From: Peter van Oene (pvo@xxxxxxxxxxxx)
Date: Wed Apr 24 2002 - 17:34:00 GMT-3
Actually, I didn't read the whole thread. In this case, the /24 should
disappear when the virtual link comes up.
In the example, with R2 ---R5 ---- R4 as the topology, before the virtual
link from R4 comes up, R4 is an internal router from an OSPF
perspective. As such, it is able to use the type 3 from R5 which describes
the /24 network. However, as soon as the virtual link comes up, R4 becomes
an ABR. ABR's are only allowed to use type 3's from area 0, and thus the
type 3 it was using from R5 becomes unusable. Hence, the /24
disappears. R4 then learns the /28 via the type 1 that it picks up in area 0.
The order of preference between OSPF routes is irrelevant in this case as
is the relative depth of the subnet mask. The fact that ABR's cannot use
type 3's from non backbone areas is the primary lesson here. This behavior
is specified in the RFC, and is there to prevent looping.
Pete
At 03:24 PM 4/24/2002 -0500, Ambern, Jeff wrote:
>Please explain your reasoning. I'm don't see
>why the behavior should be different.
>
>Jeff
>
>-----Original Message-----
>From: Peter van Oene [mailto:pvo@usermail.com]
>Sent: Wednesday, April 24, 2002 1:36 PM
>To: Sam.MicroGate@usa.telekom.de; jambern@MerchantWired.com
>Cc: ccielab@groupstudy.com
>Subject: RE: How does OSPF enter routes into the routing table?
>
>
>In this case, both should go into the table.
>
>At 12:19 PM 4/24/2002 -0400, Sam.MicroGate@usa.telekom.de wrote:
> >I think you are correct. The same prefixes with two different subnet masks.
> >R4 receives 172.10.7.0/24 O IA and 172.10.7.0/28 O and then installs the
>/28
> >in the routing table. That is because, of cource, it is O.
> >
> >Sam
> >
> >-----Original Message-----
> >From: Ambern, Jeff [mailto:jambern@MerchantWired.com]
> >Sent: Wednesday, April 24, 2002 11:25 AM
> >To: 'Sam.MicroGate@usa.telekom.de'
> >Cc: 'ccielab@groupstudy.com'
> >Subject: RE: How does OSPF enter routes into the routing table?
> >
> >
> >I still believe that it has to do with the prefix being the same length
> >and the first route is an intra-area route. Try making the /28 something
> >other than subnet-zero and see the /24 shows up in the routing table.
> >
> >Jeff
> >
> >-----Original Message-----
> >From: Sam.MicroGate@usa.telekom.de [mailto:Sam.MicroGate@usa.telekom.de]
> >Sent: Wednesday, April 24, 2002 10:18 AM
> >To: jambern@MerchantWired.com
> >Subject: RE: How does OSPF enter routes into the routing table?
> >
> >
> >What about the scenario below? did you take a look at it?
> >
> >Sam
> >
> >-----Original Message-----
> >From: Ambern, Jeff [mailto:jambern@MerchantWired.com]
> >Sent: Wednesday, April 24, 2002 11:14 AM
> >To: 'Sam.MicroGate@usa.telekom.de'
> >Subject: How does OSPF enter routes into the routing table?
> >
> >
> >Ospf will insert Intra-Area routes into the routing table befor Inter-Area
> >routes.
> >The order of preference is:
> >O
> >IA
> >E1
> >E2
> >Jeff
> >
> >
> >--------------------------------------------------------------------------
> >Hello all,
> >
> >Last night I worked in a distribution lab from fatkid.com. In this lab I
>had
> >the following problem:
> >Area 2 is configured a virtual area between routers R5 and R4. I used area
>0
> >range command to summarize the /28 subnet into area 2. Now the problem is
>R4
> >receive both routes the 172.10.7.0/28 as O and 172.10.7.0/24 as O IA and
> >strangely choose the first to install in the routing table. The /24 subnet
> >gets suppressed and does not make it to the routing table. Shouldn't I see
> >both routes in R4 because OSPF supports VLSM? Why OSPF choose the route
>with
> >the longer mask and ignored the one with shorter mask? If this the default
> >behavior, How can I make the summary address /24 reach R4. Thank you all
>for
> >your help.
> >
> >
> >
> >172.10.7.0/28 172.10.128.0/26 LB0:172.10.5.4/24
> >R2------------------------------R5------------------------------R4---------
>-
> >-------------------
> >area 0 area 2 area 3
> >Frame Relay Ethernet LB of R4
> >
> >At R5:
> >area 0 range 172.10.7.0 255.255.255.0
> >
> >
> >
> >
> >Elsayed Mohamed
> >Sr. Network Consultant
> >Microgateds, Inc.
> >732-936-4413
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