implications of using different ospf prcocesses in ospf domain.

Answered Question
Aug 16th, 2009

Hi every body

I have few questions.

1) If we have three routers and all are running ospf 1. Since link data base is created on per process bases, therefore there will be only one link state database. Is it correct?

thanks and have a nice weekend.

I have this problem too.
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Correct Answer by Jon Marshall about 7 years 3 months ago

Sarah

"Routers have no way knowing that these lsa are from different companies."

Correct and that is the key point ie. routers are totally unaware of organisational setups, they merely deal in routes.

So with some IP addresses lets say it looks like this -

Company A network -> (192.168.5.1/24) RA (192.168.6.1/30) -> (192.168.6.2/30) RB (192.168.7.1/24) -> Company B network

note in the above "Company A network" and "Company B network" can include many routers/switches etc..

Now if lets say your config is like this

RA

router ospf 1

network 192.168.5.0 0.0.0.255 area 0

network 192.168.6.0 0.0.0.3 area 1

RB

router ospf 2

network 192.168.7.0 0.0.0.255 area 0

network 192.168.6.0 0.0.0.3 area 1

if you used the above config RA & RB are not seen as ASBR's at all because they don't know they are dealing with different OSPF processes. In fact they are not dealing with 2 OSPF processes, just the one because as previously discussed process ID is only locally significant.

So you have merged the 2 companies together but not by using ASBR's. Note that the above is not really a way to do things and is only used for illustration because you would end up with 2 area 0's, one in each company network that were not joined together. Not good.

So to merge them at first you would do something like

RA

router ospf 1

network 192.168.5.0 0.0.0.255 area 0

router ospf 2

network 192.168.6.0 0.0.0.3 area 1

RB

router ospf 1

network 192.168.7.0 0.0.0.255 area 0

router ospf 2

network 192.168.6.0 0.0.0.3 area 1

Now RA & RB have 2 processes each. You would now need to redistribute routes between these 2 processes ie. on RA & RB

1) redistribute OSPF 1 into OSPF 2 so that RA & RB receives each others internal routes

2) redistribute OSPF 2 into OSPF 1 so that each companies routes are propogated throughout each companies networks.

Now both RA & RB are ASBR's.

One final note. You need to be careful with mutual redisribution so that you don't get into a redistribution loop.

Jon

Correct Answer by Jon Marshall about 7 years 3 months ago

Sarah

I think the confusion is coming from the fact the process ID is only significant on the local router.

So if you have 2 routers and on one you have defined "router ospf 1" and on the other "router ospf 2" they will form a neighborship and exchange LSA's and have the same LSDB, providing they are in the same area as Giuseppe points out.

However a router with 2 processs is diffent ie. on the same router

router ospf 1

router ospf 2

now there are 2 different processes running because they are on the same router.

Does this make sense ?

Jon

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Giuseppe Larosa Sun, 08/16/2009 - 11:18

Hello Sarah,

actually a LS DB for each area-id is created.

ABR routers have multiple DBs one for each area they connect to.

OSPF process-id is used only on the device and it is not exported in OSPF v2.

So if you use different process-ids on the three routers nothing changes until they agree on the OSPF area-id in the hello messages

Hope to help

Giuseppe

sarahr202 Sun, 08/16/2009 - 11:44

Thanks Giuseppe.

It has been long time since i heard from you. I hope you are doing fine.

In my earlier post titled " asbr in ospf". one of net pro, Rick says

"If the same router is configured with OSPF 1 and with OSPF 2 then this is an example of multiple AS. The router with both OSPF 1 and OSPF 2 will have all the routes from both ASs in its routing table. But it will maintain a separate link state database for OSPF 1 and for OSPF 2. Without redistribution no LSAs from OSPF 1 will be advertised to OSPF 2. And no other router connected in OSPF 2 will learn routes from OSPF 1."

Here is what my book says:

"The process-id does not have to match on each router and it can be any integer between 1 and 65,535"

======================================

According to book, routers running different ospf proccesses can exchange routes without redistribution.

