Re: preferring OSPF inter-area compared to intra-area

Narbik,
try to link R2 fa0/0 to R3 fa0/0 and put them in area 1
what I was trying to do, the thing that triggered this thread, was to reach
R2 L0 from R1 but through R3.
According to Brian this is not possible..since I'm preferring inter-area
instead of intra-area. With a route-map I'm going to filter RIB.
If I've understood well I can filter RIB as much as I can but OSPF won't
change his decision.
I should filter ospf db instead, but this is not possibile since I'm in the
same area and I have lsa type1 and type2.

easiest workaround for sure is move the loopback 0 from area 1 to area 0.

did I write something not correct? am I in the correct path now?

On Tue, Jan 8, 2013 at 5:14 AM, Narbik Kocharians <narbikk_at_gmail.com> wrote:

> *SORRY FOR THE LONG POST.*
>
> *
> *
>
> *Lets say we have a hub and spoke topology, where R1 is the hub router,
> and R2, R3, and R4 are the spoke routers. All the links are configured as
> P2P using the following IP addressing scheme:*
>
>
>
>
R1(S0/0.12)12.1.1.1/24--------------12.1.1.2(S0/0.21)R2<http://12.1.1.1/24---
-----------12.1.1.2%28S0/0.21%29R2>
>
>
R1(S0/0.13)13.1.1.1/24--------------13.1.1.3(S0/0.31)R3<http://13.1.1.1/24---
-----------13.1.1.3%28S0/0.31%29R3>
>
>
R1(S0/0.14)14.1.1.1/24--------------14.1.1.4(S0/0.41)R4<http://14.1.1.1/24---
-----------14.1.1.4%28S0/0.41%29R4>
>
>
>
> *R2 is running OSPF on its Lo0 and Lo1 with IP addresses of 2.2.2.2/8 and
> 200.2.2.2/24 respectively.*
>
> * *
>
> *R3 is running ospf on its Lo0 with an IP address of 3.3.3.3/8*
>
> *R4 is running ospf on its Lo0 with an IP address of 4.4.4.4/8*
>
> * *
>
> *Everything is running in OSPF area 0. Lets begin:*
>
>
>
> *On R1*
>
>
>
> R1#Show ip route ospf | I O
>
> *O 2.0.0.0/8 [110/65] via 12.1.1.2, 00:01:24, Serial0/0.12*
>
> O 3.0.0.0/8 [110/65] via 13.1.1.3, 00:00:44, Serial0/0.13
>
> O 4.0.0.0/8 [110/65] via 14.1.1.4, 00:00:14, Serial0/0.14
>
> *O 200.2.2.0/24 [110/65] via 12.1.1.2, 00:01:24, Serial0/0.12*
>
>
>
> *You can see that R1 is receiving two routes from R2, networks 2.0.0.0/8and
> 200.2.2.0/24.*
>
>
>
> *Lets filter all routes coming through S0/0.12:*
>
>
>
> *On R1*
>
>
>
> R1(config)#*Route-map tst deny 10*
>
> R1(config-route-map)#*match interface s0/0.12*
>
> R1(config)#*route-map tst permit 90*
>
>
>
> R1(config-route-map)#*router ospf 1*
>
> R1(config-router)#*distribute-list route-map tst in*
>
>
>
> R1#*Show ip route ospf | I O*
>
> O 3.0.0.0/8 [110/65] via 13.1.1.3, 00:00:38, Serial0/0.13
>
> O 4.0.0.0/8 [110/65] via 14.1.1.4, 00:00:38, Serial0/0.14
>
>
>
> *Lets remove the previous solution and filter network 2.0.0.0/8 coming
> through S0/0.21 sub-interface:*
>
>
>
> R1(config)#*No route-map tst*
>
>
>
> R1(config)#*router ospf 1*
>
> R1(config-router)#*No distribute-list route-map tst in*
>
>
>
> R1#*Show ip route ospf | I O*
>
> O 2.0.0.0/8 [110/65] via 12.1.1.2, 00:00:19, Serial0/0.12
>
> O 3.0.0.0/8 [110/65] via 13.1.1.3, 00:00:19, Serial0/0.13
>
> O 4.0.0.0/8 [110/65] via 14.1.1.4, 00:00:19, Serial0/0.14
>
> O 200.2.2.0/24 [110/65] via 12.1.1.2, 00:00:19, Serial0/0.12
>
>
>
> *The previous solution will NOT work here; to filter network 2.0.0.0/8coming
through S0/0.21 we need to do the following:
> *
>
>
>
> R1(config)#*access-list 2 permit 2.0.0.0 0.255.255.255*
>
>
>
> R1(config)#*route-map tst deny 10*
>
> R1(config-route-map)#*match interface s0/0.12*
>
> R1(config-route-map)#*match ip addr 2*
>
> R1(config)#*route-map tst permit 90*
>
>
>
> R1(config-route-map)#*router ospf 1*
>
> R1(config-router)#*distribute-list route-map tst in*
>
>
>
> R1#*Show ip route ospf | I O*
>
> O 3.0.0.0/8 [110/65] via 13.1.1.3, 00:00:23, Serial0/0.13
>
> O 4.0.0.0/8 [110/65] via 14.1.1.4, 00:00:23, Serial0/0.14
>
> O 200.2.2.0/24 [110/65] via 12.1.1.2, 00:00:23, Serial0/0.12
>
>
>
> *Perfect it worked, obviously the same task can be configured using
> another method.*
>
> * *
>
> *Lets remove the configuration from the previous solution and filter
> network 200.2.2.0/24 coming from R2, but this time we are going to use
