OSPF Multi area route inefficiency

[George-Sl]

Hi,

In a multi area like this where if we have a direct link from R5 to R7 and R6, we will have a shorter faster path to those routers, how should we fit it in with ospf protocol definition?

I can't come up with any solution other than merging area 1 and 2?!

my problem with this solution is what if we wanted to add another 7 routers to a new area, and that new area also has to be connected to all areas directly, then what?

[Filip Knezevic]

Even if you would have a direct link to those routers, packets from R5 would go to R2 which is the ABR with area 0. Routers in different areas will always chose a route over area 0 as it helps OSPF to avoid loops. 

So in my opinion you don't really have to merge those two areas. It's more up to your design. 

The problems start when you have 10+ areas per production router :). I opened a thread for that issue:

https://community.cisco.com/t5/routing/ospf-area-design-question/m-p/3857325#M315135

Maybe you find it useful :)

[Joseph W. Doherty · Accepted solution]

"Even if you would have a direct link to those routers, packets from R5 would go to R2 which is the ABR with area 0."

BTW, I believe that's not always correct. If the R5<>R6/R7 links were area 2 links, R5 would have a copy of the area 2 topology and traffic going anywhere in area 2 would take the link if it was the "better" path. This only applies to traffic transiting R5, as R5 would not share area 2 routes with its area 1 neighbors. Also, R6 and R7 wouldn't have any area 1 routes.

If you want two way, you would need to have the R5<>R6/R7 links have logical connections in both area 1 and area 2, but again, the redirection would only apply to traffic on those specific routers, not to any other same area routers.

[Filip Knezevic]

Thanks for the remark, Joseph.

[Joseph W. Doherty · Accepted solution]

Other solutions would be to include those to be interconnected routers in area zero or merge all your routers into just one area (e.g. area zero, unless you have lots and lots of them). Or, perhaps switch from OSPF to EIGRP.

[George-Sl]

Yeah
I just wanted to make sure

OSPF has this lack of full mesh connectivity in between Campuses/ASs, so any connection to a major network has to go over the backbone, for these designs you either better have to switch to eigrp or if you have a very big topology, Use Internal BGP with Eigrp

[Filip Knezevic · Accepted solution]

You can also use eBGP with private AS numbers to route between sites. That makes the failover easy and you can manipulate your own paths the way you want.

[luis_cordova]

Hi @George-Sl 

If any change in the topology occurs, it is the responsibility of the network manager to evaluate the necessary changes.
Having a static topology is never a good idea, since changes, such as those you propose, require dynamism, whether modifying the areas, reallocating the areas, changing the routing protocol, etc.

I am sure that before changes in your network you can make the best decision.
Anyway, if you have doubts about the necessary changes, you just have to post them and we will try to help you.

Regards