[NE Router-Troubleshooting] The router Receives Two LSAs with the Same LS ID but Fails to Calculate a Route Based on One of the LSAs

Fault Symptom

Traffic is unevenly distributed between the path from Router A to the BAS and the path from Router B to the BAS. You need to configure load balancing between the path Router A -> BAS -> destination and the path Router A -> RouterB -> BAS-> destination for the traffic transmitted from Router A to the network segment to which the BAS is connected.

The following takes traffic to destination network segment 10.1.1.0 as an example.

On Router B, a static route to 10.1.1.0 is configured and OSPF is configured to import static routes. Router A receives an ASE LSA with the LS ID being 10.1.1.0 from Router B and an ASE LSA with the LS ID also being 10.1.1.0 from the BAS. Router A can calculate a route based on the LSA received from the BAS, but fails to calculate a route based on the LSA received from Router B.

Fault Analysis

The possible causes are as follows:

1. The configurations of devices are incorrect.

2. The FA field in the LSA sent by Router B is 10.1.2.26. The LSA is not calculated because the FA field of the LSA is incorrect.

3. The conditions of generating routes for load balancing are not met.

Based on the analysis of the preceding possible causes, it can be concluded:

1. The configurations of the devices are normal.

2. The LSA whose FA field meets the condition of route calculation.

<RouterA> ping 10.1.3.1

  PING 10.1.3.1: 56  data bytes, press CTRL_C to break

    Reply from 10.1.3.1: bytes=56 Sequence=1 ttl=255 time=1 ms

    Reply from 10.1.3.1: bytes=56 Sequence=2 ttl=255 time=1 ms

    Reply from 10.1.3.1: bytes=56 Sequence=3 ttl=255 time=1 ms

    Reply from 10.1.3.1: bytes=56 Sequence=4 ttl=255 time=1 ms

    Reply from 10.1.3.1: bytes=56 Sequence=5 ttl=255 time=1 ms

  --- 10.1.3.1 ping statistics ---

    5 packet(s) transmitted

    5 packet(s) received

    0.00% packet loss

    round-trip min/avg/max = 1/1/1 ms

<RouterA> display ip routing-table 10.1.3.1

Route Flags: R - relay, D - download to fib

------------------------------------------------------------------------------

Routing Table : Public

Summary Count : 2

Destination/Mask    Proto  Pre  Cost     Flags NextHop         Interface

     10.1.3.1/32  O_ASE  150  1           D  10.1.2.45     GigabitEthernet1/0/5

                  O_ASE  150  1           D  10.1.2.49     GigabitEthernet1/0/6

<RouterA> ping 10.1.2.26

    Reply from 10.1.2.26: bytes=56 Sequence=1 ttl=254 time=1 ms

    Reply from 10.1.2.26: bytes=56 Sequence=2 ttl=254 time=1 ms

    0.00% packet loss

    round-trip min/avg/max = 1/1/1 ms

<RouterA> display ip routing-table 10.1.2.26

    10.1.2.24/30  OSPF   10   101         D  10.1.2.45     GigabitEthernet1/0/5

                  OSPF   10   101         D  10.1.2.49     GigabitEthernet1/0/6

3. On this network, the costs of LSAs are 1. You need to compare the cost of the route to the ASBR and the cost of the route to the FA.

For Type 2 ASE LSAs, OSPF equal-cost routes can be generated when the following conditions are met:

a. The costs of LSAs are the same.

b. The cost of the route to the ASBR is the same as the cost of the route to the FA.

On the network, the cost of the route to the FA is 101.

· For the LSA with the FA field being 0.0.0.0, the cost of the route to ASBR at 10.1.3.1 is 1.

· For the LSA with the FA field not being 0.0.0.0, the cost of the route to the FA being 10.1.2.26 is 101.

The LSA with the FA field being set is not calculated because the priority of the LSA is lower. As a result, equal-cost routes cannot be formed.

Procedure

1. To form equal-cost routes on the network, do as follows:

On the BAS, run the network command to enable OSPF on the next-hop interface of the route to 10.1.1.0. Run the ospf cost command to set the cost on the interface to 100 so that the interface advertises LSAs with the FA field being its address.

Then, you can find two LSAs with FA fields on Router A. The cost of the route to one FA and the cost of the route to the other FA are both 101. Thus, equal-cost routes can be formed.