Re: OSPF Distance Command .!

Hey team,

Not to change the topic too much ... but I had a practice lab pretty
similar, in which I had a router with two neighbors, off of two different
interfaces. Two equal routes showed up in the routing table, one from each
neighbor, and the lab asked me to prefer the path out of only one of the
neighbors.

I changed the cost on one interface and the desired outcome was achieved.
To be sure, i cleared ospf, cleared the routing table and even rebooted the
routers at the end of the night. Next day, the desired behavior was still
present, and the router was still preferring the routes from only one of the
neighbors. Nothing too fancy ... and after reading this long thread, I
realize that I need to practice setting the AD for a particular neighbor ...
;-)

An interesting caveat is this ... if another router has redist into OSPF and
used a default type E2. This default type, E2, can not change it's cost
when it gets advertised across the network. So the method os using
interface cost will not work.

To be quick about it, I changed and redid the redistribution and this time
chose type 1 instead of the deafult type. I never bothered with worrying
about the AD ... although perhaps that was the preferred solution. HTH,

Andrew Lee Lissitz

On Fri, May 22, 2009 at 12:07 PM, Paul Cosgrove <paul.cosgrove_at_gmail.com>wrote:

> It might be clearer for other people if you replied to my email about that
> topology. When you are testing OSPF I recommend you shut interfaces between
> all routers before clearing the process on each. Otherwise, in many
> instances you can see very different behaviour (especially with external
> routes).
>
> I do not know if the behviour which I see in my lab applies to all
> releases, which was why I included the IOS version and router model.
> Perhaps it is different, perhaps not. The version of IOS you are using
> may contain a bug, perhaps mine does. Try the same release as me if you
> wish to like: c7200-k91p-mz.122-25.S15. The point is that selection of
> external routes follows very different rules to that of internal routes, and
> you don't need to take my word for it as the rfc's contains the details.
>
> Paul.
>
> Divin Mathew John wrote:
>
>> Paul Cosgrove,
>>
>> I tried your topology..
>> R3
>> / \
>> R1 R2
>> \ /
>> R0
>>
>> R3 does not run any routing protocols, but has Lo0: 3.3.3.3 for testing.
>> R1 and R2 both have static routes to R3's loopback.
>> R0, R1 and R2 run OSPF between each other.
>>
>> this is my config
>> ***********************
>> R0********************
>> ***********************
>> interface Loopback0
>> ip address 100.100.100.100 255.255.255.0
>> !
>> interface Serial0/0
>> ip address 10.1.1.1 255.255.255.252
>> serial restart-delay 0
>> !
>> interface Serial0/1
>> ip address 20.1.1.1 255.255.255.252
>> serial restart-delay 0
>> !
>> interface Serial0/2
>> no ip address
>> shutdown
>> serial restart-delay 0
>> !
>> interface Serial0/3
>> no ip address
>> shutdown
>> serial restart-delay 0
>> !
>> router ospf 1
>> router-id 100.100.100.100
>> log-adjacency-changes
>> network 10.1.1.1 0.0.0.0 area 0
>> network 20.1.1.1 0.0.0.0 area 0
>> network 100.100.100.100 0.0.0.0 area 0
>> !
>> ip http server
>> ip forward-protocol nd
>> !
>>
>> ***********************
>> R1********************
>> ***********************
>> interface Loopback0
>> ip address 1.1.0.1 255.255.255.0
>> !
>> interface Serial0/0
>> ip address 10.1.1.2 255.255.255.252
>> serial restart-delay 0
>> !
>> interface Serial0/1
>> ip address 13.1.1.1 255.255.255.252
>> serial restart-delay 0
>> !
>> interface Serial0/2
>> no ip address
>> shutdown
>> serial restart-delay 0
>> !
>> interface Serial0/3
>> no ip address
>> shutdown
>> serial restart-delay 0
>> !
