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
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
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!