But the Cisco book contradicts Cisco doc. It also cotradicts Rick ' statement.

http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080531fd2.shtml

If i trust the book, then i need to answer this question.

Since link state base is created on per area , per processes. Router runs Spf on these link database then installs the routes in its routing table.

Now keep the above logic in mind, Let say we have router R1 running ospf 1 receives an update(10.0.0.0) from R2 running ospf 2.

According to my book, R1 will install the route advertised by R2. The question is how? because R1 is not running ospf 2, so there is no link data base for ospf 2 on R1, since there is no link data base for ospf 2 on R1, router1 can not run spf and installs the route in its routing table.

Thanks and have a nice weekend.

Correct Answer
Jon Marshall Sun, 08/16/2009 - 11:51

Sarah

I think the confusion is coming from the fact the process ID is only significant on the local router.

So if you have 2 routers and on one you have defined "router ospf 1" and on the other "router ospf 2" they will form a neighborship and exchange LSA's and have the same LSDB, providing they are in the same area as Giuseppe points out.

However a router with 2 processs is diffent ie. on the same router

router ospf 1

router ospf 2

now there are 2 different processes running because they are on the same router.

Does this make sense ?

Jon

sarahr202 Sun, 08/16/2009 - 12:13

Thanks Jon.

Please consider the following case.

Let say we have different two companies, companyA and company B . Both are companies are running ospf as their igp.

Both companies decided to merge .

Let say RouterA at company A is chosen as Asbr and RouterB at comany B is chosen as Asbr . The end result looks like that:

(companA)RA----RB( company B)

company A is running ospf 1

company B is running ospf 2

How does RA decide LSa being advertised by router B are from company B not just from different ospf process,so needs to be propagated as lsa type 5?

As we already established two routers though running different ospf processes, can exchange routes without redistribution in same area. If these two routers RA and RB are connected by serial int both are in same area say 2, then RA will sipmly receives lsa from Rb and store them in database and vice versa.Routers have no way knowing that these lsa are from different companies.

Thanks a lot and have a nice weekend.

Correct Answer
Jon Marshall Sun, 08/16/2009 - 12:39

Sarah

"Routers have no way knowing that these lsa are from different companies."

Correct and that is the key point ie. routers are totally unaware of organisational setups, they merely deal in routes.

So with some IP addresses lets say it looks like this -

Company A network -> (192.168.5.1/24) RA (192.168.6.1/30) -> (192.168.6.2/30) RB (192.168.7.1/24) -> Company B network

note in the above "Company A network" and "Company B network" can include many routers/switches etc..

Now if lets say your config is like this

RA

router ospf 1

network 192.168.5.0 0.0.0.255 area 0

network 192.168.6.0 0.0.0.3 area 1

RB

router ospf 2

network 192.168.7.0 0.0.0.255 area 0

network 192.168.6.0 0.0.0.3 area 1

if you used the above config RA & RB are not seen as ASBR's at all because they don't know they are dealing with different OSPF processes. In fact they are not dealing with 2 OSPF processes, just the one because as previously discussed process ID is only locally significant.

So you have merged the 2 companies together but not by using ASBR's. Note that the above is not really a way to do things and is only used for illustration because you would end up with 2 area 0's, one in each company network that were not joined together. Not good.

So to merge them at first you would do something like

RA

router ospf 1

network 192.168.5.0 0.0.0.255 area 0

router ospf 2

network 192.168.6.0 0.0.0.3 area 1

RB

router ospf 1

network 192.168.7.0 0.0.0.255 area 0

router ospf 2

network 192.168.6.0 0.0.0.3 area 1

Now RA & RB have 2 processes each. You would now need to redistribute routes between these 2 processes ie. on RA & RB

1) redistribute OSPF 1 into OSPF 2 so that RA & RB receives each others internal routes

2) redistribute OSPF 2 into OSPF 1 so that each companies routes are propogated throughout each companies networks.

Now both RA & RB are ASBR's.

One final note. You need to be careful with mutual redisribution so that you don't get into a redistribution loop.

Jon

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