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New Member

I am a beginner - OSPF issue

Hi expert,

I am a beginner in ospf, below are few issues, hope you can help to solve. Thanks !!!

(1) Is it OSPF will choose DR and BDR in "EACH" LAN segment ?

(2) According to the attach diagram,

(2.1)From the view of "LAN Segment A", is it RTC will be elected as DR and RTD will be elected as BDR ?

(2.2)From the view of "LAN Segment B", is it RTE will be elected as DR and RTG will be elected as BDR ?

(2.3)Then if answer of 2.1 and 2.2 are correct, out of RTC, RTD, RTE, RTG, other routers will be DROTHER ?

(2.4) How about the DR/BDR status of RTC if RTC is a newly add router ? or we need to avoid to add RTC if there is a DR and BDR already exist in the network.

(3) How to define the Router ID in OSPF environment. ( use loopback address ? how about if there is two loopback address in two loopback interface respectively ?)

(4) I seldom see the sample configuration showing the "ip ospf network {broadcast | non-broadcast | point-to-multipoint}"

is it if not specify, the default is ip ospf network braodcast ?

(5) I know the different of E1(external cost) and E2 (external + internal cost ), but how can I choose what type of External route is the best to fit the network

(6) why injected default route or redistributed route need to specify the metric value

(7) why need to generate dafult route by using "default-information originate" , why not just simply add ip route x.x.x.x ?

(8) is it redistribute connected subenet is same as advestise the network under the ospf domain


Re: I am a beginner - OSPF issue

I believe below design guide can reply most questions. Please try to read it.

Hope this helps.

Re: I am a beginner - OSPF issue

Hi ,

To answer your queries ,

Yes , there would be a DR and BDR in each LAN segment and your answer is correct . In ospf , when a new router is added , it would not get involved in DR and BDR election in turn would take the already existing DR and BDR . It can get involved in election , when there is a DR or BDR election again if one goes down.

Always E1 route has a higher priority than E2 route and you have interchanged E1 and E2

E2(external cost) and E1 (external + internal cost ).

The route advertised with the network under the ospf would be available as O - Intra Area routes and O IA - Interarea routes

and redistributed routes would be available as O E2 routes .

Others are very well explained in the OSPF design guide as suggested by Jack.

Hope this helps


vanesh k

New Member

Re: I am a beginner - OSPF issue

Thanks ! vanesh k and jackyoung, but after read the link, I still can't found the answer of Point 5 & 7. Could you kindly help !

Re: I am a beginner - OSPF issue


regarding point 7 , redistribution of the default happens only " when you have a default-information originate " under router ospf : It might be the coding done just to have a control of whether to give a default in my IGP eventhough I have multiple default routes via other protocols like static , bgp...

Regarding question 5 , usually its left to the default type-2 , so that the forward metric(IGP metric to reach the ASBR ) takes care of choosing the best E2 route . Type-1 might be used , when you want to decide between two routes even comparing the both the external and internal metrics.

Hope this helps.


vanesh k

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