Loopback+OSPF+RID= Confused on a Sunday

Greetings:

I am doing a self study for the CCNA and I have a question about RID's, LoopBacks, and OSPF DR and how they all play into effect. Please feel free to correct me if I am wrong.

In a network (lets just say an ethernet network) using OSPF during the election process the router with the highest RID will become the DR. The RID is determined by the highest ip address on the interfaces on the router. This part I think I understand. But my question is this: How does a loopback make things different? I am reading the CCNA Study Guide 6th edition by Todd Lammle. In this chapter he established the DR as having the ip address of 10.1.12.1. He then added a loopback interface of 172.16.10.1 to another router and rebooted that router. Now the router with the loopback interface 172.16.10.1 is now the DR. How is 172.168.10.1 higher the 10.1.12.1? Is there something I am missing? What exactly is the highest ip address? Is 1.1.1.1 higher than 224.1.1.1 or am I missing something? Please Help?

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Comments

blackninja

knwminus wrote: »

Greetings:

I am doing a self study for the CCNA and I have a question about RID's, LoopBacks, and OSPF DR and how they all play into effect. Please feel free to correct me if I am wrong.

In a network (lets just say an ethernet network) using OSPF during the election process the router with the highest RID will become the DR. The RID is determined by the highest ip address on the interfaces on the router. This part I think I understand. But my question is this: How does a loopback make things different? I am reading the CCNA Study Guide 6th edition by Todd Lammle. In this chapter he established the DR as having the ip address of 10.1.12.1. He then added a loopback interface of 172.16.10.1 to another router and rebooted that router. Now the router with the loopback interface 172.16.10.1 is now the DR. How is 172.168.10.1 higher the 10.1.12.1? Is there something I am missing? What exactly is the highest ip address? Is 1.1.1.1 higher than 224.1.1.1 or am I missing something? Please Help?

Highest ip address = numerical highest, i.e. 224 is higher than 1 (224 is not assignable to an interface, as multicast (class D))

If there is a loopback interface this will be used OVER the physical interface, if two or more loopback interfaces exist the highest wins.

Bl8ckr0uter

blackninja wrote: »

Highest ip address = numerical highest, i.e. 224 is higher than 1 (224 is not assignable to an interface, as multicast (class D))

If there is a loopback interface this will be used OVER the physical interface, if two or more loopback interfaces exist the highest wins.

Ok. That makes since. BTW I only said 224 because I happened to be looking in my book and it said 224. I understand. So it is literally the highest number. I got it. Now I feel like a tard but I'll live.

linux44

u can also specify the router id by typing router-id 'a.b.c.d' which will be used over loopback and physical int

Bl8ckr0uter

I saw that while I was doing the command. I think I was just very tired yesterday because I understand it now. Now I am working on Spanning Tree Protocol.

What it seems to me that every protocol of any value keeps multiple databases. I wonder if this will hold true...

stlsmoore

The way I look at is that you can use a loop back address if you want to specifically set the RID to be higher or lower for determining the DR/BDR.

mrblackmamba343

During the DR/BDR election the OSPF chooses the router with the highest LOGICAL IP ADDRESS over a router with a highest physical IP address.

In other words loopback interfaces get preference over physical interfaces during the election

jovan88

The DR is chosen by the router priority, the RID is only used as a tie breaker. <- keep that in mind