Solution to sub-optimal path after multi-point bidirectional redistribution

Concept preview:

What is multi-point bidirectional routing re-release?
Different routing protocols send route information to each other through re-publishing. The number of ASBR routers for re-publishing is more than one. in each routing protocol, the ASBR can receive the route information re-published by each other.
I. Experiment topology:

 

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Ii. Purpose:
Dual-point: for the sake of redundancy of Route entries, removing any ASBR router will not affect the communication between the two;
Bidirectional: routes of different AS instances can learn from each other.
Iii. Experiment description:
Run OSPF on R1, R2, and R3 and re-distribute 1.1.1.1 to OSPF [redistribute connected subnet].
R2, R3, and R4 are run with the MongoDB 4.4.4 re-distributed to [redistribute connected subnet]
Iv. Lab requirements:
R1 is able to learn the routes of the ivertices, And to 4.4.4.4 is the Server Load balancer.
R4 can learn the OSPF route, and 1.1.1.1 is the Server Load balancer.
V. experiment process:
1: Basic Interface Configuration:
R1 (config) # int s0/1
R1 (config-if) # ip add 12.0.0.1 255.255.255.0
R1 (config-if) # no sh
R1 (config-if) # int s0/2
R1 (config-if) # ip add 13.0.0.1 255.255.255.0
R1 (config-if) # no sh
R1 (config-if) # int lo 0
R1 (config-if) # ip add 1.1.1.1 255.255.255.0

R2 (config) # int s0/1
R2 (config-if) # ip add 12.0.0.2 255.255.255.0
R2 (config-if) # no sh
R2 (config-if) # int e1/0
R2 (config-if) # ip add 10.0.234.2 255.255.255.0
R2 (config-if) # no sh
R2 (config-if) # int lo 0
R2 (config-if) # ip add 2.2.2.2 255.255.255.0
R2 (config-if) # ^ Z
R3 (config) # int s0/2
R3 (config-if) # ip add 13.0.0.3 255.255.255.0
R3 (config-if) # no sh
R3 (config-if) # int e1/0
R3 (config-if) # ip add 10.0.234.3 255.255.255.0
R3 (config-if) # no sh
R3 (config-if) # int lo 0
R3 (config-if) # ip add 3.3.3.3 255.255.255.0

R4 (config) # int e0/0
R4 (config-if) # ip add 10.0.234.4 255.255.255.0
R4 (config-if) # no sh
R4 (config-if) # int lo 0
R4 (config-if) # ip add 4.4.4.4 255.255.255.0
R4 (config-if) # no sh

2: Route Configuration:
R1 (config) # router OS 1
R1 (config-router) # net 12.0.0.0 0.0.255 a 0
R1 (config-router) # net 13.0.0.0 0.0.255 a 0
R1 (config-router) # redistribute connected subnets

R2 (config) # router OS 1
R2 (config-router) # net 12.0.0.0 0.0.255 a 0
R2 (config-router) # net 2.2.2.0 0.0.0.255 a 0
R2 (config-router) # exit
R2 (config) # router VPN 1
R2 (config-router) # net 10.0.234.0 0.0.255
R2 (config-router) # no au

R3 (config) # router OS 1
R3 (config-router) # net 13.0.0.0 0.0.0.255 a 0
R3 (config-router) # net 3.3.3.0 0.0.255 a 0
R3 (config-router) # exit
R3 (config) # Fig 1
R3 (config-router) # net 10.0.234.0 0.0.255
R3 (config-router) # no au

R4 (config) # router VPN 1
R4 (config-router) # net 10.0.234.0 0.0.255
R4 (config-router) # redistribute connected

R1 # sho ip route
1.0.0.0/24 is subnetted, 1 subnets
C 1.1.1.0 is directly connected, Loopback0
2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/65] via 12.0.0.2, 00:01:58, Serial0/1
3.0.0.0/32 is subnetted, 1 subnets
O 3.3.3.3 [110/65] via 13.0.0.3, 00:01:58, Serial0/2
12.0.0.0/24 is subnetted, 1 subnets
C 12.0.0.0 is directly connected, Serial0/1
13.0.0.0/24 is subnetted, 1 subnets
C 13.0.0.0 is directly connected, Serial0/2

R4 # sho ip route
4.0.0.0/24 is subnetted, 1 subnets
C 4.4.4.0 is directly connected, Loopback0
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.234.0 is directly connected, Ethernet0/0
// It is found that R1 and R4 cannot learn routes from each other. Next we will implement bidirectional redistribution on E2 and R3.
3: bidirectional redistribution Configuration:
R2 (config) # router OS 1
R2 (config-router) # redistribute VPN 1 subnets
R2 (config-router) # exit
R2 (config) # router VPN 1
R2 (config-router) # redistribute ospf 1 metric 100000 1 255 1 1500

