OSPF Network Types

Answered Question
Jul 4th, 2008

Hi,

I'm getting confused with the kinds of OSPF network in Frame-relay. Well I know of course what is a point-to-point link but I want to know the difference between these:

RFC-Compliant:

Non-Broadcast Multiaccess

Point-to-Multipoint

Modes from Cisco:

Point-to-Multipoint nonbroadcast

Broadcast

Point-to-point

What is the difference of point-to-multipoint nonbroadcast and point-to-multipoint? Both modes doesn't elect a DR/BDR.

From the CCIE R&S Exam Quick Reference Sheet, I can only see that their main difference is the neighbor statement. Is that their only difference? Thanks.

John

I have this problem too.
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Correct Answer by Edison Ortiz about 8 years 5 months ago

John,

Correct on both counts.

__

Edison.

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Edison Ortiz Fri, 07/04/2008 - 17:03

The non-broadcast option will instruct the router not to use Multicast for OSPF packets, therefore that's why you see the Quick Reference Sheet mentioning the neighbor statement.

The neighbor statements instruct OSPF to use unicast packets to discover its neighbors.

HTH,

__

Edison.

Giuseppe Larosa Sat, 07/05/2008 - 11:14

Hello John,

just a little addition to what Edison had alread written.

With point-to-multipoint non-broadcast the neighbor statement is required and accepts a cost option.

With this network type a per-neighbor cost can be specified, whereas in the point-to-multipoint network type the cost is the usual outgoing OSPF interface cost so all neighbors are seen at the same cost.

The reason for this is when PVCs with different bandwidth parameters lead to two or more remote routers you can provide a preferred path without using two different interfaces or subinterfaces.

Hope to help

Giuseppe

John Patrick Lopez Sun, 07/06/2008 - 13:28

So this is how it works if I understood it correctly.

ip ospf network non-broadcast (default)

- elects DR/BDR

- requires neighbor command to establish adjacency

- does not multicast hello but rather unicasts

ip ospf network point-to-multipoint

- does not elect DR/BDR

- multicasts hellos to establish neighbor adjacency

ip ospf network point-to-multipoint non-broadcast

- does not elect DR/BDR

- does not multicast hellos and neighbor command is required to establish adjacency. utilizes unicast as hello.

The "ip ospf network" command only applies for ospf hellos and adjacency and also determines if there will be a DR/BDR. This is completely different with frame-relay map ip broadcast keyword right? Thanks for all your replies.

John

Edison Ortiz Sun, 07/06/2008 - 14:46

This is completely different with frame-relay map ip broadcast keyword right?

The broadcast keyword in the frame-relay mapping enables the transmission of multicast packets on the link.

If you need to run an IGP routing protocol and you are missing the broadcast keyword, then you have to use the neighbor option in the IGP routing protocol for unicast packet delivery.

HTH,

__

Edison.

John Patrick Lopez Sun, 07/06/2008 - 16:13

Thank you Sir Edison. What about the other things that I just wrote? Are they correct?

In a point-to-multipoint configuration where neighbor command is not needed, you will need to specify frame-relay map ip broadcast command since neighbor command is not necessary. By configuring frame-relay map ip broadcast, the multicast hellos of OSPF will be able to cross the link.

In point-to-multipoint non-broadcast, even if the broadcast keyword is in place in the frame-relay map ip command, OSPF hellos will not be sent right?

Thanks,

John

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