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AR Router Troubleshooting Guide

This Product Documentation provides guidance for maintaining AR Enterprise Router, covering common information collection and fault diagnostic commands, typical fault troubleshooting guide, and troubleshooting.
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The router Receives Two LSAs with the Same LS ID but Fails to Calculate a Route Based on One of the LSAs

The router Receives Two LSAs with the Same LS ID but Fails to Calculate a Route Based on One of the LSAs

Fault Symptom

On the network shown in Figure 19-9, traffic is unevenly distributed between the path from Router A to the BAS and the path from Router B to the BAS. Load balancing between the path Router A -> BAS -> destination and the path Router A -> RouterB -> BAS-> destination must be configured for the traffic transmitted from Router A to the network segment to which the BAS is connected.

Figure 19-9  Network diagram of the router receiving two LSAs with the same LS ID but fails to calculate a route based on one of the LSAs

The following uses traffic sent to 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 10.1.1.0 from Router B and an ASE LSA with the same LS ID 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. Device configurations 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 required to generate 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
    ------------------------------------------------------------------------------
    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     GigabitEthernet1/0/0
                      O_ASE  150  1           D  10.1.2.49     GigabitEthernet2/0/0
    <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     GigabitEthernet1/0/0
                      OSPF   10   101         D  10.1.2.49     GigabitEthernet2/0/0
  3. On this network, the costs of LSAs are 1. 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:

    1. The costs of LSAs are the same.
    2. 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 0.0.0.0, the cost of the route to ASBR at 10.1.3.1 is 1.
    • For the LSA with an FA field other than 0.0.0.0, the cost of the route to the FA at 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 of the interface to 100 so that the interface advertises LSAs with the FA field as the address of the interface.

    Then, there will be 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. Therefore, equal-cost routes can be formed.

Summary

To form equal-cost routes, set the same cost on the interfaces so that the interfaces advertise LSAs with the same FA field, the addresses of the interfaces.

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Updated: 2019-05-10

Document ID: EDOC1000079719

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