R3 (config) # router OS 1
R3 (config-router) # redistribute VPN 1 subnets
R3 (config-router) # router VPN 1
R3 (config-router) # redistribute ospf 1 metric 100000 1 255 1 1500

R4 # sho ip route VPN
1.0.0.0/24 is subnetted, 1 subnets
D ex 1.1.1.0 [170/281856] via 10.0.234.3, 00:04:57, Ethernet0/0
[170/281856] via 10.0.234.2, 00:04:57, Ethernet0/0
2.0.0.0/24 is subnetted, 1 subnets
D 2.2.2.0 [90/409600] via 10.0.234.2, 00:06:15, Ethernet0/0
3.0.0.0/24 is subnetted, 1 subnets
D 3.3.3.0 [90/409600] via 10.0.234.3, 00:06:15, Ethernet0/0
12.0.0.0/24 is subnetted, 1 subnets
D ex 12.0.0.0 [170/281856] via 10.0.234.3, 00:04:57, Ethernet0/0
[170/281856] via 10.0.234.2, 00:04:57, Ethernet0/0
13.0.0.0/24 is subnetted, 1 subnets
D ex 13.0.0.0 [170/281856] via 10.0.234.3, 00:04:57, Ethernet0/0
[170/281856] via 10.0.234.2, 00:04:57, Ethernet0/0
// It is found that there is a route for load balancing to R1 on R4.
R1 # sho ip route
1.0.0.0/24 is subnetted, 1 subnets
C 1.1.1.0 is directly connected, Loopback0
2.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
O 2.2.2.2/32 [110/65] via 12.0.0.2, 00:01:34, Serial0/1
O E2 2.2.2.0/24 [110/20] via 13.0.0.3, 00:01:34, Serial0/2
3.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
O 3.3.3.3/32 [110/65] via 13.0.0.3, 00:01:34, Serial0/2
O E2 3.3.3.0/24 [110/20] via 12.0.0.2, 00:01:34, Serial0/1
4.0.0.0/24 is subnetted, 1 subnets
O E2 4.4.4.0 [110/20] via 12.0.0.2, 00:01:33, Serial0/1
10.0.0.0/24 is subnetted, 1 subnets
O E2 10.0.234.0 [110/20] via 13.0.0.3, 00:01:34, Serial0/2
[110/20] via 12.0.0.2, 00:01:34, Serial0/1
12.0.0.0/24 is subnetted, 1 subnets
C 12.0.0.0 is directly connected, Serial0/1
13.0.0.0/24 is subnetted, 1 subnets
C 13.0.0.0 is directly connected, Serial0/2
// Found that R1 does not have a route to the Server Load balancer of 4.4.4.0, Why?
Let's take a look at the route tables on the two ASBR Routers:
R3 # sho ip route
1.0.0.0/24 is subnetted, 1 subnets
O E2 1.1.1.0 [110/20] via 13.0.0.1, 00:02:47, Serial0/2
2.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
O 2.2.2.2/32 [110/129] via 13.0.0.1, 00:02:47, Serial0/2
D ex 2.2.2.0/24 [170/281856] via 10.0.234.2, 00:08:55, Ethernet1/0
3.0.0.0/24 is subnetted, 1 subnets
C 3.3.3.0 is directly connected, Loopback0
4.0.0.0/24 is subnetted, 1 subnets
O E2 4.4.4.0 [110/20] via 13.0.0.1, 00:02:47, Serial0/2
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.234.0 is directly connected, Ethernet1/0
12.0.0.0/24 is subnetted, 1 subnets
O 12.0.0.0 [110/128] via 13.0.0.1, 00:02:47, Serial0/2
13.0.0.0/24 is subnetted, 1 subnets
C 13.0.0.0 is directly connected, Serial0/2
It is found that there is no MongoDB route 4.4.4 on R3 and it is replaced by an OSPF route. It should be well-formed by using OSPF, but now OSPF is selected, the reason is that the route 4.4.4.0 learned by R3 is an external route, the management distance is 170, and R3 re-distributes it from R2) the route management distance from OSPF to 4.4.4.0 is 110, and R3 takes the route table to which the management distance is small, as a result, when R3 re-distributes it to OSPF, the local OSPF table does not have a route of 4.4.4.0. Therefore, R1 does not have a route to the Server Load balancer of 4.4.4.4.0.

 

Solution: Modify the AD
The Management Distance between the OSPF route learned by R2 from R3 is higher than that of the external OSPF (170,
You can change the Management Distance of the routes learned by R3 from R2.

