Question of impact.

Answered Question
Sep 21st, 2008

Hi,

i have to change the ospf reference bandwidth in our collapsed backbone. Does anybody know what impact i should expect, or is it better to do this at a inspection window?

Tia,

Stephan

I have this problem too.
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Correct Answer by Giuseppe Larosa about 8 years 2 months ago

Hello Stephan,

there was recently another thread about this: the question was changing the ospf reference bandwidth can cause OSPF adjacencies to go down or other effects.

OSPF adjacencies shouldn't be affected because the interface cost (that changes) is not a parameter that must match in the hello packets.

I did this change and I didn't notice OSPF neighbors to go down.

Be aware that all interfaces where the ip ospf cost is present will keep the current manually configured cost and this can lead to some suboptimal routing.

It is better to do this in a maintainance window but the impact should be minimum to null.

Of course you need to do it on all routers/multilayer switches that are in the OSPF domain.

OSPF interface cost is a 16bit integer and route cost is a 24bit integer so you can use a reference bandwidth of 100 Gbps to accomodate 10 Gbps links.

Hope to help

Giuseppe

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Correct Answer
Giuseppe Larosa Sun, 09/21/2008 - 22:57

Hello Stephan,

there was recently another thread about this: the question was changing the ospf reference bandwidth can cause OSPF adjacencies to go down or other effects.

OSPF adjacencies shouldn't be affected because the interface cost (that changes) is not a parameter that must match in the hello packets.

I did this change and I didn't notice OSPF neighbors to go down.

Be aware that all interfaces where the ip ospf cost is present will keep the current manually configured cost and this can lead to some suboptimal routing.

It is better to do this in a maintainance window but the impact should be minimum to null.

Of course you need to do it on all routers/multilayer switches that are in the OSPF domain.

OSPF interface cost is a 16bit integer and route cost is a 24bit integer so you can use a reference bandwidth of 100 Gbps to accomodate 10 Gbps links.

Hope to help

Giuseppe

paul.matthews Mon, 09/22/2008 - 00:46

Bang on. Unless you have only two routers, and can change them at the same time, the effect of the change of reference bandwitdth will diffuse aross the area as you configure each router, so there is the potential for strange routing decisions as devices disagree. Impact should be minimal, but it does need to be done in a change widow - any change may have unforseen effects.

Paul.

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