OSPF DR election

Hi,

I am a little confused over the DR election issue.

I understand that if the priorities are equal "1" the key factor in the election (as long as all routers boot up at the same time) is the IP address, loopback < physical due to stability issues.

And it is the highest IP on the ENTIRE router, not just the interface involved in the exchange of hello packets which is then the deciding factor.

So if thats all in order how does this line taken from Cisco materials work...
"Because priorities are an interface-specific value, they provide better control of the OSPF multiaccess networks. They also allow a router to be the DR in one network and a DROther in another."

How can priorities be interface specific when the interfaces are not specific to the election process?

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billscott92787

nezzy456 wrote: »

Hi,

I am a little confused over the DR election issue.

I understand that if the priorities are equal "1" the key factor in the election (as long as all routers boot up at the same time) is the IP address, loopback < physical due to stability issues.

And it is the highest IP on the ENTIRE router, not just the interface involved in the exchange of hello packets which is then the deciding factor.

So if thats all in order how does this line taken from Cisco materials work...
"Because priorities are an interface-specific value, they provide better control of the OSPF multiaccess networks. They also allow a router to be the DR in one network and a DROther in another."

How can priorities be interface specific when the interfaces are not specific to the election process?

Just remember the Loopback is the highest priority over any other interface. But, this is only if there is an active loopback on the router. If not then the highest IP address.

The statement you are referring to, "Because priorities are an interface-specific value, they provide better control of the OSPF multiaccess networks. They also allow a router to be the DR in one network and a DROther in another." Is indicating to you that you can change the OSPF priorities on a PER INTERFACE level. You do this with the ip ospf priority "value here 0-255." If an interface is set to priority of 0, that interface does not participate in the election. The higher the priority will be selected over a lower priority. When you say this statement,

"How can priorities be interface specific when the interfaces are not specific to the election process?" They are specific to the election process because by default the interfaces have a priority value equal to 1. Which is the default value, than the election (if it ties) it goes to look at the IP ADDRESSES. The highest address will be selected.

You can change the interface priorities to modify how the election process will take place.

Say you have Router A, Router B, and Router C, which I have provided an example. You need Router A to be the DR. You want no BDR because you have a frame-relay hub and spoke topology. So you want all information to pass through the Hub. You could configure the interface on B (s0/0) with ip ospf priority 0, which would prevent it from participating in the election. Along with the same for router C's s0/0 interface (ip ospf priority 0), which would prevent it from taking place in the election. This would ensure that only Router A is the DR and there are no BDR's.

Hope this clears things up for you.

Example.JPG

Bl8ckr0uter

nezzy456 wrote: »

Hi,

I am a little confused over the DR election issue.

I understand that if the priorities are equal "1" the key factor in the election (as long as all routers boot up at the same time) is the IP address, loopback < physical due to stability issues.

And it is the highest IP on the ENTIRE router, not just the interface involved in the exchange of hello packets which is then the deciding factor.

So if thats all in order how does this line taken from Cisco materials work...
"Because priorities are an interface-specific value, they provide better control of the OSPF multiaccess networks. They also allow a router to be the DR in one network and a DROther in another."

How can priorities be interface specific when the interfaces are not specific to the election process?

Taken from cisco

When two routers attached to a network both attempt to become the designated router, the one with the higher router priority takes precedence. If there is a tie, the router with the higher router ID takes precedence. A router with a router priority set to zero is ineligible to become the designated router or backup designated router. Router priority is configured only for interfaces to multiaccess networks (in other words, not to point-to-point networks).

You can put priority on individual interfaces

ip ospf priority To set the router priority, which helps determine the designated router for this network, use the ip ospf priority command in interface configuration mode. To return to the default value, use the noform of this command.
ip ospf priority number-value
no ip ospf priority

nezzy456

Cheers Guys,

That clears it nicely.

Bl8ckr0uter

nezzy456 wrote: »

Cheers Guys,

That clears it nicely.

I havent labbed since I started my CCNA:S studies so this looks like something to get me going again

ilcram19-2

They routers have the same priority by default which is 1, when they boot up the use the hight ip address on the device which alot of routers also used as the router ID, you can manioulate the router ID as well as the priority of the ODPF interface, if you dont want a router to be a DR just set it to 0, but on a point to popint link you will get somehing like this

Neighbor ID Pri State Dead Time Address Interface
10.1.100.1 0 FULL/ - 00:00:39 10.1.100.1 Tunnel99

also notice that i set the priority as 0 on purpose, so it wont become a DR but still woudnt matter.

ilcram19-2

When two routers attached to a network both attempt to become the designated router, the one with the higher router priority takes precedence. If there is a tie, the router with the higher router ID takes precedence. A router with a router priority set to zero is ineligible to become the designated router or backup designated router. Router priority is configured only for interfaces to multiaccess networks (in other words, not to point-to-point networks).

But there is a big but check this below
Neighbor ID Pri State Dead Time Address Interface
10.1.3.1 1 FULL/BDR 00:00:30 10.1.3.1 GigabitEthernet0/0
10.1.3.4 1 FULL/DR 00:00:34 10.1.3.4 GigabitEthernet0/0
10.1.2.1 100 FULL/BDR 00:00:36 10.1.2.3 GigabitEthernet0/1

as you can see 10.1.2.1 has the highest priority but is not a DR, why? because it went offline and while comming back up 10.1.3.1 and 10.1.3.4 were fighter for the its place and since 10.1.3.4 has biggest ip it tool the place as new DR. If you want 10.1.2.1 to be DR again and use the priority of 100 you will need to reboot the other two routers or/ change the priority to 0 so they will never be DRs.

kryolla

nezzy456 wrote: »

How can priorities be interface specific when the interfaces are not specific to the election process?

The election process is per interface/subnet since an interface can only belong to one subnet not per router