Quick question on Route redistribution between two routing protocols

[sateeshk10]

I am testing scenario in the LAB , here is the my setup

A (pair) < eBGP > B(pair) <iBGP>C (pair) < OSPF stub area> D (pair)

Looking for best configuration  for C switch pair where the IBGP to OSPF and OSPF to IBGP takes place and should avoid any kind of route looping and also, I am fine protocols mutual  redistribution with tags.

I dont need prefix list for route control because i  am advertising few 100`s  from A pair and few 100`s of routes from OSPF (D pair)  and planning to control routes with tags or community ?

I have collected sample configuration from internet but still looking for best

Appreciated quick response

Thanks

Kumar

[sateeshk10]

Hi  Giuseppe

As soon as i  enable stub area on C and D pair default route getting over written by OSPF because the default route which i am originating  from BGP(top pair) to IBGP with Ad is 200  so, OSPF is winning?

Without OSPF stub area

C pair routers(config-router-ospf)#show ip route 0.0.0.0/0 (receiving from upstream which is my core)

Gateway of last resort:

 B I      0.0.0.0/0 [200/0] via 1.1.1.13, Ethernet10

                            via 1.1.1.15, Ethernet11

As soon as I enable totally STUB area.

C Pair router#show ip rou 0.0.0.0/0 (receiving from downstream)

Gateway of last resort:

 O IA     0.0.0.0/0 [110/24] via 1.1.1.21, Ethernet14

                             via 1.1.1.25, Ethernet15

Thanks

Kumar

[Giuseppe Larosa]

Hello Kumar,

in our last posts we were speaking of using RIP on C1, C2 routers and not OSPF.

When using OSPF and OSFP stub area between C1,C2 and D1,D2 routers you don't need to send an iBGP default route to C1,C2.

Because C1 and C2 will send out a default route in stub area regardless the presence of a default route in IP routing table.

Your understanding is correct OSPF O IA has lower admin distance (110) then iBGP route (200).

How to fix this on C1 and C2 routers if you want to keep the iBGP default route installed ?

We could use a distribute-list in OSPF to deny the default route

access-list 11 deny 0.0.0.0

access-list 11 permit any

router ospf 10

distribute-list 11 in

or using a prefix-list if supported

ip prefix-list DENY-OIA-DEFAULT seq 5 deny 0.0.0.0/0

ip prefix-list DENY-OIA-DEFAULT seq 10 permit 0.0.0.0/0 ge 1 le 32

router ospf 10

distribute-list prefix DENY-OIA-DEFAULT in

Note:

being OSPF a link state protocol the distribute-list command only prevents the installation of filtered routes in the local IP routing table, but it does not change / filter the link state database.

This change has to be implemented on both C1 and C2 devices.

Hope to help

Giuseppe

[sateeshk10]

I fixed in other way..

i am learning default route as OIA so, i have option to set OIA administrative distance to 210 for inter area routes and  got fixed. just avoided few lines of configuration :)

router ospf 1
distance ospf inter-area 210

Thanks and appreciated.

One more basic question : When we need mutual redistribution means, scenario with example! 

Kumar

[Giuseppe Larosa]

Hello Kumar,

OK you changed O IA AD on C1,C2.

mutual redistribution is really needed in cases like:

- IGP migration : for example you want to replace EIGRP with OSPF. At beginning you have only EIGRP running on all routers. You start adding configuration of OSPF on selected routers that form a block. At the border you may need to use mutual redistribution between OSPF and EIGRP to make the two routing domains to communicate.

This mutual redistribution is only needed during migration in the final network state only OSPF is running on all routers and these redistributions are not needed anymore and are removed and also the EIGRP is removed.

-  a case similar to yours but using a default route in one protocol is not acceptable, because for example your default route needs to point to a different direction / different devices.

Hope to help

Giuseppe

[paul driver]

Hello

If applicable cam you post a topology of your lab so its clear as to what actual setup you do have?

[sateeshk10]

Appreciated paul for your response, i have attached the diagram.

Thanks