From: Daniel Fredrick (dfredrick@gmail.com)
Date: Thu Oct 05 2006 - 16:07:20 ART
Thanks so much... I forgot all about using RFC as resouces... I usually
avoid them... or made it a habit when I study study a couple of years ago,
so I just forgot about them. I will use them more often... Thanks.
Oh.. if any one is interested... here is the part Scott is talking about. He
is right it is buried in there good...
" If the newly added vertex is a transit network, the routing
table entry for the network is located. The entry's
Destination ID is the IP network number, which can be
obtained by masking the Vertex ID (Link State ID) with its
associated subnet mask (found in the body of the associated
network-LSA). If the routing table entry already exists
(i.e., there is already an intra-area route to the
destination installed in the routing table), multiple
vertices have mapped to the same IP network. For example,
this can occur when a new Designated Router is being
established. In this case, the current routing table entry
should be overwritten if and only if the newly found path is
just as short and the current routing table entry's Link
State Origin has a smaller Link State ID than the newly
added vertex' LSA."
Thanks,
Dan
On 10/5/06, Scott Morris <swm@emanon.com> wrote:
>
> 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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