>> router ospf 1
>> router-id 1.1.0.1
>> log-adjacency-changes
>> redistribute static subnets
>> network 1.1.0.1 0.0.0.0 area 0
>> network 10.1.1.2 0.0.0.0 area 0
>> !
>> ip http server
>> ip forward-protocol nd
>> ip route 3.3.0.3 255.255.255.255 Serial0/1 13.1.1.2
>> !
>> ***********************
>> R2********************
>> ***********************
>> interface Loopback0
>> ip address 2.2.0.2 255.255.255.0
>> !
>> interface Serial0/0
>> ip address 20.1.1.2 255.255.255.252
>> serial restart-delay 0
>> !
>> interface Serial0/1
>> ip address 23.1.1.1 255.255.255.252
>> serial restart-delay 0
>> !
>> interface Serial0/2
>> no ip address
>> shutdown
>> serial restart-delay 0
>> !
>> interface Serial0/3
>> no ip address
>> shutdown
>> serial restart-delay 0
>> !
>> router ospf 1
>> router-id 2.2.0.2
>> log-adjacency-changes
>> redistribute static subnets
>> network 2.2.0.2 0.0.0.0 area 0
>> network 20.1.1.2 0.0.0.0 area 0
>> !
>> ip http server
>> ip forward-protocol nd
>> ip route 3.3.0.3 255.255.255.255 Serial0/1 23.1.1.2
>> !
>> ***********************
>> R3********************
>> ***********************
>> !
>> interface Loopback0
>> ip address 3.3.0.3 255.255.255.0
>> !
>> interface Serial0/0
>> ip address 13.1.1.2 255.255.255.252
>> serial restart-delay 0
>> !
>> interface Serial0/1
>> ip address 23.1.1.2 255.255.255.252
>> serial restart-delay 0
>> !
>> interface Serial0/2
>> no ip address
>> shutdown
>> serial restart-delay 0
>> !
>> interface Serial0/3
>> no ip address
>> shutdown
>> serial restart-delay 0
>> !
>> ip http server
>> ip forward-protocol nd
>> !
>> !
>> !
>>
>> and this is my routing table at R0
>> R0#sir
>> Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
>> D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
>> N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
>> E1 - OSPF external type 1, E2 - OSPF external type 2
>> i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS
>> level-2
>> ia - IS-IS inter area, * - candidate default, U - per-user static
>> route
>> o - ODR, P - periodic downloaded static route
>>
>> Gateway of last resort is not set
>>
>> 1.0.0.0/32 is subnetted, 1 subnets
>> O 1.1.0.1 [110/65] via 10.1.1.2, 00:00:01, Serial0/0
>> 2.0.0.0/32 is subnetted, 1 subnets
>> O 2.2.0.2 [110/65] via 20.1.1.2, 00:00:01, Serial0/1
>> 100.0.0.0/24 is subnetted, 1 subnets
>> C 100.100.100.0 is directly connected, Loopback0
>> 3.0.0.0/32 is subnetted, 1 subnets
>> O E2 3.3.0.3 [110/20] via 20.1.1.2, 00:00:01, Serial0/1
>> [110/20] via 10.1.1.2, 00:00:01, Serial0/0
>> 20.0.0.0/30 is subnetted, 1 subnets
>> C 20.1.1.0 is directly connected, Serial0/1
>> 10.0.0.0/30 is subnetted, 1 subnets
>> C 10.1.1.0 is directly connected, Serial0/0
>>
>> It does Load balancing.!
>>
>> R0#show ip ospf neighbor
>>
>> Neighbor ID Pri State Dead Time Address
>> Interface
>> 1.1.0.1 0 FULL/ - 00:00:38 10.1.1.2
>> Serial0/0
>> 2.2.0.2 0 FULL/ - 00:00:34 20.1.1.2
>> Serial0/1
>>
>> i did try shutting interafces... clear ip route * , clear ip ospf process
>>
>> no change in the OUTcome.!
>>
>> Thanking You
>>
>> Yours Sincerely
>>
>> Divin Mathew John
>> divinjohn_at_gmail.com
>> divin_at_dide3d.com
>> http://www.dide3d.com
>> +91 9945430983
>> +91 9846697191
>> +974 5008916
>> PGP PUBLIC KEY BLOCK @ http://www.dide3d.com/divin_Public_PGP_key.txt
>> Sent from Bangalore, KA, India
>>
>> On Fri, May 22, 2009 at 8:59 PM, Paul Cosgrove <paul.cosgrove_at_gmail.com
>> >wrote:
>>
>>
>>
>>> Hi Sadiq,
>>>
>>> I would have to recommend trying it again yourself, as it does work:
>>>
>>> R1 ----- R2
>>> \ /
>>> \ /
>>> \ /
>>> R0
>>>
>>> All links are network type point to point. Shutdown interfaces, cleared
>>> ospf process, no shut and waited for neighbors to establish.
>>>
>>> R0 then shows the following:
