Missing OSPF routes to subnets [GNS3]

[AK0]

I created a little topology in GNS3 with 3 routers. And the routing tables on R1 and R3 should have two routes to a subnet:

• R1 should have two equal cost routes to subnet 10.1.5.0/24
• R3 should have two equal cost routes to subnet 10.1.4.0/24

R1 routing table:

R1#sh ip route ospf
     10.0.0.0/24 is subnetted, 6 subnets
O       10.1.3.0 [110/65] via 10.1.6.3, 00:06:29, Serial0/1
O       10.1.2.0 [110/65] via 10.1.4.2, 00:05:45, Serial0/0
O       10.1.5.0 [110/128] via 10.1.4.2, 00:05:45, Serial0/0

R3 routing table:

R3#sh ip route ospf     
     10.0.0.0/24 is subnetted, 6 subnets
O       10.1.2.0 [110/65] via 10.1.5.2, 00:05:37, Serial0/1
O       10.1.1.0 [110/65] via 10.1.6.1, 00:06:13, Serial0/0
O       10.1.4.0 [110/128] via 10.1.5.2, 00:05:37, Serial0/1

R2 routing table:

R2#sh ip route ospf
     10.0.0.0/24 is subnetted, 6 subnets
O IA    10.1.3.0 [110/65] via 10.1.5.3, 00:05:37, Serial0/1
O IA    10.1.1.0 [110/65] via 10.1.4.1, 00:05:37, Serial0/0
O IA    10.1.6.0 [110/128] via 10.1.5.3, 00:05:37, Serial0/1
                         [110/128] via 10.1.4.1, 00:05:37, Serial0/0

R2 does have two equal cost routes to 10.1.6.0/24. Why don't R1 and R3 have two equal cost routes to 10.1.5.0/24 and 10.1.4.0/24 respectively, but instead have only one? Network topology screenshot is included.

[MHM Cisco World · Accepted solution]

From R2 view it see 10.1.6.0 from two routers lsa3 in same area

But for R1/3 it see 10.1.4/5.0 from two routers in different area 

Area 0 and Area 1

And ospf prefer prefix intra area  lsa1 over inter area lsa3.

So R1/2 prefers prefix via area 1 and hence there is no equal cost multi path

MHM

[AK0]

Ok that was simple. Thanks for the explanation.

[Harold Ritter]

Hi @MHM Cisco World ,

Just one small precision. It is the route via area 1 that will be preferred as this is the one seen as an intra area route. The route via area 0 is seen as an inter area route.

Regards,

[MHM Cisco World]

I See below so it intra area not inter area inter area must be O IA not O

O       10.1.5.0 [110/128] via 10.1.4.2, 00:05:45, Serial0/0

MHM

[Harold Ritter]

Hi @MHM Cisco World ,

Exactly my point. R1 sees the route to 10.1.5.0/24 as an intra area route via R2. R1 sees the same route via R3 as inter area and therefore prefers the one via R2.

Regards,

[MHM Cisco World]

That what I was explain to Mr @AK0 

R1/2 see prefix 10.1.4/5.0 one time via lsa1 and other via lsa3 and ospf always prefer lsa1 over lsa3. That why he see one path not two.

[Harold Ritter]

Hi @MHM Cisco World ,

My comment came from what you said here.

> So R1/2 prefers prefix via area 0 and hence there is no equal cost multi path

R1 and R3 do not prefer the path via area 0, but rather the one via area 1.

Regards,

[MHM Cisco World]

Sorry it typo I correct it.

Thanks 

MHM

[AK0]

Yes I understand now. The "O" in R1 routing table next to 10.1.5.0/24 means INTRA-area route, I explain it to myself as: the subnet is part of an area of which the local router (R1) is part of.

In this case 10.1.5.0/24 is part of Area 1, and next-hop 10.1.4.2 (R2) is also in Area 1, so to reach a subnet in Area 1, stay in Area 1, no need to forward to another area (no need to forward to Area 0 first, and then back into Area 1).

[MHM Cisco World]

No need config friend I see what @Harold Ritter meaning it typo and correct it.

Thanks all

MHM