[nsp] Newbie OSPF Q.

[Stork, D.H. (Duncan)]

I just wanted to add a little extra info to the answer of Oliver:

OSPF uses 3 databases: neighbour, topology and routes.
Neighbour-table keeps all neighbouring routers.
Topology-table keeps all routers in the area/network.
Routing-table keeps all routes to the destinations.

So when a link goes down, it will set the link to "down" in it's link-state
db, the SPF-algorythm recalculates a new route and puts the new route in
it's routing-table.

Best regards,

Duncan

-----Oorspronkelijk bericht-----
Van: Oliver Boehmer (oboehmer) [mailto:oboehmer at cisco.com]
Verzonden: dinsdag 20 april 2004 15:36
Aan: Gould, Aaron M (NRSW N61CR1W)
CC: Fredrik.Jacobsson at enskilda.se; cisco-nsp at puck.nether.net
Onderwerp: RE: [nsp] Newbie OSPF Q.


> 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.

A route doesn't "die" all for sudden. A route can go away if a
link/router goes down (topology change) or if the announcing router
withdraws his announcement (ex: external, inter-area). Both events go
along with new/updated LSAs being flooded throughout the area, so each
OSPF node detects the change, runs SPF (and/or PRC), finds and installs
the new route.

	oli

> -----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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