cancel
Showing results for 
Search instead for 
Did you mean: 
cancel
364
Views
6
Helpful
10
Replies

Missing OSPF routes to subnets [GNS3]

AK0
Level 1
Level 1

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.

1 Accepted Solution

Accepted Solutions

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

View solution in original post

10 Replies 10

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

Ok that was simple. Thanks for the explanation.

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,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

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

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,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

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.

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,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México

Sorry it typo I correct it.

Thanks 

MHM

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).

 

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

Thanks all

MHM

Review Cisco Networking for a $25 gift card