OSFT Designated Router election
TIA568B
Member Posts: 31 ■■□□□□□□□□
in CCNA & CCENT
OSPF DR election is one of the few concepts in CCNA that I'm really struggling to get my head around and I'm hoping for a bit of assistance with it.
My understanding of the election process is it's based on router ID which will automatically be the highest IP address assigned to an interface on a router if it's not set manually or there's no loopback interface set. So the DR will normally be the highest ID and the BDR will normally be the second highest ID, at least that's how I understand it. Is this wrong?
I've been doing a few practice exams (boson, via CISCO net academy, on various websites) and I always seem to be getting "which router will be the DR?" style questions wrong and I'm really struggling to see why.
My understanding of the election process is it's based on router ID which will automatically be the highest IP address assigned to an interface on a router if it's not set manually or there's no loopback interface set. So the DR will normally be the highest ID and the BDR will normally be the second highest ID, at least that's how I understand it. Is this wrong?
I've been doing a few practice exams (boson, via CISCO net academy, on various websites) and I always seem to be getting "which router will be the DR?" style questions wrong and I'm really struggling to see why.
Certs:
Cisco Certified Network Associate (640-802)
Microsoft Licensing Sales Specialist (MLSS) Lite
Cisco SMB SMB Specialization for Engineers (650-195)
Cisco Certified Network Associate (640-802)
Microsoft Licensing Sales Specialist (MLSS) Lite
Cisco SMB SMB Specialization for Engineers (650-195)
Comments
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pham0329
Member Posts: 556
The DR election is based on the Router Priority, with the RID being used as the tie breaker. The priority range is from 0 - 255, with 1 being the default priority. If you want to exclude a router from participating in the election, you can set its priority to 0.
Once a router is enabled for OSPF, it will start sending out Hello messages with the DR and BDR address set to 0.0.0.0, and it will also start the wait timer, which is equal to that of the dead timer. As long as the wait timer is active, the router will look at the DR and BDR field of incoming Hellos to learn about the DR and BDR.
If the wait time expires and no DR or BDR is found, it will start the election process.