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I have a router in an NSSA area attached, via an ABR, to area 0; this is done through a broadcast link with a point-to-point address (31 bit prefix), so there are basically only two routers on this network segment.

Is it ok to have the NSSA router act as the DR for the link between the ABR and itself? Or should it only be the ABR router that is in Area 0 that acts as the BR for the link between the NSSA router and itself?

*NOTE No please allow me to clear up any confusion on the wording, because there are people out there that get easily confused by the words, VERY easily confused.

When I say, "I have a router in an NSSA area attached, via an ABR, to Area 0" What I mean exactly is that there is a router that has all interfaces in the NSSA area, and no interfaces in any other area. It is attached to some cables that run through some switches and ultimately connects to a router that has interfaces in more than one area. This type of router is known as an ABR. This ABR has one of its interfaces in Area 0, and another interface in the aforementioned NSSA area.

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  • Your question is still a little confusing. ...this is done through a multipoint link with a point-to-point address. Do you mean your interfaces are configured as point to multipoint? Or point to point? – Ron Trunk Feb 19 '19 at 18:12
  • @Ron, you actually cleared up my confusion on the the resource usage between the DR/BDR in another post of mine, which actually formed the basis for this question. I now know it would be useless to have the NSSA router have perform the role of DR, because that will not lighten the resource load for the BDR, which would the ABR in the question. – Shōgun8 Feb 19 '19 at 18:37
  • Actually, that was me, not @RonTrunk, to whom you are replying. In any case, the scenario you describe in this question would not use DR/BDR because it is the wrong link type for that. – Ron Maupin Feb 19 '19 at 18:38
  • multipoint with a 31 bit address; the kind of addressing used in point-to-point links – Shōgun8 Feb 19 '19 at 18:46
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    @Octavian Sadly, SE doesn't work that way. The best way for others to understand your question and learn from your answer is to make things clear by creating a new question and deleting this one. Future readers shouldn't have to plow through all this to get to the answer. – Ron Trunk Feb 19 '19 at 20:25
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New answer for the edited question:

Is it ok to have the NSSA router act as the DR for the link between the ABR and itself? Or should it only be the ABR router that is in Area 0 that acts as the BR for the link between the NSSA router and itself?

In such a situation, there are several possibilities:

  • Everything at the default: It doesn't matter which device is the DR and which is the BDR because each router will form a full adjacency with the other router.
  • One router with the priority set to 0 (never be DR or BDR): It doesn't matter which device is the DR because each router will form a full adjacency with the other router.
  • The broadcast interfaces configured as OSPF point-to-point interfaces: There will be no DR/BDR, and each router will form a full adjacency with the other router.

In any scenario, each of the two routers on the network will need to form a full adjacency with the other router on the link. In such a case, it would be normal to configure the interfaces on the routers to set the OSPF network type to be point-to-point using the ip ospf network point-to-point interface command because there is no real need for a DR/BDR.

The idea behind using a DR/BDR is for links where there are multiple OSPF routers to prevent the need for each router on the link from having to form a full adjacency with every other router on the link. That can quickly get out of hand (exponentially). By using a DR, then every router only needs to create one full adjacency with the DR (and one with a BDR, if there is a BDR, which is not a requirement). That can drastically cut back on the number of adjacencies (router resources) and OSPF control traffic required on a broadcast link.

If there are only two routers on a link, then it makes sense to set the OSPF network type to point-to-point to let them simply form full adjacencies with each other, which is going to happen in any scenario. In any of the scenarios, the router resources used for adjacencies will be the same, and eliminating the need for DR/BDR makes sense because that is one less thing for a router to do.

You can even use an OSPF point-to-point network on smaller prefixes, e.g. /24, if there are only two OSPF router in the network.


Original answer for the original question:

Your question is moot because there will be no DR/BDR for point-to-point or point-to-multipoint links. There will be no DR in your scenario.

OSPF Design Guide

Adjacencies on Point-to-Point Interfaces

OSPF will always form an adjacency with the neighbor on the other side of a point-to-point interface such as point-to-point serial lines. There is no concept of DR or BDR. The state of the serial interfaces is point to point.

-and-

Point-to-Multipoint Interfaces

An OSPF point-to-multipoint interface is defined as a numbered point-to-point interface having one or more neighbors. This concept takes the previously discussed point-to-point concept one step further. Administrators do not have to worry about having multiple subnets for each point-to-point link. The cloud is configured as one subnet. This should work well for people who are migrating into the point-to-point concept with no change in IP addressing on the cloud. Also, they would not have to worry about DRs and neighbor statements. OSPF point-to-multipoint works by exchanging additional link-state updates that contain a number of information elements that describe connectivity to the neighboring routers.