R2 (config) # router ospf 1
R2 (config-router )#Distance 180 3.3.3.3 0.0.0.0

// The OSPF route Modification Management Distance learned by R2 from 3.3.3.3 is 180.

R3 (config) # router OS 1
R3 (config-router )#Distance 180 2.2.2.2 0.0.0.0

// The route Modification Management Distance learned by R3 from 2.2.2.2 is 180.

View the route table on R2 and R3:
R2 # sho ip route

1.0.0.0/24 is subnetted, 1 subnets
O E2 1.1.1.0 [110/20] via 12.0.0.1, 00:00:58, Serial0/1
2.0.0.0/24 is subnetted, 1 subnets
C 2.2.2.0 is directly connected, Loopback0
3.0.0.0/24 is subnetted, 1 subnets
D 3.3.3.0 [90/409600] via 10.0.234.3, 00:16:50, Ethernet1/0
4.0.0.0/24 is subnetted, 1 subnets
D ex 4.4.4.0 [170/281856] via 10.0.234.4, 00:00:35, Ethernet1/0
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.234.0 is directly connected, Ethernet1/0
12.0.0.0/24 is subnetted, 1 subnets
C 12.0.0.0 is directly connected, Serial0/1
13.0.0.0/24 is subnetted, 1 subnets
O 13.0.0.0 [110/128] via 12.0.0.1, 00:00:59, Serial0/1
R3 # sho ip route
1.0.0.0/24 is subnetted, 1 subnets
O E2 1.1.1.0 [110/20] via 13.0.0.1, 00:01:21, Serial0/2
2.0.0.0/24 is subnetted, 1 subnets
D 2.2.2.0 [90/409600] via 10.0.234.2, 00:13:37, Ethernet1/0
3.0.0.0/24 is subnetted, 1 subnets
C 3.3.3.0 is directly connected, Loopback0
4.0.0.0/24 is subnetted, 1 subnets
D ex 4.4.4.0 [170/281856] via 10.0.234.4, 00:01:20, Ethernet1/0
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.234.0 is directly connected, Ethernet1/0
12.0.0.0/24 is subnetted, 1 subnets
O 12.0.0.0 [110/128] via 13.0.0.1, 00:01:21, Serial0/2
13.0.0.0/24 is subnetted, 1 subnets
C 13.0.0.0 is directly connected, Serial0/2
// It is found that the problem of sub-optimal routing has been solved on the two ABR servers. Let's look at the Load Balancing Problem of R1 and R4.
R1 # sho ip route ospf
2.0.0.0/24 is subnetted, 1 subnets
O E2 2.2.2.0 [110/20] via 13.0.0.3, 00:17:02, Serial0/2
[110/20] via 12.0.0.2, 00:17:02, Serial0/1
3.0.0.0/24 is subnetted, 1 subnets
O E2 3.3.3.0 [110/20] via 13.0.0.3, 00:17:02, Serial0/2
[110/20] via 12.0.0.2, 00:17:02, Serial0/1
4.0.0.0/24 is subnetted, 1 subnets
O E2 4.4.4.0 [110/20] via 13.0.0.3, 00:02:29, Serial0/2
[110/20] via 12.0.0.2, 00:02:29, Serial0/1
10.0.0.0/24 is subnetted, 1 subnets
O E2 10.0.234.0 [110/20] via 13.0.0.3, 00:17:02, Serial0/2
[110/20] via 12.0.0.2, 00:17:02, Serial0/1
R4 # sho ip route VPN
1.0.0.0/24 is subnetted, 1 subnets
D ex 1.1.1.0 [170/281856] via 10.0.234.3, 00:02:42, Ethernet0/0
[170/281856] via 10.0.234.2, 00:02:42, Ethernet0/0
2.0.0.0/24 is subnetted, 1 subnets
D 2.2.2.0 [90/409600] via 10.0.234.2, 00:18:45, Ethernet0/0
3.0.0.0/24 is subnetted, 1 subnets
D 3.3.3.0 [90/409600] via 10.0.234.3, 00:18:45, Ethernet0/0
12.0.0.0/24 is subnetted, 1 subnets
D ex 12.0.0.0 [170/281856] via 10.0.234.3, 00:02:42, Ethernet0/0
[170/281856] via 10.0.234.2, 00:02:42, Ethernet0/0
13.0.0.0/24 is subnetted, 1 subnets
D ex 13.0.0.0 [170/281856] via 10.0.234.3, 00:03:05, Ethernet0/0
[170/281856] via 10.0.234.2, 00:03:05, Ethernet0/0
// Load Balancing has been achieved.

 

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