From: Daniel Fredrick (dfredrick@gmail.com)
Date: Thu Oct 05 2006 - 16:22:53 ART
Yes... I was dealing with external routes... and it preferred the lower
ones... I haven't tried it with inter or intra area routes.
On 10/5/06, Victor Cappuccio <cvictor@protokolgroup.com> wrote:
>
> More interesting yet, if you redistribute the information then the lower
> router-id is chosen
>
> R1(config-router)#no maximum-paths 1
> R1(config-router)#do show ip route 33.33.33.0
> Routing entry for 33.33.33.0/24
> Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric
> 64
> Last update from 150.1.123.3 on Serial1/2, 00:00:02 ago
> Routing Descriptor Blocks:
> * 150.1.123.2, from 2.2.2.2, 00:00:02 ago, via Serial1/2
> Route metric is 20, traffic share count is 1
> 150.1.123.3, from 3.3.3.3, 00:00:02 ago, via Serial1/2
> Route metric is 20, traffic share count is 1
>
> R1(config-router)#maximum-paths 1
> R1(config-router)#do show ip route 33.33.33.0
> Routing entry for 33.33.33.0/24
> Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric
> 64
> Last update from 150.1.123.2 on Serial1/2, 00:00:00 ago
> Routing Descriptor Blocks:
> * 150.1.123.2, from 2.2.2.2, 00:00:00 ago, via Serial1/2
> Route metric is 20, traffic share count is 1
>
> Thanks
> Victor.-
>
>
> -----Mensaje original-----
> De: Victor Cappuccio [mailto:cvictor@protokolgroup.com]
> Enviado el: Jueves, 05 de Octubre de 2006 03:03 p.m.
> Para: 'Scott Morris'; 'Daniel Fredrick'; 'Cisco certification'
> Asunto: RE: 2nd try: OSPF best path selection
>
> Hello Scott,
>
> I've tried this and the Higher Router ID seems to be winning the
> election--
>
> R1(config-router)#maximum-paths 1
> R1(config-router)# do show ip route 33.33.33.33
> Routing entry for 33.33.33.33/32
> Known via "ospf 1", distance 110, metric 65, type intra area
> Last update from 150.1.123.3 on Serial1/2, 00:00:04 ago
> Routing Descriptor Blocks:
> * 150.1.123.3, from 3.3.3.3, 00:00:04 ago, via Serial1/2
> Route metric is 65, traffic share count is 1
>
> R1(config-router)# no maximum-paths 1
> R1(config-router)# do show ip route 33.33.33.33
> Routing entry for 33.33.33.33/32
> Known via "ospf 1", distance 110, metric 65, type intra area
> Last update from 150.1.123.2 on Serial1/2, 00:00:00 ago
> Routing Descriptor Blocks:
> * 150.1.123.3, from 3.3.3.3, 00:00:00 ago, via Serial1/2
> Route metric is 65, traffic share count is 1
> 150.1.123.2, from 2.2.2.2, 00:00:00 ago, via Serial1/2
> Route metric is 65, traffic share count is 1
>
> R1(config-router)#
>
>
> -----Mensaje original-----
> De: nobody@groupstudy.com [mailto:nobody@groupstudy.com] En nombre de
> Scott
> Morris
> Enviado el: Jueves, 05 de Octubre de 2006 02:41 p.m.
> Para: 'Daniel Fredrick'; 'Cisco certification'
> Asunto: RE: 2nd try: OSPF best path selection
>
> It's a bit on the "mucked up" side of things... But if you examine
> section
> 16 of the OSPF RFC (2328), you'll find a bit of information on how the
> algorithm's for path calculation take place. One of the things that it
> uses
> in building the Dykstra tree is a "vertex ID" which essentially is the
> router-id. Being that this is a piece of the puzzle, and we always go for
> the lower information, when you reduct the maximum paths to one, this
> would
> be the only differentiator between two routes.
>
> And therefore the lowest number will win.
>
> HTH,
>
> Scott Morris, CCIE4 (R&S/ISP-Dial/Security/Service Provider) #4713, JNCIE
> #153, CISSP, et al.
> CCSI/JNCI-M/JNCI-J
> IPExpert VP - Curriculum Development
> IPExpert Sr. Technical Instructor
> smorris@ipexpert.com
> http://www.ipexpert.com
>
>
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> Daniel Fredrick
> Sent: Thursday, October 05, 2006 2:27 PM
> To: Cisco certification
> Subject: 2nd try: OSPF best path selection
>
> OK... this seems like an easy question, but is difficult of find
> documentation on it.
>
> If you have a setup like the following...
>
> (R4) (R3)
> \ /
> \ /
> (R5)
>
> R4 and R3 are sending the same route to R5. On R5, they both have an equal
> cost... so by default ospf on R5 will load balance across the two paths,
> since the metric-type and metric are equal.
>
> Now this is part that I cannot find documentation on...
>
> If I go under the ospf process on R5 and set the maximum-paths to 1. Now
> load balancing is disabled.
>
> How does R5 determine which route to prefer? What if I change the Admin
> distance for the routes coming from R4 to 100? Will that influence
> anything?
>
> I noticed it goes by lowest-router ID... but is there some documentation
> on
> this... or an order of steps that ospf will follow for the best path
> selection?
>
> Thanks,
> Dan
>
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