Below are the OSPF network types, including the Cisco extensions. Please note the multipoint links (what you ask about in the question) do not elect DR/BDR, which is what makes your question moot.

OSPF elects DR/BDR on broadcast or non-broadcast links:

enter image description here


Perhaps you will believe RFC 2328, OSPF Version 2 (emphasis is mine):

C.6 Point-to-MultiPoint network parameters

On Point-to-MultiPoint networks, it may be necessary to configure the set of neighbors that are directly reachable over the Point-to-MultiPoint network. Each neighbor is identified by its IP address on the Point-to-MultiPoint network. Designated Routers are not elected on Point-to-MultiPoint networks, so the Designated Router eligibility of configured neighbors is undefined.

Alternatively, neighbors on Point-to-MultiPoint networks may be dynamically discovered by lower-level protocols such as Inverse ARP ([Ref14]).

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  • This answer doesn't help at all, in fact it confuses things for you even further. There IS a DR/BDR selection in this scenario because it is a mulltipoint link, as stated directly in the question in the first sentence; " multipoint link with a point-to-point address (31 bit prefix)". So a multi-point link with a 31 bit address still has an election. – Shōgun8 Feb 19 '19 at 18:45
  • I guess you didn't read the part about point-to-multipoint links where is explains: "Also, they would not have to worry about DRs and neighbor statements." I really do not understand why it is so hard for you to understand that point-to-point and point-to-multipoint links do not have DR/BDR. It is all over the Cisco documentation. You simply need to read it. I have been doing OSPF on Cisco devices since probably before you were born. Ron Trunk is a CCIE, and I doubt you will find anyone here that agrees with your position. – Ron Maupin Feb 19 '19 at 18:51
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    @Octavian It seems you are casually using terms like "multipoint" and "point to point" that have a special meaning for OSPF. You now appear to be describing a broadcast network. These terms have special meaning to OSPF and they affect the answer to your question. That's why your question is confusing. We're trying to get clarification so we can give you the answer you're looking for. – Ron Trunk Feb 19 '19 at 19:01
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    BTW the choice of subnet mask has no bearing on the interface type, and therefore neither on the answer. – Ron Trunk Feb 19 '19 at 19:03
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    @Octavian Also, you seem to lack a lot of respect to those who answer your questions. We're just trying to help you, there's no reason to be rude about it. – user36472 Feb 19 '19 at 19:13
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The router in the NSSA area can take the role of the DR with no averse consequences. That is because in this scenario, the are only two routers on the broadcast link, as it has a 31-bit prefix.

Both routers will consume the same amount of resources and both routers will exchange the topologies with only each other. So since the same amount of resources are consumed by the router acting as the DR as are consumed by the router acting as the BDR, there is no advantage gained in specifying the ABR as the BDR. Therefore there is also no advantage gained in specifying the NSSA (non-ABR router) as the DR.

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  • This answer is incorrect because multipoint OSPF networks simply do hot have DRs. If you believe otherwise, despite, and contrary to, the RFC and Cisco documentation, please share the source documentation describing that with us. – Ron Maupin Feb 19 '19 at 19:23
  • I guess I will as soon as you tell me what a 'multipoint OSPF network' is – Shōgun8 Feb 20 '19 at 1:55
  • In OSPF parlance, a multipoint network is an OSPF point-to-multipoint type of network. That is what you will lead any network engineer to understand when you speak of multipoint OSPF networks, as you have seen here, on NESE. We sometimes have problems with novices and the misuse of terminology as you demonstrated, and we put the question on hold as unclear what you're asking until we can determine the correct terminology. You would do well to delete this question, then ask a new question, as it would remove your down votes, and you could provide a correct answer to a correct question. – Ron Maupin Feb 20 '19 at 2:01
  • You may even get some up votes on both the correct question and correct answer – Ron Maupin Feb 20 '19 at 2:17
  • I beleve that the term 'multipoint', when used by itself, refers to a network that resembles a 'broadcast' network. merriam-webster.com/dictionary/multipoint techdifferences.com/… en.wikipedia.org/wiki/HVLAN – Shōgun8 Feb 20 '19 at 18:11

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