OSPF and Testout

Not many people know this. 244.0.0.5 is the multicast address each router uses to send hello packets. This is the drother to drother IP address. Then you have your 224.0.0.5 and 224.0.0.6.

For those of you who are in Cisco Academy classes, I suggest you purchase Testout to supplement your classes. What I do is read the cirriculum, go through Testout, reread the Cisco cirriculum, and go through Testout a second time. Once you go through it that second time, go to the Practice Exams section and take the practice exam for the domain you just studied. There you will have an all in one exam to study for your chapter test rather than taking 4-7 of the section exams.

Also, why can't Cisco just say this is how the election process works:
Interface priority
Router ID
Highest IP address on a loopback interface
Highest IP address on a physical interface

I don't know why the interface priority gets their own special section instead of explaining how it ties into the other 3 tie breakers below.

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Comments

marlon23

you have it mixed. Interface priority and Router ID comes to play during election of DR on a multiaccess network (aka segment). Configured Router ID, highest loopback IP, highest interface IP its a process to select Router ID which would uniquely identify router in a network when OSPF process starts.

BroadcastStorm

I have a little note here from a while back, I don't guarantee the consistency of it.

DR and BDR election is accomplished via the Hello protocol. Hello packets are exchanged via IP multicast packets on each segment.

However, only segments that are broadcast and nonbroadcast multi-access networks (examples are Ethernet and Frame Relay) will perform DR and BDR elections. Point-topoint links, like a serial WAN for example, will not have a DR election process.

On a broadcast or nonbroadcast multi-access network, the router with the highest OSPF priority on a segment will become the DR for that segment. This priority is shown with the show ip ospf interface command.

The default priority for a router interface is one, If all routers have the default priority set, the router with the highest Router ID (RID will win.

The RID is determined by the highest IP address on any interface at the moment of OSPF startup. This can be overridden with a loopback (logical) interface.

If you set a router's interface to a priority value of zero, that router won't participate in the DR or BDR election on that interface. The state of the interface with priority zerio will then be DROTHER.

This is useful for the following scenario:

- old model and slow router
- router with existing roles
- frame-relay spoke router

"The default priority of an OSPF-enabled interface is 1. The interface with the highest priority becomes the DR, and the interface with the second-highest priority will become the BDR"

The following command has to be issued on the router you intend to become as a non DR or BDR.

R2(config-subif)#ip ospf priority ?
<0-255> Priority
R2(config-subif)#ip ospf priority 0

chrisone

Sorry no need to purchase "testout" , this information is clearly in the CCNP BSCI self study and exam certification guide books. Also MOST people know this who are CCNP.

abdulwart

I was just reading the discussion and the answers and comes to my mind to ask something around the topic discussed.

Is it possible that one router will be both DR in one shared network and the same router will be BDR for another shared network. I do know it is possible but would like more illestration on this.

Appreciate it.

dtlokee

R1-------+-------R2
         |
         R3
         |
R4-------+-------R5
 
Loopback addresses:
R1 1.1.1.1/32
R2 2.2.2.2/32
R3 3.3.3.3/32
R4 4.4.4.4/32
R5 5.5.5.5/32

Assuming the default priorities and only 1 loopback interface per router.

the upper segment between R1, R2 and R3 - R3 will become the DR becasue it has the highest loopback IP address and R2 will become the BDR because it has the second highest loopback IP address. R1 will become 'DROTHER'.

The lower segment between R3, R4, adn R5 - R5 will become the DR because it has the highest loopback ip address and R4 will become the BDR because it has the second highest loopback IP address. R3 will be 'DROTHER'

If you increase the interface priority on R3's interface connected to the lower segment you could foce it to become the DR, or if you never want it to become a DR or BDR you can set it to zero - the default priority is 1.