Redistribution and Route Tagging

[Cisconew]

Hi all,

I am studying for my ENARSI exam. Finally i came through exam question bank. There i encounter some questions which seems answer is wrong. Can anyone clarify the below question.

Please find attached the topology for your reference.

There is a loop in the network(R5).Which is the correct configuration to avoid this loop.

Redistribution done on R3 and R4.

Question bank says Answer is B. But i have doubted it with answer B.

A],

R3

route-map SET-TAG permit 10

 set tag 10

!

router ospf 1

 redistribute eigrp 1 subnets route-map SET-TAG

!

R4

router eigrp 1

 redistribute ospf 1 metric 2000000 1 255 1 155 route-map FILTER-TAG

!

route-map FILTER-TAG deny 10

 match tag 10

!

route-map FILTER-TAG permit 20

B]

R3

router eigrp1

 redistribute ospf 1 route-map SET-TAG

!

route-map SET-TAG permit 10

 set tag 10

!

R4

router eigrp 1

 redistribute ospf 1 metric 2000000 1 255 1 155 route-map FILTER-TAG

network 10.1.24.4 0.0.0.0

!

route-map FILTER-TAG deny 10

 match tag 10

!

route-map FILTER-TAG permit 20

[paul driver]

Hello
Actually both don’t look correct , But A actually is the closest as its show redistribution of ospf into eigrp/eigrp-ospf  but its not matching correctly via the route-maps , I would have expected to see the same on both routers regards denying and set of tags so any eigrp routes that were tagged with 10 are being filtered at both redistribution points  but i assume the question is showing you is an extract of the full config.

[Cisconew]

I forgot to mention D EX (external EIGRP routes) also available in EIGRP topology.

Thanks for your detailed reply.

Jason

[Giuseppe Larosa]

Hello @Cisconew ,

it is possible to find questions with wrong answer(s)  in study material.

Sometimes even the question is wrong.

In this case you have a topology using two routing protocols EIGRP and OSPF.

Both R3 and R4 can perform mutual redistribution that is both are redistributing  OSPF into EIGRP and EIGRP into OSPF.

This can create the potential for routing problems in some cases.

Each protocol is able to discriminate internal routes from external routes.

EIGRP internal routes are D with AD 90 and EIGRP external routes are D EX with AD 170  ( default values)

OSPF considers route type   O routes are preferred over O IA routes that are preferred over O E1 that are preferred over O E2.

With a stable topology there is no potential for issues ( EIGRP external routes in EIGRP domain see later)

Let us suppose that the link  R2 to R3 starts to flap .

When the link is down R3 has no EIGRP active neighbor and learns all the routes from the OSPF domain and in particular it will install a set of OSPF external routes that describe all the subnets in the EIGRP routing domain,  as a result of redistribution performed by R4 on link R4 to R5.

When the link between R2 and R3 comes back up the EIGRP neighborship is rebuilt and R3 learns via EIGRP all internal EIGRP subnets.

These internal EIGRP subnets are then installed in IP routing table as type D with AD 90 and replace the OSPF learned external routes that have AD 110.

There could be an issue if external routes would exist in the EIGRP domain.

If both routers are doing mutual redistrubution the use of route tags should be done on both in the same way.

Example:

Both redistribute EIGRP routes into OSPF  with a route-tag of 10 and both when redistributing OSPF routes into EIGRP deny routes with a route-tag of 10.

The solution a) and b) that you have reported appear both incomplete for this reason and I think they are equivalent.

There is a potential issue only if EIGRP external routes exist in the EIGRP domain as these routes D EX with AD 170 would not win a competition with OSPF O E2 routes coming from the other ASBR node as the last one have AD 110.

Final note : with the exception of connected routes a route coming from a protocol X can be redistributed into another protocol Y only if it is installed in the IP routing table.  It is not enough to be in the protocol database.

Hope to help

Giuseppe