topic Re: OSPF specific route change path in Routing and SD-WAN

OSPF specific route change path

Timohamoto — Wed, 25 Jul 2018 21:04:26 GMT

Hi Guys!

How to prioritize route 2.2.2.2 thru R8? Does it possible to match some policy and to put better metric then it now? For current situation for 2.2.2.2 it's multipath coming from R8 and R7 with OSPF metric 6.

But if I set maximum path to 1, it comes from R7. So I want exactly 2.2.2.2 from R8 and everything else no matter, let it go from R7 or R8 independenly. If I play with ip ospf cost on interface it will make all paths comes from prioritezed router. So I need something like policy, to match exactly route and set ospf cost to it. Does it possible?

Re: OSPF specific route change path

Troy Jackson — Wed, 25 Jul 2018 22:49:46 GMT

I'm not sure if you are thinking about using a route-map here. But you can't because the OSPF database needs to be consistent on all the routers within an area. You could use PBR or static routes. I just can't think of another way.

Re: OSPF specific route change path

Francesco Molino — Thu, 26 Jul 2018 04:26:15 GMT

Hi

You don't have too much choices.
Is there any mpls in this design?

If not, you can play with costs along the path or use PBR to force the traffic out through certain specific interfaces like @troy said.

EDIT: just to add for information. Here it seems that you have the same area across all routers and you can't do any policy because the ospf database must be the same on all routers

Re: OSPF specific route change path

Calin C. — Thu, 26 Jul 2018 06:47:31 GMT

R9# conf t

R9(config)# interface fa0/1

R9(config-if)# ip ospf cost 10000

Does this do the trick? I'm pretty sure it does it, but there are multiple things to take into consideration...

Calin

Re: OSPF specific route change path

Timohamoto — Thu, 26 Jul 2018 11:47:57 GMT

Hi Francesco!

Sorry that did't draw clearly, but there are 3 areas. With area 0 in the middle, also thera OSPF only.

So Yes, I did research and there are PBR solution only. Worked for me on that LAB.

So Stil trying to Understand -

- Why with maximum path set to 1. From R10 to R11 It takes route thru router ABR 6.6.6.6 and not 9.9.9.9? I did change router id of R9 to 2.0.0.1 (make ID lower than ABR 6.6.6.6) and cleared process OSPF but it does't help 😐 Seems like "Tie breaker" with all links and costs are the same is ABR?

Re: OSPF specific route change path

Calin C. — Thu, 26 Jul 2018 15:56:01 GMT

Did you clear the OSPF process after changing the router-id?

Re: OSPF specific route change path

Timohamoto — Thu, 26 Jul 2018 16:12:35 GMT

Hi, @Calin C.

Yeap, after configuration made processes restarting.

So it's doen't make sence to put cost - this is works.

I'm Asking in curent topology - what is TIE BREAKER for choosing path thru R6 and not thru R9

Re: OSPF specific route change path

Calin C. — Thu, 26 Jul 2018 16:57:08 GMT

@Timohamoto - Basically you're right, the router-id is the tie break, but I think there are some exceptions.

Can I see an output of the on R7:

show ip ospf neighbors

show ip route ospf (with 1 path and 2 paths)

I want to see how is the prefix known to OSPF.

Thanks,

Calin

Re: OSPF specific route change path

Timohamoto — Thu, 26 Jul 2018 17:22:26 GMT

So there are NSSA in Area2. Here is what we have from two possible ways thru R6 and R9. And what is makes preferable R6... 😞 I did try to change R9 to ID less than R6's 6.6.6.6 but it's still went thru R6.

Re: OSPF specific route change path

Francesco Molino — Thu, 26 Jul 2018 18:20:54 GMT

When you changed router-id on R9, did you cleared the ospf process? If so, did you do it on R9 and R8?

Re: OSPF specific route change path

Timohamoto — Thu, 26 Jul 2018 19:05:49 GMT

@Calin C.@Francesco Molino

Finally Highest router ID makes priority. Thank you guys

(@Calin C. not lowest how I wrote you in private message 😐 some confusion there)

Re: OSPF specific route change path

Francesco Molino — Thu, 26 Jul 2018 19:10:56 GMT

Yeah that's right, Higher router-id makes priority. I didn't noticed you were talking about lower and on your print-screens, R9 have a higher RID.