nbma and ospf

Unanswered Question
Jun 28th, 2007

say you have frame-relay configured in hub and spoke. You've set your broadcast up in the interface section using: frame relay map <ip> <wildcard> <DLC> BROADCAST, that way OSPF traffic will flow across the links.

Now, you go to configure OSPF. You don't want a BDR so you set the spokes to IP OSPF PRIORITY 0. However, I've read that on the hub you still have to set a manual neighbor statement for all your spokes. Why is that? If broadcast has been turned on...isn't the neighbor adjacency formed by the now (presumably) broadcasted OSPF packets?

I have this problem too.
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Amit Singh Thu, 06/28/2007 - 09:31

Hello,

There are two modes of NBMA deployment on the frame-relay using the OSPF.

1. Point-point: In this the OSPF routers will become the neighbor and you dont need any farme-relay map command as the links are treated as point-point. You have to give command " ip ospd network point-point " under the serial interfaces connected and running point-point FR.There will be no DR and BDR election in this and the OSOF packets are multicast over the network.

2. Point-multipoint: In this config since all the neighbors are not directly connected you have to give the static frame-relay mapping command on the serial interface running FR. You have to give mappings for all the neighbors running in the hub-spoke topology. You have to configure the OSPF as point-multipoint. Its not only HUB you have to configure the mapping on the Spoke routers as well.

-amit singh

Paolo Bevilacqua Thu, 06/28/2007 - 10:28

Hello,

My recommendation would be for the point-to-point subinterfaces. You would have many advantages, controlling each interface separately for ACL and QoS, etc.

My experience is that no "ip ospf network point-point" is necessary, as the code is smart enough to figure that out automatically.

Best luck, and rate this post if it helps!

jkillion Thu, 06/28/2007 - 09:33

This depends on the type of OSPF interface you use. If you don't want to specify the neighbors, use "ip os network broadcast" under the frame interfaces.

Even if you use "broadcast" w/ your frame map statement, you would still have to specify neighbors by default. This is b/c a frame non-subinterface defaults to OSPF type "NBMA". This mode still elects a DR/BDR (you've got down how to ensure the hub is the DR), but it doesn't use multicast for neighbor detection - hence the name "Non Broadcast Multi Access".

The "broadcast" in the frame map statement simply allows broadcast / multicast to be sent over the frame link. You still have to tell OSPF that it can use multicast over the interface.

Jay Killion, CCIE #17873

sundar.palaniappan Thu, 06/28/2007 - 10:59

" "However, I've read that on the hub you still have to set a manual neighbor statement for all your spokes. Why is that?" "

The default physical interface type is non-broadcast and therefore OSPF wouldn't send multicast packets out that interface. In that case the only way OSPF can discover neighbor(s) is by configuring 'neighbor' statements.

" "If broadcast has been turned on...isn't the neighbor adjacency formed by the now (presumably) broadcasted OSPF packets?" "

The frame relay map with 'broadcast' keyword makes the DLCI (PVC) capabale of carrying broadcasts and doesn't mean the router would send OSPF multicasts out that DLCI.

HTH

Sundar

shivlu jain Fri, 06/29/2007 - 04:23

on nbma mode we have to manually set the neighbor statement just becasue it can find the neighbor directly.

shivlu

sundar.palaniappan Fri, 06/29/2007 - 04:28

That's correct!!

An alternative to that would be to configure the int as point-to-multipoint or broadcast type for OSPF and there would be no need to configure manual neighbors.

HTH

Sundar

Richard Burts Fri, 06/29/2007 - 06:36

I believe that there is one thing to be aware of when considering configuring it as OSPF broadcast. If I read the original post correctly the network is a hub and spoke. Configuring the interface as broadcast assumes that the Frame Relay provides a full mesh connection. When this assumption is not true we should proceed carefully in configuring the interface as OSPF broadcast.

HTH

Rick

jkillion Mon, 07/02/2007 - 04:49

So long as you ensure the hub is the DR (which the OP has done) there aren't any issues w/ OSPF broadcast over Frame.

thx

Jay Killion, CCIE #17873

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