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OSPF Election Process

Just want to clarify the DR/BDR election process.

Does it select the DR in this order? -Highest priority becomes DR, if all same priority then: -Highest Router ID, if no RID then: -Highest IP on loopback interface, if not set then: -Highest IP on physical interface

Hope that make sense.

Appreciate the help :)

10 comments
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level 1
IT depends2 points · 8 months ago

Does it select the DR in this order? -Highest priority becomes DR, if all same priority then: -Highest Router ID

Till there you are correct.

if no RID then: -Highest IP on loopback interface, if not set then: -Highest IP on physical interface

You can't reach the DR/BDR election if there is no RID.

level 2
CCNAOriginal Poster1 point · 8 months ago

Ahh yes, what I meant by "no RID" is no RID set by the user using the router id command. I can see now it should be a question of priority vs RID - however I've added/mixed the rid selection process in there as well. Thank you !

level 3
Just 'cause it ain't in my flair doesn't mean I don't have certs1 point · 8 months ago

It's just simply highest priority, then highest RID. If you have two routers with the same RID, shit's broken. Same if you don't have an RID at all.

level 4
[deleted]
1 point · 8 months ago

Adjacency wouldn't even form with same RID therefore no DR/BDR election process. If you get to the point where you've got two devices with the same RID in an election process you've hit the part of the code which generally has a comment like

/* shit, you should never get to this point - abort, abort, abort */
level 1
Now with more Cisco!1 point · 8 months ago

Keep in mind that DR/BDR election based on Priority and RID is an OSPF concept. The device conveniently selecting a RID from a loopback or physical address if none is supplies is a Cisco feature. OSPF doesn't care where your RID comes from, just that you have one, and it's unique. Cisco noticed that those rules are generally followed for interface addresses--especially loopbacks--and so tried to make your life easier.

level 1
Meow 🐈🐈Meow 🐱🐱 Meow Meow🍺🐈🐱Meow A+!1 point · 8 months ago

Close, except that OSPF must have a router id

level 2
CCNAOriginal Poster1 point · 8 months ago

I understand that the router will always have an RID (whether you set it or it uses an interface IP) but if you set the priority higher than other routers, will the routers use that to select the DR over the highest RID?

level 3
IT depends3 points · 8 months ago

Correct, it goes like this:

  1. Highest OSPF priority

  2. Highest RID

level 4
CCNAOriginal Poster1 point · 8 months ago

Excellent thanks guys :)

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