[nsp] Newbie OSPF Q.

[Matthew Crocker]

OSPF maintains a database (show ip ospf database) which is a complete 
map of all links and announcements from routers in the area.  When a 
link goes down the router 'owning' the link will update its 
announcement in the database.  All routers will run the Shortest Path 
First (SPF) algorithm on the database after an update.  SPF determines 
the best route and enters that into the Forward Information Base (FIB). 
  The FIB is used by Cisco Express Forwarding (CEF) to forward packets.  
The FIB is what the router uses to route traffic.   OSPF maintains the 
alternate paths in the database so when a link gets removed from the 
database and SPF is run again it will pick the alternate path and enter 
it into the FIB.

debug ip ospf spf   will show the SPF process running and  the 
decisions it makes.

-Matt

On Apr 20, 2004, at 9:27 AM, Gould, Aaron M (NRSW N61CR1W) wrote:

> oliver and group, if ospf discards the alternate paths and only enters 
> the
> least costly (most preferred) route into its rib, how does a ospf 
> process
> running in a router add a new (more costly/less preferred but good
> nonetheless) route into its rib when the most preferred one goes down?
> ...i mean if it has no other routes to add to its rib because it 
> previously
> "discarded" them how will it accomodate an outage in the case of the 
> more
> preferred route dieing.
>
> aaron
>
> p.s.  i'm not sorry for asking a newbie question.  ha (just kidding)
>
>
>
>
> Aaron Gould (CACI)
> Network Engineer, PACSW/CNRSW/RITSC
> 619-545-2569
> 619-981-4900 cell
> <><
>
>
> -----Original Message-----
> From: Oliver Boehmer (oboehmer) [mailto:oboehmer at cisco.com]
> Sent: Tuesday, April 20, 2004 2:08 AM
> To: Fredrik.Jacobsson at enskilda.se
> Cc: cisco-nsp at puck.nether.net
> Subject: RE: [nsp] Newbie OSPF Q.
>
>
> Hi,
>
>> I'd like to see all possible paths to a network announced by OSPF.
>> In my case I have three possible links, not having equal costs.
>>
>> Is there a way to see all paths (as with sho ip route) where one can
>> see paths with unequal costs. And the costs listed as well.
>
> No, there is no way to see the alternate paths. You can take a look 
> into
> the LSA's and see which alternate node advertised the prefix, but this
> doesn't give you what you're looking for.
> When we perform the SPF algorithm, only the best path(s) are entered
> into the RIB, the alternate paths are discarded.
>
> In IOS' ISIS implementation, there is a hidden knob
> ("display-route-detail") which shows you the backup paths (rather: the
> LSP which contains it and the metric), but we don't do this for OSPF, 
> as
> far as I know.
>
> 	oli
>
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