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Beginner

OPSF cost

What traffic does the interface command ip ospf cost ##. Traffic incoming to the interface or outgoing of the interface when a router is connected to two ABR routers. Switch X is connected to two ABR switches A & B. IP OSPF cost when configured on interface connected to A, i can see incoming traffic increased but output rate is zero

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VIP Mentor

Re: OPSF cost

Hello

ospf interface cost is only locally significant , So it wont influence its neighboring ospf peer adjacency return path for that to happen you need to apply the costing on both ends of the peer



kind regards
Paul

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Re: OPSF cost

Its easy to understand this concept.

 

Take an example of 4 routers connected in diamond shape. 

 

 

R1--------------b-R2 -a------------------R3        // a,b,c,d are the interface name like fa0/0 or fa0/1

|                                                                         |

|---------------c--R4--d-----------------|

 

Running ospf in area 0. 

 

R3 has 3.3.3.3 and R1 has 1.1.1.1 in the loopback.

 

If i am going to trace the path, for example, its taking path via R2 with no changes in cost. 

If I configure "ip ospf cost 100" inside interface a and clear the ospf process from R1 the path will change to R4.

If I configure "ip ospf cost 100" inside interface b and clear the ospf process from R1 the path will not have any change. 

 

Its just matter how  am learning the route from R3. as the Dataplane and control plane works in opposite direction. 

When I get the 3.3.3.3 route from R3 it will take the "a" cost not the "b" cost.

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Re: OPSF cost

Can someone tell me why my traffic, takes one path and how its deciding the path too.

Please do not hesitate to click the STAR button if you are satisfied with my answer.
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VIP Mentor

Re: OPSF cost

Its long explanation how this is decides the paths.  but in short below URL help you understand better.

 

https://blog.ine.com/2010/05/14/traffic-engineering-challenge

BB
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VIP Expert

Re: OPSF cost

OSPF interface cost is used to determine best path, i.e. egress interface, to destination.

OSPF ingress traffic is determined by other side's interface egress cost.

What you've described can be caused by asymmetrical costing. However, when dealing with ABRs, area types and area address summarization can also cause asymmetrical path selection.
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Beginner

Re: OPSF cost

Thank you all..and i agree with you, but may be i did not describe my complete problem too.

I having exact issue as described here..

https://community.cisco.com/t5/routing/ospf-problem-with-two-abr/td-p/2086740

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VIP Advocate

Re: OPSF cost

Hi,

This command is locally significant and it will increase ospf cost (metric) to the particular interface and it will directly putt effect on outgoing traffic from that interface.

 

Regards,

Deepak Kumar

Regards,
Deepak Kumar,
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