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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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Routes Are Abnormal Because the FA Fields in Type 5 LSAs Are Set Incorrectly

Routes Are Abnormal Because the FA Fields in Type 5 LSAs Are Set Incorrectly

Fault Symptom

On the network shown in Figure 19-8, Router C is a non-Huawei device. Router A and Router B are two routers. Router A and Router B have two upstream GE interfaces and are configured with two static routes.

  • Router A

    [RouterA] ip route-static 0.0.0.0 0.0.0.0 192.168.0.69
    [RouterA] ip route-static 0.0.0.0 0.0.0.0 192.168.0.65
  • Router B

    [RouterB] ip route-static 0.0.0.0 0.0.0.0 192.168.0.5
    [RouterB] ip route-static 0.0.0.0 0.0.0.0 192.168.0.1

Router A and Router B advertise default routes to Router C in an unforced manner. Normally, Router C has a default external route to Router A and another default external route to Router B. Router C, however, has a route to only one of Routers A and B in the following situations:

  • The static route 192.168.0.65 on Router A is deleted, and other configurations remain unchanged. In this case, Router C has an OSPF default route to only Router B.
  • The static route 192.168.0.1 on Router B is deleted, and other configurations remain unchanged. In this case, Router C has an OSPF default route to only Router A.
Figure 19-8  Network diagram of the networking where routes on a device are abnormal

Fault Analysis

  1. Run the undo ip route-static 0.0.0.0 0.0.0.0 192.168.0.65 command on Router A, and then view the details about the corresponding LSA on Router C. The FA field of the LSA is incorrectly set by Router A. In this case, Router C has an OSPF default route to only Router B, because Router C finds that the route to address 192.168.0.69 is unreachable when performing SPF calculation.

  2. Run the undo ip route-static 0.0.0.0 0.0.0.0 192.168.0.1 command on Router B, and then view the details about the corresponding LSA on Router C. The FA field of the LSA is incorrectly set by Router B. In this case, Router C has an OSPF default route to only Router A, because Router C finds that the route to address 192.168.0.5 is unreachable when performing SPF calculation.

  3. The preceding analysis shows that the root cause of the fault is that Router A and Router B incorrectly set the FA fields in the corresponding LSAs.

    The rules the router uses to fill in the FA fields of LSAs and calculate routes are as follows:

    • When the value of the FA field of a Type 5 LSA is 0.0.0.0, the router that receives the LSA knows that the router sending the LSA is an advertising router (that is, an ASBR), and calculates the next hop.

    • When all of the following conditions are met, an ASBR fills in an address other than 0.0.0.0 in the FA field of a Type 5 LSA, and the router that receives the LSA calculates the next hop based on the value of the FA field:

      1. The route to the interface of the ASBR's next hop is reachable.
      2. The interface connecting the ASBR to an external network is not configured as a silent interface.
      3. The network type of the interface connecting the ASBR to an external network is not P2P or P2MP.
      4. The address of the interface connecting the ASBR to an external network is within the network address range advertised by OSPF.

      If none of the preceding conditions are met, the FA field of an LSA is set to 0.0.0.0.

Procedure

  1. Do as follows to rectify the fault:

    • Check the data configuration on Router A and Router B, the following information can be found:
      • The network 192.168.0.68 0.0.0.3 command rather than the network 192.168.0.64 0.0.0.3 command is run in the OSPF process on Router A.

      • The network 192.168.0.4 0.0.0.3 command rather than the network 192.168.0.0 0.0.0.3 command is run in the OSPF process on Router B.

    • In the OSPF process on Router A, delete the network command used to advertise the network segment to which the next hop of the configured static route corresponds. Perform the same operation on Router B. Then, the fault is rectified.

    • Run the ospf network-type p2p command on the interface specified in the network command run on the Router A to change the network type of the interface. Then, perform the same operation on Router B. After that, the fault is rectified.

    • Set the corresponding interface on Router A to be a silent interface, or enable the routes from Router C to all the next hops of the static routes of Router A to be reachable. Perform the same operation on Router B. Then, the fault is rectified.

Summary

The network segment addresses and interface types of OSPF interfaces must be correctly configured. This allows the router to correctly fill in the FA field in a Type 5 LSA and calculate routes based on defined rules.

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Updated: 2019-05-10

Document ID: EDOC1000079719

Views: 453430

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