cost OSPF on directed connected interface

Answered Question
May 8th, 2009

hello,

to simplify my pb i want to know if it is possible to prioritize the information of cost compared to the fact of being directed connected.

I have 4 routers connected in "circle" and i use OSPF in this wan (A-B-C-D-A). The 4 routers are connecte with microwave, each connection is an E1 connected to a serial interface. I have pb on the link C-D, that is not stable. So i put a cost of 1000 on this link and pass through others links.

The pb is that, even if i put this cost, all flow between C and D are passing through this link.

Have you got an idea (exept changing OSPF by BGP) to solve this problem, and keep the link C-D only in backup mode.

Thanks

I have this problem too.
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Correct Answer by Giuseppe Larosa about 7 years 7 months ago

Hello Yas,

what is the destination address of your traceroute ?

be aware that if you are on RC and you try traceroute to RD's ip address in link C-D what you see is correct.

to verify your change has been successful you need to try to reach a ip address that is not on Link c-d subnet.

for example if

10.1.1.3 is a loopback in RD advertised in OSPF the traceroute from RC should make all the ring without using the direct link.

This is what you should check

Hope to help

Giuseppe

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guruprasadr Fri, 05/08/2009 - 06:28

HI Yas,

pb (Policy Based Routing) will override the OSPF Cost metrics.

The LAN/any IP Segment i.e. matched under PBR and to which interface it is applied,

The Router assumes the exit interface for the matched LAN/any IP Segment will be the defined next-hop interface/ip-address i.e. in PBR.

Hope I am Informative.

Best Regards,

Guru Prasad R

Giuseppe Larosa Fri, 05/08/2009 - 06:45

Hello Yas,

if you have modified the OSPF cost on both sides of the link other factors may drive OSPF to still use the link:

OSPF prefer intra area routes (O in sh ip route) to OSPF inter-area routes (O IA in sh ip route) regardless of total cost.

The hierarchy is:

O > O IA > O E1 > O E2

so if the link C-D is the only one in the same area it makes visible O routes that would be not usable when it is broken.

However, link C-D in single area ring is not used if the cost associated with the link is higher then the sum of all other costs.

Depending on your auto-reference bandwidth under ospf process the cost of each link is:

REf. BW / linkBW

ref- BW is 100 Mbps,

linkBW is expressed in kbps and can be seen in

sh int type x/y

the resulting OSPF cost can be seen in

sh ip ospf int typex/y

For example an E1 link with default OSPF settings is

100000/2048 truncated to 48 for E1 and 64 for T1.

Buf if autoreference-bandwidth is 1000 Mbps each E1 link has cost 488 !

if you are using a single area OSPF domain you can find the way to have link C-D unused in both directions

you may need to increase the OSPF cost on both directions of link C-D

Hope to help

Giuseppe

yas.cisco Fri, 05/08/2009 - 07:38

Hello Giuseppe, thanks for helping me.

The different cost on links are auto configurated, and it is 48 when i do a sh ip ospf int.

i am using a single ospf area domain and i increased the cost to 1000 on both interface C and D. but when i make a traceroute it always shows the using of link C-D.

Prasad send a message which talking about PBR, but i don't understand well the PBR. I must make researches to know how it works and if it is possible with 3600 and 2600 routers running 12.1 and 12.3 IOS.

Could you tell me more about it or send me a link ?

Thanks.

Yas

Correct Answer
Giuseppe Larosa Fri, 05/08/2009 - 08:23

Hello Yas,

what is the destination address of your traceroute ?

be aware that if you are on RC and you try traceroute to RD's ip address in link C-D what you see is correct.

to verify your change has been successful you need to try to reach a ip address that is not on Link c-d subnet.

for example if

10.1.1.3 is a loopback in RD advertised in OSPF the traceroute from RC should make all the ring without using the direct link.

This is what you should check

Hope to help

Giuseppe

yas.cisco Fri, 05/08/2009 - 09:02

Giuseppe,

You were totally right ! I was doing a traceroute on the wrong interface, those for the C-D link !

When i do a traceroute on the lan interface of C and D, i see that i'am passing through the others link.

Your help can permit me to enjoy this week end !

Thanks to all of people who answer my topic and i hope it will help other people.

Yas

Joseph W. Doherty Fri, 05/08/2009 - 08:33

Without knowing how your other links are costed, it's possible your explict setting for C-D isn't sufficient to keep it from being used for transit traffic.

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