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Regarding installing multiple routes with unequal costs to get to ABR/ASBR

In the following OSPF network topology, is it correct that RTA installs multiple inter-area routes over two links(atm3072, atm3073) with costs, 10 and 1 respectively? If so, does it comply with the OSFP standard, RFC2328?

Any response will be greatly appreciated.

Yongjun.

(10.0.0.0/24)

<---atm3072--->

(cost: 10)

RTA RTB <----------------> RTC

<---atm3073--->(ABR) (20.0.0.0/24)

(74.0.0.0/24)

(cost: 1)

172.31.0.4 172.31.0.1 172.31.0.2

<------------ Area 0 --------> <------- Area 100 ---------->

============ OSPF network routing table ============

N 10.0.0.0/24 [1] area: 0.0.0.0

N IA 20.0.0.0/24 [2] area: 0.0.0.0 /* ?? */

via 10.0.0.1, atm3072

via 74.0.0.1, atm3073

N IA 38.0.0.0/24 [3] area: 0.0.0.0 /* ?? */

via 10.0.0.1, atm3072

via 74.0.0.1, atm3073

N 74.0.0.0/24 [2] area: 0.0.0.0

via 10.0.0.1, atm3072

via 74.0.0.1, atm3073

N 172.31.0.1/32 [2] area: 0.0.0.0

via 10.0.0.1, atm3072

via 74.0.0.1, atm3073

N IA 172.31.0.2/32 [3] area: 0.0.0.0 /* ?? */

via 10.0.0.1, atm3072

via 74.0.0.1, atm3073

N 172.31.0.4/32 [1] area: 0.0.0.0

directly attached to loopback0

-----------------------------------------------------------------

4 REPLIES

Re: Regarding installing multiple routes with unequal costs to g

I am not able to understand the network topology.

New Member

Re: Regarding installing multiple routes with unequal costs to g

Sorry for the corrupted topology.

In the routing table of RTA below, RTA keeps two routing entries over two links, atm3072 and atm3073 with unequal costs, 10 and 1 respectively,

for inter-area destinations and ABR itself. What I'm asking about is whether if RTA complies with the OSPF standard, RFC2328. Otherwise, does RTA work incorrectly?

(10.0.0.0/24)

<---atm3072--->

RTA RTB <----------------> RTC

<---atm3073---> (ABR) (20.0.0.0/24)

(74.0.0.0/24)

<--------- Area 0 --------> <------- Area 100 ---------->

RTA lo: 172.31.0.4, RTB lo: 172.31.0.1, RTC lo: 172.31.0.2

============ OSPF network routing table in RTA ============

N 10.0.0.0/24 [1] area: 0.0.0.0

N IA 20.0.0.0/24 [2] area: 0.0.0.0 /* ?? */

via 10.0.0.1, atm3072

via 74.0.0.1, atm3073

N IA 38.0.0.0/24 [3] area: 0.0.0.0 /* ?? */

via 10.0.0.1, atm3072

via 74.0.0.1, atm3073

N 74.0.0.0/24 [2] area: 0.0.0.0

via 10.0.0.1, atm3072

via 74.0.0.1, atm3073

N 172.31.0.1/32 [2] area: 0.0.0.0

via 10.0.0.1, atm3072

via 74.0.0.1, atm3073

N IA 172.31.0.2/32 [3] area: 0.0.0.0 /* ?? */

via 10.0.0.1, atm3072

via 74.0.0.1, atm3073

N 172.31.0.4/32 [1] area: 0.0.0.0

directly attached to loopback0

Re: Regarding installing multiple routes with unequal costs to g

I guess this is your topology. RTA and RTB are connected via two links (atm 3072, atm 3073) and these links fall in area 0. Link between B and C is in area 100.

Have you modified the ospf cost ? where have you applied the ospf cost command ?

New Member

Re: Regarding installing multiple routes with unequal costs to g

Your understanding on my topology is exactly right.

We have applied the ospf costs with 1and 10 for atm3072 and atm3073, respectively in RTA.

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