OSPF DR/BDR Election explained

Lessons Discussion
143

why does on the video says that router-id will influence the DR/BDR election rather than Ospf priority?

144

Hello Don

The election process for the DR and BDR begins with the OSPF priority. By default, the OSPF priority is the same on all OSPF routers. Therefore, it is the OSPF router with the highest router ID that becomes the DR. So, in the video, Rene is assuming that the priority is the same for all routers.

For a more detailed look at this election process, take a look at this NetworkLessons note:

https://notes.networklessons.com/ospf-dr-bdr-election-process

I hope this has been helpful!

Laz

1 Like
145

we know for the point to point network we don t have OSPF DR/BDR election. but why ??

146

Hello Azondekond

You can configure a particular interface to operate as a point-to-point OSPF topology. This will indeed result in no DR/BDR elections. Why?

Well, remember that DR/BDR elections take place on a per-network-segment basis. In other words, an election takes place between OSPF routers that exist within a broadcast domain, or within a VLAN, or within a single subnet. The OSPF routers will have interfaces with IPs in the same subnet. The purpose of the election is to minimize the number of neighbor relationships that are necessary within such a topology. Such an arrangement is found only when multiaccess technologies are used, such as Ethernet. This is all detailed in the lesson.

However, if you have a point-to-point link between two OSPF routers, then by definition you know that there can only be two OSPF routers on the subnet of that particular link. Serial connections are point-to-point connections that can only have a single device on the other end, unlike Ethernet, which can potentially have hundreds.

So when we define a particular link of a router as a point-to-point network, no DR/BDR elections need to take place because, by definition, only a single OSPF peer can exist on that link. Allowing elections to take place would be a waste of CPU and memory since such a mechanism has no purpose on a point-to-point link.

I hope this has been helpful!

Laz

147

Hi Andrew, There is one query regarding the election of DR/BDR.

The first point is all routers should’ve different R’ids in case OSPF neighborship to be started or established, in case we have it same for 2 or more routers then Neighborship will not form then why do we have criteria after Router ID? If neighborship is not forming then loopback and Highest IP address must not come into the picture.

I think it can be used in this scenario:-

““Because of this, two OSPF routers with the same router ID will not become neighbors but you could still have duplicated router IDs in the network with routers that are not directly connected to each other.””

148

Hello Ajeet

The way that Andrew states this list in his post can be a bit misleading. Let me try to clarify.

Strictly speaking, the DR/BDR election takes place with only two criteria, with the following order of precedence:

  1. The OSPF priority is examined and the highest wins the election. If there is a tie, then…
  2. The highest router ID is used.

That’s it.

Now because the router ID MUST be unique, it will always be a tiebreaker. Now, why did Andrew mention loopback and IP address on an interface? Well, these are the criteria used to determine the router ID on a device. So a router ID is chosen based on:

  1. a manually configured router ID value
  2. the highest loopback address
  3. the highest IP address on an interface of the router

For more info, check out this NetworkLessons Note on OSPF DR/BDR election criteria.

I hope this has been helpful!

Laz

PS I’ve updated Andrew’s answer to be clearer. Thanks!

1 Like
149

Thanks Lazaros, I got the answer.

1 Like
150

Good Lesson!

Thank you Mr. Rene!

1 Like
151

I have a question here related to OSPF DR/BDR election. I have 2 routers and when I checked who is the DR and who is the BDR it shows me that both of them are DR and BDR is area 0.0.0.0 ? network is normal but what is the explanation of this behavior ?

152

Hello Ramy

There are a few things you should keep in mind about the DR/BDR elections.

  • First, remember that a DR/BDR election takes place on a per-segment basis. That means that a particular router may be a DR on one interface, and a BDR on another. Just saying an OSPF router is a DR is not enough information. We must know if it is a DR or a BDR on each OSPF adjacency it has on every interface.
  • Initially, all OSPF routers consider themselves DRs, and set the BDR to 0.0.0.0. That means that no BDR has yet been chosen. Once a neighbor relationship has been successfully achieved, then the DR and BDR for each segment should also have been chosen. If the BDR remains 0.0.0.0, then it is likely the neighbor adjacency has not been established. Use the show ip ospf neighbor command to ensure that the neighbor adjacency has been successfully created.

Use this information to continue your troubleshooting, and if you need further help, just let us know.

I hope this has been helpful!

Laz

153

I have a question related to ECMP

Router R6 advertises the prefix 172.10.3.0/24 using a routing protocol that supports ECMP. All routers are configured with an ECMP value of 4. All links shown in the diagram have the same cost. How many entries with a destination prefix of172.10.3.0/24 are in router R1’s routing table?

image
image1572×668 77.5 KB

154

Hello Ramy

The quick answer is that R1 will have two routes in the routing table for 172.10.3.0/24, one via R2, and one via R3. If the routing protocol supports ECMP, and the metric to the destination is the same, then both will appear in the routing table.

One thing I didn’t understand is this:

What is an ECMP value? Are you talking about the variance command in EIGRP? Can you clarify so that we can help further?

I hope this has been helpful!

Laz

155

Hello Laz.
The ECMP used by OSPF . So the metric will be the cost . what will be the whole path through R2 and R3 ? . ECMP value of 4 means that all Routers are configured to support up to 4 Paths with equal cost and ECMP is enabled on all routers .
Also , if you can please explain further why 2 Paths ?

which two paths will be in the routing table of R1 ?
R1-R2-R4-R6
R1-R3-R5-R6
R1-R2-R5-R6
R1-R3-R4-R6

Thanks for your response .

156

Hello Ramy

Thanks for the clarification. By default, OSPF will install up to four paths in the routing table. The maximum-paths command is used under OSPF configuration mode to change this value anywhere from 1 to 32 paths.

Now the terminology can be confusing. The maximum-paths command will limit the number of routing table entries. Remember that a routing table tells the router the next hop to be used. You are correct that there are actually four possible paths as you stated in your post, but the routing table doesn’t care about what happens after the next hop. It only cares about the next hop.

So for your topology, even though there are indeed four possible routes from end to end that can be taken to reach 172.10.3.0/24, R1 really only has two choices as to where to send the packet, either to R2 or to R3. That’s why there will be two entries in the routing table and not four.

Now R2 and R3 each will have two entries in their routing tables as well, one to R4 and one to R5 since those are equal cost routes as well. However, R4 and R5 will each have only one route to the destination, via R6. Does that make sense?

I hope this has been helpful!

Laz

157

it makes sense . thanks for the clarification

1 Like
158

Hi (Laz),

I have a question that has come up after I did some labbing.

My scenario is this:

  1. I have routers R1 and R2 connected to a switch and I configure OSPF on them
  2. The DR/BDR election takes place and R2 is elected DR (higher RID) and R1 is elected the BDR
  3. Network is stable, with only hello packets going back and forth.
  4. Then, at a later time, I introduce R3.
  5. A 2-WAY state is established between R1-R3 and R2-R3, and, as expected a DR/BDR election takes place immediately on all three switches (R3 doesn’t wait for the 40s timer to expire because hello packets already contain a DR/BDR).

Everything so far is as expected, however, after a FULL adjacency is established between R2 (DR) / R1 (BDR) and R3 (DROTHER), two additional DR/BDR elections take place on both R2 and R1.

The result is the same because the DR/BDR have already been selected, but why they are being triggered is what is confusing me. RFC 2328, ‘9.2. Events causing interface state changes’ contains NeighborChange events that can trigger a BDR/DR election, but I can’t see how any apply to what I am witnessing.

I am seeing the elections using debug ip ospf adj.

Thanks very much.

Sam