FW: [nsp] Newbie OSPF Q.

[Stork, D.H. (Duncan)]

The group wasn't included in the receipients, so I forwarded this one...

-----Oorspronkelijk bericht-----
Van: Hudson Delbert J Contr 61 CS/SCBN
[mailto:Delbert.Hudson at LOSANGELES.AF.MIL]
Verzonden: dinsdag 20 april 2004 17:24
Aan: 'Stork, D.H. (Duncan)'
Onderwerp: RE: [nsp] Newbie OSPF Q.



duncan is correct. actually it can take even longer for convergenge
to happen if the link-state info for a route to a dedicated router 
goes down as dr and bdr election related tasks kick-off along with
all the other activities mentioned previously in this thread.
if this router happens to be a redist point for a igp like eigrp
then one will see console message like 'stuck in dual' or if bgp
then route flaps can occur if one hasnt told the protocols not to wait 
if waiting (synch=on of course this is a bad idea
but i see it lot)

/* Del Hudson Contr 61CS/SCBN */


-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net
[mailto:cisco-nsp-bounces at puck.nether.net]On Behalf Of Stork, D.H.
(Duncan)
Sent: Tuesday, April 20, 2004 7:04 AM
To: 'cisco-nsp at puck.nether.net'
Subject: RE: [nsp] Newbie OSPF Q.


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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