[c-nsp] OSPF Cost

[Adam Vitkovsky]

Hi,

That is because when you increase an OSPF cost on the R2 interface you are essentially telling the router to advertise LSAs out that interface using this higher cost 100.
So to the neighbouring router R3 the link to R2 over which it received the LSAs will appear with higher cost 100.
That is why traffic from R3 to R1 goes via R2.

However since you did not increment the metric on R3 interface towards R2 - R3 is still advertising LSAs to R2 with a standard metric so that is why trace from R2 goes via direct link.

This is sometimes referred to as asymmetric link costing and might be desirable in some situations. 
 
adam
> -----Original Message-----
> From: cisco-nsp [mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of
> M K
> Sent: 12 March 2015 13:59
> To: cisco-nsp at puck.nether.net
> Subject: [c-nsp] OSPF Cost
> 
> I have the below setup
> R2 -- R3|     /R1
> Each router has a PC connected to itThe setup implement a flat OSPF setup
> (area 0)
> Now , the trace from PC1 (Connected to R1) to PC3 (Connected to R3) will go
> through the link between R1 and R3 which is normalNow , I have raised the
> cost from the interface of R3 connecting to R1 (ip ospf cost 100) and the trace
> still go through the link between R1 and R3When I do the same but from R1
> side , the trace will follow the path R1 - R2 - R3Why ?
> 
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