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ospf small issue,ps help

Unanswered Question
Apr 10th, 2012
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Hi Experts

Pls have a look to the attached diagram


i could not able to acheive  load sharing on links running ospf between r1 , r2 , r3 , r4 from the subnet 172.30.12.144/28 towards the Internet the traffic only use r3 r1 link to go to the Internet r4 r2 is idle link


thanks


jamil

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Overall Rating: 4 (3 ratings)
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Joseph W. Doherty Tue, 04/10/2012 - 09:51
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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


Assuming equal costed links and using R3 as the gateway, that would likely be expected.


It might load balance if you used R4 as the gateway, but can't be certain without knowing interface costing.

neerajjagga Tue, 04/10/2012 - 10:02
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OSPF does not support unequal cost load-balancing.

use ip ospf cost command to change interface cost and also, under OSPF congfigurations, configure maximum-paths 2 command.

Hope it helps.

Ibrahim Jamil Tue, 04/10/2012 - 10:24
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Hi Joseh , hi neerajjagga


all my interfaces are Gig speed so unequal coat load-balancing doesnt exist! am i right?


in my situation what cost should i assign for gig interface , ip ospf cost ???


thanks

Audrius Jonavicius Tue, 04/10/2012 - 10:57
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Ibrahim,


Because you have only one connection to the internet I would suggest to remove the R2 router (IP .5) from topology and create a new link between routers R1 (IP .1) and R4 (IP .6).

Next you can you use GLBP for link load balancing.

http://www.cisco.com/en/US/docs/ios/12_2t/12_2t15/feature/guide/ft_glbp.html


This would be a good solution in your case, with no need to change ospf cost.


Regards,

Audrius

Ibrahim Jamil Tue, 04/10/2012 - 11:07
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hi Audrius


i thing neeraga is right on his proposed solution , he maybe from cisco tac

Audrius Jonavicius Tue, 04/10/2012 - 11:23
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Ibrahim,


yes, neerajjagga is right on proposed solution, you can play with "ip ospf cost" command, but my advise is to change your design, because it is not efficient.

Your proposed design with some corrections is usually used for High Availability. But because you have only one exit point, I do not see any reason to have so many routers.


Regards,

Audrius

Kishore Chennupati Tue, 04/10/2012 - 19:00
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Jamil,


As the other felllow engineers have advised, there is unequal costs across the network. If you want to have ECMP then you have tune the costs which as mentioned here using "ip ospf cost" command.


do a  "sh ip ospf interface brief" and it should give you the details of costs associated with the interfaces participating in the ospf process


BTW, the hostnames are so fudgy, i could barely see which one is what. I nrealy went blind

Joseph W. Doherty Tue, 04/10/2012 - 17:10
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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


Ibrahim Jamil wrote:


Hi Joseh , hi neerajjagga


all my interfaces are Gig speed so unequal coat load-balancing doesnt exist! am i right?


in my situation what cost should i assign for gig interface , ip ospf cost ???


thanks

If all your links are gig and running default costs (1), then you would have unequal costed paths from R3 to R1, but not from R4 to R1, via R2 and R3.  (It's hard for me to make out the routers designations on the diagram, it appears R1, R2, R3, and R4, top to bottom and left to right, correct?)


PS:

OSPF should default to 4 equal cost routes, so shouldn't be a need to change max paths.

neerajjagga Wed, 04/11/2012 - 11:35
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Jamil,

On R3, run this command show ip route ospf and paste the output here. if you do not want to paste the output, look of the cost to networks via interface gi0/0 and gi1/0. the cost appears as highlighted belwo in the routing table:


O 10.0.0.0 [110/3] via x.x.x.x, 00:00:09, Gi0/0  cost is 3 here

O 10.0.0.0 [110/1] via x.x.x.x, 00:00:09, Gi0/1  cost is 1 here

in this scenario, go to interface config mode for gi1/0 and run the command ip ospf cost 3, so the cost to network 10.0.0.0 becomes equal annd do not forget to run maximum-paths 2 under ospf.


Hope it helps.


Neeraj Jagga

khannanaval Wed, 04/11/2012 - 12:22
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Hi,

'


I was going through the above discussion and i believe what is said by Joseph is correct, and while in my datacenter i faced the same situation which i got solved through my trainer at http://networkerszone.com/
he is genius and CCIE in routing and switching and Security, feel free to drop him an email for such queries at [email protected]. he seldom visits forums, but can anser emails.....gud luck

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