How to configure the ASBR to that each AS can backup each other

[Steph1963]

Hi to Everybody,

The following topology was presented during my T Route class and the following question was asked:

You want to configure the ASBRs so that each AS can back up the other. You also want to prevent routing loops between the ASes. What should we do

used prefix-based filtering

or

listed tagged-based filtering

The good answer is that prefix-based filtering should be used but I just cannot find the reason why it is better than listed tagged filtering.

1) Can somebody confirm that the following reasoning make sense and

1) RIP Route 10.2.1.0/24 from R4 arrive at R1 first

2) At R1, route gets tag with tag 2 and redistibute into OSPF AS 1

3) R1 install the route since it`s not tag with 2

4) RIP Route 10.2.1.0/24 from R4 arrives at R2 second

5) At R2, route gets tag with tag 2 and redistribute into OSPF

6) R1 receive the route with a tag 2 and it is filter so because of tag 2, the route is not redistribute back into AS2

2) Can we achieve the goal of having each AS backup the other and preventing loop by using a redistrubute ospf route-map instead of using prefix-based filtering to decide what is redistributed or not.

The following configurations were suggests:

Listed Tag-Based filtering

router ospf 1

redistribute ospf 2 subnet tag 1

distribute-list 1 route-map as2-filter in

!

route-map as2-filter deny 10

match tag 2

route-map as2-filter permit 20

!

router ospf 2

rredistribute OSPF 1 subnets tag 2

distribute-list 1 route-map as1-filter in

!

route-map as1-filter deny 10

match tag 1

route-map-as1-filter permit 20

Prefix-Based filtering

router ospf 1

redistribute ospf 2 subnet route-map as2-filter

distance ospf external 200

!

route-map as2-filter permit 10

match ip address 1

!

router ospf 2

redistribute ospf 1 subnet route-map as1-filter

distance ospf external 200

!

route-map as2-filter permit 10

match ip address 1

Thanks for your help

Stéphane

[Thotsaphon Lueangwattanaphong]

Hi,

   I just want to make sure that your picture and your description are the same. Are you redistribuing between RIP and OSPF or OSPF process1 and OSPF process2?

Toshi

[Steph1963]

Hi Toshi,

I am only redistributing between OSPF 1 and OSPF 2.

Thanks for your help

Stéphane

[Thotsaphon Lueangwattanaphong · Accepted solution]

Hi Stephane,

          We're doing a 2-way redistribution between 2 ospf processes. First off we need to know that we cannot compare ospf routes between ospf processes as a normal way. We normally prefer Intra-area > Inter-area > External.  But we can use Admistrative Distance to solve this problem. As per your question, we might get sub-optimal routes or routing loop if you don't use route filtering. Ospf routes will be installed separately each process. So who's going to win? It depends on which process installs ospf routes first. So you might see R1 have ospf routes installed by only one process. Let's say Process1 won , R1 will install O routes for domain AS1 and O*E2 for domain AS2 for OSPF process1. This is an example for sub-optimal routes.

    Let's talk about how to beat these problems.

1. Use tag

         If we use TAG to filter O*E2 routes(After doing a redistribution) to not install O*E2 routes into the routing table at the ASBR. Let's say we do a TAG on R1 and filter it on R2 and vise versa. The problem is when Fa0/1 interface on R1 failed, R1 cannot use AS2 to get the rest networks on  AS1 because O*E2 routes have not been installed because of filtering TAG. So this solution has no backup routes.

2. Change AD for external routes (O*E2 in this case)

          If we change to AD for O*E2 routes at the ASBR. Let's say we do a redistribution on R1 and change AD the R2 and vise versa. R2 will prefer AD-110 that is O routes. R2 can also use sub-optimal routes in case the link failed because O routes is gone, O*E2 will be installed into RIB instead.

    Hope I can help you some.

Toshia

[Steph1963]

Hi Toshia,

Very useful and helpful answer.

Many thanks for your precious help

Stéphane