OSPF suboptimal path

[rafael2gc]

Hi,

I´m having an issue with ospf related to the fact that OSPF always prefer an INTRA-area route over an INTER-area.

The point is that the INTER-area path is the optimal path couse it is 10G.

There is any way, except put all elements on area 0, to solve this issue?

Regards,

Rafael

[Reza Sharifi]

Hi,

A. According to section 11 of RFC 2328 , the order of preference for OSPF routes is:

• intra-area routes, O

• interarea routes, O IA

• external routes type 1, O E1

• external routes type 2, O E2

This rule of preference cannot be changed.

http://www.cisco.com/en/US/tech/tk365/technologies_q_and_a_item09186a0080094704.shtml#q13

HTH

[cadet alain]

Hi Reza,

in order to circumvent this order of preference which can't be changed, couldn't we use longest match traffic engineering?

Regards.

Alain.

[Reza Sharifi]

Hi Alain,

This can possibly be provided by using MPLS/RSVP and building bidirectional tunnels.  This way, the IGP path is not followed.

Regards,

Reza

[rafael2gc]

Hi Reza Sharifi, thank you for your time.

I already knew this rules,  i need re-design this OSPF solution.

Imagine two rings, one is 10G  with 3 elements in area 0 and another is 1G with 60 elements in area 1.

The 3 10G elements are ABR with area 1, it has tengig interface in area 0 and 1gig interface in area 1.

The problem is when i advertise the loopback of elements in area 1.

Imagine one element that is one 1gig hop from the 10G ring, instead of goes through 1gig hop then take the 10G hop to the destination, it goes through many 1gig hops through the 1G ring to the same destination.

It happen becouse of the order preference you show us.

The point is, how make the traffic use the optimal path by the 10G ring? Put all in area 0 is not desirable.

thanks

Rafael

[Reza Sharifi]

Rafael,

Can you provide a simple diagram showing how you are planning to design this network with 1Gig and 10Gig?

Reza

[cadet alain]

Hi Reza,

isn't there a simpler solution than MPLS/RSVP? using the longest match forwarding principle depending on topology of course.

Regards.

Alain.

[rafael2gc]

Reza,

There we go.

Lets take sw-1 as an example.

The traffic from CE connected to SW-1 goes through the 1G ring to reach resources on DC.

I know why. I need a way to overcome this behavior.

[Reza Sharifi]

Rafael,

How about putting each section around area 0 a different area?

Area 0 in the middle

lower section area 1

left side area 2

right side are 3

HTH

[rafael2gc]

Reza,

I understod what you said but this is not a definitive solution.

Lets take the right side. Imagine the last switch in this section.

To go to DC it will take the path along the 1G ring, not the path through the ABR and them the 10G ring.

Got it?

Rafael

[Reza Sharifi]

If you put each section in a different area, then in your example the last switch in the right side will take the 1Gig way up until it gets to the ABR.  From there, it will take the 10Gig path (shortest path) to DC.

[rafael2gc]

Sorry Reza but a have to disagree.

Think about the loopback of this switch being advertised to the CORE-Router that is connected to DC.

The CORE-Router will always preffer the INTRA-area route over the INTER-area, becouse of this, CORE-Router will always prefer the 1Gig path over the 10Gig, unless the 1Gig ring is open.

agree?

Rafael

[Joseph W. Doherty]

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Posting

I believe what Reza proposed (one area per diagramed ring) would work as long as you weren't using summaries and DC destinations were either in area zero or different area from the rings.

"The CORE-Router will always preffer the INTRA-area route over the  INTER-area (correct), becouse of this, CORE-Router will always prefer the 1Gig  path over the 10Gig, unless the 1Gig ring is open. (correct with your diagramed topology - not so if rings are different areas, as I believe is what Reza proposed)"

[Joseph W. Doherty]

Disclaimer

The   Author of this posting offers the information contained within this   posting without consideration and with the reader's understanding that   there's no implied or expressed suitability or fitness for any purpose.   Information provided is for informational purposes only and should not   be construed as rendering professional advice of any kind. Usage of  this  posting's information is solely at reader's own risk.

Liability Disclaimer

In   no event shall Author be liable for any damages whatsoever (including,   without limitation, damages for loss of use, data or profit) arising  out  of the use or inability to use the posting's information even if  Author  has been advised of the possibility of such damage.

Posting

Another approach, you can trunk the two areas across the 10gig link.  I've used it.  Works fine.

This approach also handles the situation if you summarize a non-zero area.

This approach also supports different non-zero areas.  If, for instance, if you wanted to partition your three area 1 segments into different areas.