From: Victor Cappuccio (cvictor@protokolgroup.com)
Date: Thu Oct 05 2006 - 16:20:10 ART
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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