OSFP Multiarea

acbenny · ‎09-14-2020

Dear Expert,

Sorry that I can't revise the previous diagram to make it clear and I have create a separate post to describe the situation.

I know that the ospf area (backbone area 0) must not broken inside ospf domain. For other ospf area (e.g. area 10) attached in different boundary in ospf area 0 shown in below. May I know if the link between A-DR2 and B-DR1 is broken (orange link).

will there any ospf route connectivity will be disconnected ultimately to turn down the network ?

OSPF Multi-area 10 ver2.JPG ,

pmckenzie · ‎09-14-2020

Hi there Benny not sure I understand your problem

According to your diagram you have a two OSPF ABR's connected to Area 10.

If the link between ABR1 and A-DR1 is broken ABR1 will still have a path to A-DR1 through ABR-2.

Initially ABR-1 will have shortest path to A-DR1. When link goes down ABR-1 is notified (hardware fault or lack of hellos/ dead timer)

ABR-1 will then update routing table to show ABR-2 as next hop for A-DR1.

There are worlds out there where the sky is burning, where the sea's asleep and the rivers dream, people made of smoke and cities made of song. Somewhere there's danger, somewhere there's injustice and somewhere else the tea is getting cold" Dr Who

acbenny · ‎09-15-2020

Hi Pmckezie,

If the link between A-DR2 and B-DR1 is broken (orange link), the Area 10 will be discontinuous to different protons. Will this impact to the disconnection/ospf down in this case?

pmckenzie · ‎09-15-2020

You are doing strange things with your OSPF.

All traffic has to go through OSPF area 0.

If you had this link between A-DR2 and BDR1 it would have to be a virtual link which would go through area 0 anyways.

So in my opinion the break would not have an effect

There are worlds out there where the sky is burning, where the sea's asleep and the rivers dream, people made of smoke and cities made of song. Somewhere there's danger, somewhere there's injustice and somewhere else the tea is getting cold" Dr Who

balaji.bandi · ‎09-15-2020

As per your HLD diagram, it is hard to say what was designed since this looks like a DC environment with DR Location.

some time due to the inside interface belongs to VRF, traffic between DC to DC user internal path EASt to WEST and WEST to EAST( if the designed like this), Rather going South to North.

what is the cost also factory here, until we get some more information, its hard to understand?

BB

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Giuseppe Larosa · ‎09-15-2020

Hello @acbenny ,

the break in the orange link between A-DR2 and B-DR1 will not cause connectivity issues.

OSPF multi area actually multiple non backbone area to have the same area-id and this is the case when the orange link is broken.

Notice the following:

when orange link is up and running it is used for all traffic intra area as OSPF intra area routes are preferred over OSPF inter area routes regardless of cost.

Breaking the orange link will cause network devices in site A to see all routes of site B as O IA inter area route via backbone area 0 and viceversa site B routers will use ABR3 ABR4 as exit point to reach IP prefixes in site A.

Because LSA type 3 do not contain the area-id of the originator there is no problem. LSA type 3 will be sent into area 0 and will be regenerated by ABR3, ABR4 and sent into site B area 10.

The same apply in reverse direction.

Note: there are no issues if there is no address overlapping between site A and site B, if there is you can have issues but the orange in this case should be a L2 trunk carrying multiple VLANs.

Hope to help

Giuseppe