05-11-2010 06:30 AM - edited 03-04-2019 08:26 AM
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.
05-11-2010 07:08 AM
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
05-11-2010 09:23 AM
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.
05-12-2010 06:11 AM
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
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