>>>
>>> Gateway of last resort is not set
>>>
>>> O 192.168.12.0/24 [110/128] via 192.168.20.2, 00:00:58, Serial1/1
>>> [110/128] via 192.168.10.1, 00:00:58, Serial1/0
>>> 1.0.0.0/32 is subnetted, 1 subnets
>>> O 1.1.1.1 [110/65] via 192.168.10.1, 00:00:58, Serial1/0
>>> 2.0.0.0/32 is subnetted, 1 subnets
>>> O 2.2.2.2 [110/65] via 192.168.20.2, 00:00:58, Serial1/1
>>> C 192.168.10.0/24 is directly connected, Serial1/0
>>> C 192.168.20.0/24 is directly connected, Serial1/1
>>> 10.0.0.0/32 is subnetted, 1 subnets
>>> C 10.10.10.10 is directly connected, Loopback0
>>>
>>> R0(config)#access-list 10 permit 192.168.12.0
>>> R0(config)#router ospf 1
>>> R0(config-router)#distance 90 1.1.1.1 0.0.0.0 10
>>> R0(config-router)#do sh ip ro
>>> Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
>>> D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
>>> N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
>>> E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
>>> i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
>>> ia - IS-IS inter area, * - candidate default, U - per-user static
>>> route
>>> o - ODR, P - periodic downloaded static route
>>>
>>> Gateway of last resort is not set
>>>
>>> O 192.168.12.0/24 [90/128] via 192.168.10.1, 00:00:05, Serial1/0
>>> 1.0.0.0/32 is subnetted, 1 subnets
>>> O 1.1.1.1 [110/65] via 192.168.10.1, 00:00:05, Serial1/0
>>> 2.0.0.0/32 is subnetted, 1 subnets
>>> O 2.2.2.2 [110/65] via 192.168.20.2, 00:00:05, Serial1/1
>>> C 192.168.10.0/24 is directly connected, Serial1/0
>>> C 192.168.20.0/24 is directly connected, Serial1/1
>>> 10.0.0.0/32 is subnetted, 1 subnets
>>> C 10.10.10.10 is directly connected, Loopback0
>>> R0(config-router)#
>>>
>>> AD can and is often used to differentiate routes in the same protocol.
>>> EIGRP externals and BGP internal routes being some obvious examples.
>>> Paul.
>>>
>>>
>>>
>>> Sadiq Yakasai wrote:
>>>
>>>
>>>
>>>> Gents, a little search over GS archives would second Bryan's point! This
>>>> issue hunted me down in my first attempt!
>>>>
>>>> While some have even gone to TAC, others have tried bugging it! But from
>>>> the
>>>> research I carried out a while ago, my conclusion is you are not to use
>>>> AD
>>>> to influence prefix selection WITHIN the same routing protocol - use
>>>> metric
>>>> instead! BETWEEN 2 or more routing protocols, use AD.
>>>>
>>>> Otherwise, you are in for a long, unreliable and incoherent exercise.
>>>>
>>>> Sadiq
>>>>
>>>> On Fri, May 22, 2009 at 3:23 PM, Bryan Bartik <bbartik_at_ipexpert.com>
>>>> wrote:
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>> Hello,
>>>>>
>>>>> Divin, I think you hit it right on :) I have labbed this several times
>>>>> before and what I found was that you cannot use AD to prefer one ospf
>>>>> route
>>>>> over another in the same ospf process (There may be crazy scenarios
>>>>> where
>>>>> you can, but I don't recall finding any). Makes sense, since like you
>>>>> said
>>>>> AD is to prefer one protocol over another. You could place the link to
>>>>> R1
>>>>> in
>>>>> another process on R0 and then change the distance in that process.
>>>>>
>>>>> Bryan Bartik
>>>>> CCIE #23707 (R&S), CCNP
>>>>> Sr. Support Engineer - IPexpert, Inc.
>>>>> URL: http://www.IPexpert.com
>>>>>
>>>>>
>>>>> Blogs and organic groups at http://www.ccie.net
>>>>>
>>>>> _______________________________________________________________________
>>>>> Subscription information may be found at:
>>>>> http://www.groupstudy.com/list/CCIELab.html
>>>>>
>>>>
>
> Blogs and organic groups at http://www.ccie.net
>
> _______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html
>
>
>
>
>
>
>
>

-- 
Andrew Lee Lissitz
all.from.nj_at_gmail.com
Blogs and organic groups at http://www.ccie.net