Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

OSPF suboptimal path between areas.

I have a configuration similar to that shown in Fig.
The area 0 lies between the switches and routers with LAN links of  1Gbps and an area 1 just for WAN links with low capacity, eg 64Kbps
So when a packet is sent from a Router3 to Router4, this does not take  the most optimal path, marked in blue. This is because when the package  leaves the Router3 for a network in Router4, first reach Router1, this  router has the destination network like intra-area (“O” in routing  table) by the WAN links and chooses this path over inter-area network  with the smallest metric.
The question is whether there is any way to avoid this behavior without  making major configuration changes and maintaining the design of  existing areas.
Thanks for the attention.

Everyone's tags (3)
3 REPLIES
Hall of Fame Super Silver

Re: OSPF suboptimal path between areas.

Hello Roger,

you need to break the OSPF area

OSPF uses a strict hierarchy of route types:

O routes are preferred over O IA routes

what you see is normal and it is like OSPF works

you could move link between R2 and R4 in area 0

this would make R2 the third ABR of area 1

Hope to help

Giuseppe

New Member

Re: OSPF suboptimal path between areas.

Thank you for your fast answer.

I know that this is the behavior of OSPF, but I think that maybe this can change a bit.

I already test the configuration that you propose, and work just fine, but reading Cisco recommendation for OSPF design, it is very recommended to avoid put low Bandwidth link in area 0, to avoid the heavy traffic of OSPF backbone area in these links and to keep the backbone area  more stable and more small as possible.

I would like ask if you think it is a good idea to increase the area 1 or non-backbone area to these routers and eliminate the area 0 from the Router0 (in the middle to the left ) and Router1, so the area 0 remain between Swtich0 and Swithc1 only. I upload a figure for help.

The real configuration has like 400 WAN routers all configured in stub no-summary or NSSA not-summary areas.

Hall of Fame Super Silver

Re: OSPF suboptimal path between areas.

Hello Roger,

yes this can be a way to deal with this moving links in area 1, but the risk is that core routers can become ABR of too many areas

Hope to help

Giuseppe

1346
Views
0
Helpful
3
Replies