OSPF questions.

Answered Question
May 15th, 2007

Dear all,


Might I ask the following questions:


1. Is there a maximum limit on the number of interfaces that a single router can have in the OSPF Area 0?


2. Is it so that that the DR/BDR elections do not happen on non-broadcast point to point networks? In my current lab, I have a RT1 connected to RT2 over FR. RT2 is connected to RT3 over 802.3. I have changed the OSPF priority and the RID, however, there is still no election results, i.e. DR and BDR is 0.0.0.0.


I have configured the FR part as broadcasts.


3. Does the ip ospf network type have to match on routers?



Kind regards

Correct Answer by mounir.mohamed about 9 years 9 months ago

Dear Bennial,


1-There is no limit to attach all interfaces on one router to the backbone area or even non-backbone area it's only depending in the Memeory and CPU requeriments


2-point-to-point links has not DR/BDR ellection, DR/BDR ellection only happen on Broadcast and non-broadcast network type.


Also chaning the network type by using "ip ospf network type broadcast non-broadcast point-to-multipoint point-to-multipoint non-broadcast" will effect 2 things 1st will effect the hello style (Unicast/Multicast) 2nd will effect the DR/BDR ellection, for example DR/BDR would happen if you change the network type under the interface level to broadcast from both sides BUT TAKE care from the layer2 to layer3 mapping issue so if you using major serrial interface you need to disable invers-arp by using "No fram-relay inarp" and map the neighbor IP to specific DLCI with keywork broadcast because Braodcast network type sending multicast hello packets.


3-OSPF network type should be matched from both sides BUT in some cases it may work like if you configured one interface as point-to-point and another interface as point-to-multipoint this should work because point-to-point and point-to-multipoint both using multicast for hello packets.


Please rate helpful posts.


Best Regards,

Mounir Mohamed


  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 5 (4 ratings)
Loading.
Wilson Samuel Tue, 05/15/2007 - 05:59

1. Is there a maximum limit on the number of interfaces that a single router can have in the OSPF Area 0?


I strongly feel that there is 'no limit' on the number of interfaces that a single router can have with Area 0 (I doubt if one may use a 6500 with Sup720 with all ports in the L-3 mode, its impractical)


2. Is it so that that the DR/BDR elections do not happen on non-broadcast point to point networks? In my current lab, I have a RT1 connected to RT2 over FR. RT2 is connected to RT3 over 802.3. I have changed the OSPF priority and the RID, however, there is still no election results, i.e. DR and BDR is 0.0.0.0.


DR/BDR, elections are NON-PREMPTIVE i.e. once a DR/BDR is elected there is NO Change unless the they become un-available. changing priority or RID wont change anything on DR/BDR.


3. Does the ip ospf network type have to match on routers?


I guess its for the NBMA networks, its better that they match or else if one is with Broadcast and the other one is with NBMA then one configured with Broadcast mode expects to hear broadcast and won't be able to form adjacency.




Please rate if it helps,


Regards,

Wilson Samuel


srue Tue, 05/15/2007 - 06:40

addendum to number 2 above:

You can force an election using the "clear ip ospf process" or " clear ip ospf process_number process" commands. once the priority or RID is updated, it will use the new settings. You can also use those commands to force an election any other time as well.

Correct Answer
mounir.mohamed Tue, 05/15/2007 - 11:40

Dear Bennial,


1-There is no limit to attach all interfaces on one router to the backbone area or even non-backbone area it's only depending in the Memeory and CPU requeriments


2-point-to-point links has not DR/BDR ellection, DR/BDR ellection only happen on Broadcast and non-broadcast network type.


Also chaning the network type by using "ip ospf network type broadcast non-broadcast point-to-multipoint point-to-multipoint non-broadcast" will effect 2 things 1st will effect the hello style (Unicast/Multicast) 2nd will effect the DR/BDR ellection, for example DR/BDR would happen if you change the network type under the interface level to broadcast from both sides BUT TAKE care from the layer2 to layer3 mapping issue so if you using major serrial interface you need to disable invers-arp by using "No fram-relay inarp" and map the neighbor IP to specific DLCI with keywork broadcast because Braodcast network type sending multicast hello packets.


3-OSPF network type should be matched from both sides BUT in some cases it may work like if you configured one interface as point-to-point and another interface as point-to-multipoint this should work because point-to-point and point-to-multipoint both using multicast for hello packets.


Please rate helpful posts.


Best Regards,

Mounir Mohamed


Actions

This Discussion