> the IP Next-hop option:*
>
>
>
> R1(config)#*No route-map tst*
>
> R1(config)#*No access-list 2*
>
>
>
> R1(config)#*router ospf 1*
>
> R1(config-router)#*No distribute-list route-map tst in*
>
>
>
> R1#*Show ip route ospf | I O*
>
> O 2.0.0.0/8 [110/65] via 12.1.1.2, 00:00:29, Serial0/0.12
>
> O 3.0.0.0/8 [110/65] via 13.1.1.3, 00:00:29, Serial0/0.13
>
> O 4.0.0.0/8 [110/65] via 14.1.1.4, 00:00:29, Serial0/0.14
>
> O 200.2.2.0/24 [110/65] via 12.1.1.2, 00:00:29, Serial0/0.12
>
>
>
> *To filter using the IP next-hop option:*
>
>
>
>
>
> R1(config)#*access-list 1 permit 200.2.2.0 0.0.0.255*
>
>
>
> R1(config)#*access-list 10 permit host 12.1.1.2*
>
>
>
> R1(config)#*route-map tst deny 10*
>
> R1(config-route-map)#*match ip addr 1*
>
> R1(config-route-map)#*match ip next-hop 10*
>
> R1(config)#*route-map tst permit 90*
>
>
>
> R1(config-route-map)#*router ospf 1*
>
> R1(config-router)#*distribute-list route-map tst in*
>
>
>
> R1#*Show ip route ospf | I O*
>
> O 2.0.0.0/8 [110/65] via 12.1.1.2, 00:00:05, Serial0/0.12
>
> O 3.0.0.0/8 [110/65] via 13.1.1.3, 00:00:05, Serial0/0.13
>
> O 4.0.0.0/8 [110/65] via 14.1.1.4, 00:00:05, Serial0/0.14
>
> * *
>
> *Now.lets remove the previous solution and use the route-source
> option:*
>
>
>
> R1(config)#*No access-list 1*
>
> R1(config)#*No access-list 10*
>
> R1(config)#*No route-map tst*
>
> R1(config)#*router ospf 1*
>
> R1(config-router)#*No distribute-list route-map tst in*
>
>
>
> R1#*Show ip route ospf | I O*
>
> O 2.0.0.0/8 [110/65] via 12.1.1.2, 00:00:34, Serial0/0.12
>
> O 3.0.0.0/8 [110/65] via 13.1.1.3, 00:00:34, Serial0/0.13
>
> O 4.0.0.0/8 [110/65] via 14.1.1.4, 00:00:34, Serial0/0.14
>
> O 200.2.2.0/24 [110/65] via 12.1.1.2, 00:00:34, Serial0/0.12
>
>
>
> * *
>
> *To use the route-source option, we MUST use the RID of R2 in the
> access-list. To find out the RID of R2:*
>
>
>
> R2#*Show ip ospf | I ID*
>
> *Routing Process "ospf 1" with ID 0.0.0.2*
>
>
>
> R1(config)#*access-list 3 permit 0.0.0.2*
>
> R1(config)#*access-list 30 permit 200.2.2.0 0.0.0.255*
>
>
>
> R1(config)#*route-map tst deny 10*
>
> R1(config-route-map)#*match ip addr 30*
>
> R1(config-route-map)#*match ip route-source 3*
>
> R1(config)#*route-map tst permit 90*
>
> R1(config-route-map)#*router ospf 1*
>
> R1(config-router)#*distribute-list route-map tst in*
>
>
>
> R1#*Show ip route ospf | I O*
>
> O 2.0.0.0/8 [110/65] via 12.1.1.2, 00:00:06, Serial0/0.12
>
> O 3.0.0.0/8 [110/65] via 13.1.1.3, 00:00:06, Serial0/0.13
>
> O 4.0.0.0/8 [110/65] via 14.1.1.4, 00:00:06, Serial0/0.14
>
>
>
>
>
> I hope this helped.
>
> On Mon, Jan 7, 2013 at 7:09 PM, ccie99999 <ccie99999_at_gmail.com> wrote:
>
>> Thanks for your reply Brian..
>>
>> I see your point about filtering the RIB and not what OSPF chooses. ( I
>> didn't realize this before actually)
>>
>> What I was complaining is that route-map matching ip next-hop or matching
>> ip route-source is not working when applied with a distribute list inbound
>> to ospf.
>> Strange I don't find any official reference on this scenario.
>>
>>
>> On Mon, Jan 7, 2013 at 6:08 PM, Brian McGahan <bmcgahan_at_ine.com> wrote:
>>
>> > This filters the RIB (routing table), not the OSPF decision process. By
>> > the time you apply this OSPF has already chosen the intra area route
>> over
>> > the inter area one, so all you can do is permit or deny the intra area
>> one.
>> >
>> > The only way to really accomplish this is to make the two route types
>> > equal. If you were to change the inter area route to intra area with
>> > something like a virtual link or tunnel then you can modify which path
>> is
>> > preferred.
>> >
>>
>>
>>
>> --
>> @ccie99999
>>
>>
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>>
>>
>>
>>
>>
>
>
> --
> *Narbik Kocharians
> *CCSI#30832, CCIE# 12410 (R&S, SP, Security)
> *www.MicronicsTraining.com* <http://www.micronicstraining.com/>
> Sr. Technical Instructor
> YES! We take Cisco Learning Credits!
> A Cisco Learning Partner
>

--
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