Designing best and next best path for a network within OSPF area

This is an example of choosing best path to reach network 100.100.100.1/32 Loopback on R4 from R1.

Note: Within an area LSDB or topology table remains synchronized and identical so no manipulation can be done. But distribute-list could be used while installing routes from topology table to Routing table.

 

By default the cost to reach Lo1 100.100.100.1 on R4 from R1 will be 64+1+1.

R1 has two best equal cost path via R1->R2->R4->Lo1 or  R1->R3->R4->Lo1.

Objective:

To reach network Lo1 on R4 we want only one path either, or both the path with any one being best and second path alternative should the primary fail.

R1(config-router)#do show ip route

Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP

       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

       E1 - OSPF external type 1, E2 - OSPF external type 2

       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2

       ia - IS-IS inter area, * - candidate default, U - per-user static route

       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     1.0.0.0/30 is subnetted, 1 subnets

O       1.1.1.0 [110/65] via 2.2.2.2, 00:02:59, Serial0/0

     2.0.0.0/30 is subnetted, 1 subnets

C       2.2.2.0 is directly connected, Serial0/0

     100.0.0.0/32 is subnetted, 1 subnets

O  100.100.100.1 [110/66] via 2.2.2.2, 00:02:59, Serial0/0    //**two best path

                             [110/66] via 3.3.3.2, 00:02:59, Serial0/1

     3.0.0.0/30 is subnetted, 1 subnets

C       3.3.3.0 is directly connected, Serial0/1

     101.0.0.0/32 is subnetted, 1 subnets

O       101.101.101.1 [110/66] via 2.2.2.2, 00:03:00, Serial0/0

                                 [110/66] via 3.3.3.2, 00:03:00, Serial0/1

     4.0.0.0/30 is subnetted, 1 subnets

O       4.4.4.0 [110/65] via 3.3.3.2, 00:03:03, Serial0/1

To make R1->R2->R4->Lo1 as best path we will use distribute list gateway command on ospf

On R1:

interface Serial0/0

 ip address 2.2.2.1 255.255.255.252

 clock rate 2000000

!

interface Serial0/1

 ip address 3.3.3.1 255.255.255.252

 clock rate 2000000

!

//** using prefix list by distribution-list below, the Lo1 100.100.100.1 learned from S0/1 interface of R1 has been disallowed to go to the RIB.

router ospf 1

 log-adjacency-changes

 network 0.0.0.0 255.255.255.255 area 0

 distribute-list prefix FILTER_PREFIX gateway DENY1 in Serial0/1

 !

ip prefix-list DENY1 seq 5 permit 3.3.3.2/32

!

ip prefix-list FILTER_PREFIX seq 5 deny 100.100.100.1/32

ip prefix-list FILTER_PREFIX seq 7 permit 101.101.101.1/32

!

But the above has a disadvantage with regard to if the primary path fails at any stage the backup S0/1 on R1 will still filter the Lo1 route from being installed and thus the access to Lo1 will remain dead.

Second option is,

I have not discovered as yet any way to change the cost when R1 learns about Lo1 from R4. Tried multiple ways to change the metric via route map, also tried tagging the route at R3 and then modify the metric value on R1 via route-map set metric +/- but the output was not any better.

One more option just worked out. Within an area changing AD is the nicest way to manipulate the likelihood to become the best path. But modifying AD on R1 changes the AD for all route learned via R1 source.

R1(config-router)#distance 109 2.2.2.1 0.0.0.0   //*will change AD globally for a source

We can manipulate AD when it traverses from one process to another and that just seems to work out.

On R3:

interface FastEthernet0/0

 ip address 4.4.4.1 255.255.255.252

 duplex auto

 speed auto

!

interface Serial0/0

 ip address 3.3.3.2 255.255.255.252

 clock rate 2000000

!

router ospf 1

 log-adjacency-changes

 redistribute ospf 2 subnets tag 51          //** tagging is done for reference to check on R1

 network 3.3.3.0 0.0.0.3 area 0

!

router ospf 2

 log-adjacency-changes

 redistribute ospf 1

 network 4.4.4.0 0.0.0.3 area 0

!

Since R3 has OSPF process 1 for 3.3.3.0 R1 link and OSPF domain process 2 for 4.4.4.0 R4 link, it will not share each other routes as the processes behaves. We will redistribute them into one another.

Note: Redistribution of subnets are often the cause of loop in OSPF as it breaks broad visibility of OSPF domain. Perform it carefully.

 

At this point on R1 the we could see the route for Lo1 100.100.100.1 is learned as external, since it moved from OSPF process 2 to OSPF process 1 on R3, ignore R2 as of now.

For OSPF path preference is intra-area, then Inter-area , then External routes.

Add the below command  on R1

router ospf 1

 log-adjacency-changes

 network 0.0.0.0 255.255.255.255 area 0

 distance ospf external 111               //** change the AD of external route learned from ospf process 2                                                        on R3 to 111.

Now under normal scenario R1->R2->R4->Lo1 will have intra-area AD 110 and if it fails the R1->R3->R4->Lo1 which has two ospf process will have AD 111. 

This way R1 has two paths to reach Lo1 by manipulating their AD value with an OSPF area.

Lets Check on R1:

R1#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     1.0.0.0/30 is subnetted, 1 subnets
O       1.1.1.0 [110/65] via 2.2.2.2, 00:35:09, Serial0/0
     2.0.0.0/30 is subnetted, 1 subnets
C       2.2.2.0 is directly connected, Serial0/0
     100.0.0.0/32 is subnetted, 1 subnets
O       100.100.100.1 [110/66] via 2.2.2.2, 00:35:09, Serial0/0   //**Intra-area path preferred
     3.0.0.0/30 is subnetted, 1 subnets
C       3.3.3.0 is directly connected, Serial0/1
     101.0.0.0/32 is subnetted, 1 subnets
O       101.101.101.1 [110/66] via 2.2.2.2, 00:35:09, Serial0/0
     4.0.0.0/30 is subnetted, 1 subnets
O       4.4.4.0 [110/66] via 2.2.2.2, 00:35:10, Serial0/0

Now I will shut down the f0/0  on R4:

Lets see now if R1 has next best path to reach Lo1 100.100.100.1

R1#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     1.0.0.0/30 is subnetted, 1 subnets
O       1.1.1.0 [110/65] via 2.2.2.2, 00:00:16, Serial0/0
     2.0.0.0/30 is subnetted, 1 subnets
C       2.2.2.0 is directly connected, Serial0/0
     3.0.0.0/30 is subnetted, 1 subnets
C       3.3.3.0 is directly connected, Serial0/1
     100.0.0.0/32 is subnetted, 1 subnets
O E2    100.100.100.1 [111/2] via 3.3.3.2, 00:00:14, Serial0/1 //** external path is active now
    101.0.0.0/32 is subnetted, 1 subnets
O E2    101.101.101.1 [111/2] via 3.3.3.2, 00:00:14, Serial0/1
     4.0.0.0/30 is subnetted, 1 subnets
O E2    4.4.4.0 [111/1] via 3.3.3.2, 00:00:16, Serial0/1

Cheers!