Router) and a BDR (Backup Designated Router) are elected. All routers on that segment form adjacency with the DR and BDR only. Except on a point to point.)
Thank you.


(Rene Molenaar) #16

Hi Manushka,

When you use non-broadcast, there will be a DR/BDR election so you need to set the priority to 0 on the spoke routers.

Rene


(Nav N) #17

You can trick ospf in multi access network by “ip ospf network broadcast”.


(barzee j) #18

Hello Rene,
thanks for these amazing tutorials, i have one question
when I configured 3 routers step by step like you and the result were like the below :
except using S 0/0 I used S 1/0 on all routers

Hub#sh ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
192.168.123.2     0   FULL/DROTHER    00:01:33    192.168.123.2   Serial1/0
N/A               0   ATTEMPT/DROTHER    -        192.168.123.3   Serial1/0

what is the reason of Neighbor ID = N/A ? and the state appear ATTEMPT?


(Rene Molenaar) #19

Hi Barzee,

When you see the attempt state then it means your router has sent hello packets but hasn’t heard back from the router on the other side. If the neighbor adjacency doesn’t come up after a few seconds, check if they can reach each other. Maybe something is wrong with your frame-relay configuration.

Rene


(CEM B) #20

You’re the best Rene! Thanks for lessons. I didnt understand the point that why we need a non-broadcast network. Where can we use it? Cause we can make frame relay map broadcast as in the lesson you’ve explained (OSPF broadcast n.t over frame-relay). Is there any possibilities compared to broadcast network type over frame relay?

Excuse my ignorance,
Regards.