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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 BGP Peer Relationship Goes Down Because of Route Iteration

The BGP Peer Relationship Goes Down Because of Route Iteration

Fault Symptom

There are two links between Router A (a Huawei device) and Router B (a non-Huawei device). One link is established through GigabitEthernet 1/0/0 interfaces and the other link is established through GigabitEthernet 2/0/0 interfaces. Devices on both ends of a link establish the BGP peer relationship through loopback interfaces. After the GigabitEthernet 1/0/0 interface on Router A goes Down, the BGP peer relationship between Router A and Router B goes Down and remains in the OpenSent state. Router A, however, can successfully ping the address of the loopback interface on Router B.

Figure 19-16  Route iteration causes the BGP peer relationship to go Down

Fault Analysis

  1. After GigabitEthernet 1/0/0 on Router A goes Down, run the display ip routing-table ip-address command on Router A to view the equal-cost routes to the public network. The command output shows that there are two equal-cost routes with the same next-hop addresses, 10.0.0.1. The outbound interfaces of the equal-cost routes are GigabitEthernet 2/0/0 and Null0. Before GigabitEthernet 1/0/0 on Router A goes Down, the outbound interfaces of the two equal-cost routes are GigabitEthernet 2/0/0 and GigabitEthernet 1/0/0, and have the same next hop address, 10.0.0.1.

    Run the display bgp peer command on Router A to check the BGP peer relationship. The BGP peer with the address 10.0.0.1 is in the OpenSent state.

  2. Route iteration may cause outbound interfaces of equal-cost routes to change. If no route iteration occurs, after GigabitEthernet 1/0/0 goes Down, only one of the two equal-cost routes exists, that is, the route with the outbound interface GigabitEthernet 2/0/0.

  3. Check the configuration of Router A and analyze why the outbound interface is iterated to Null0. The configuration shows that the static routes with the 32-bit mask to the address (10.0.0.1) of the loopback interface on Router B are configured on Router A.

    ip route-static 10.0.0.1 255.255.255.255 192.168.1.1
    ip route-static 10.0.0.1 255.255.255.255 192.168.0.1

    After GigabitEthernet 1/0/0 on Router A goes Down, the preceding static route configurations cause Router A to iterate routes. Check whether there is a route to 192.168.0.1 in the routing table. The configuration file contains the following static route configuration:

    ip route-static 192.168.0.0 255.255.255.0 NULL0 preference 255

    Therefore, the outbound interface of one of the two upstream equal-cost routes becomes Null0.

  4. Analyze why the BGP peer relationship goes Down after one outbound interface becomes Null0. After GigabitEthernet 1/0/0 goes Down, two upstream routes of Router A are as follows:

    Destination/Mask    Proto  Pre  Cost       NextHop    Interface
          10.0.0.1/32   BGP    100  0          10.0.0.1   GigabitEthernet2/0/0
                        BGP    100  0          10.0.0.1   NUll0

    In this case, Router A can successfully ping the address (10.0.0.1) of the loopback interface on Router B. In normal situations, the BGP peer relationship remains Up. Because there are two links between Router A and Router B, hash calculation is triggered when packets are exchanged between the two devices. If the ping command is run without specifying the source address, the outbound interface calculated by the hash algorithm is GigabitEthernet 2/0/0, in which case the ping succeeds. If the ping command is run with loopback interface address 20.0.0.1 specified as the source address on Router A, the outbound interface calculated by the hash algorithm is GigabitEthernet 1/0/0, in which case the ping fails. Loopback interface addresses are used to establish the BGP peer relationship between Router A and Router B. GigabitEthernet 1/0/0 is now iterated to the outbound interface of Null0. Therefore, the BGP peer relationship between Router A and Router B goes Down.

To clear the fault, disable route iteration on Router A.

Procedure

  1. Run the system-view command on Router A to enter the system view.

    After GigabitEthernet 1/0/0 on Router A goes Down, the static route with the outbound interface GigabitEthernet 1/0/0 becomes unreachable and thus is deleted from the routing table. Then, all packets destined for Router B are sent through only GigabitEthernet 2/0/0.

  2. Run the undo ip route-static 10.0.0.1 255.255.255.255 192.168.1.1 and undo ip route-static 10.0.0.1 255.255.255.255 192.168.0.1 commands to delete the original static route configurations.
  3. Run the ip route-static 10.0.0.1 255.255.255.255 gigabitethernet 2/0/0 192.168.1.1 and ip route-static 10.0.0.1 255.255.255.255 gigabitethernet 1/0/0 192.168.0.1 commands to configure static routes with next hops and outbound interfaces.
  4. Run the display bgp peer command. The BGP peer with the address 10.0.0.1 is in the Established state. This indicates that the BGP peer relationship is normal. The fault is cleared.

Summary

Route iteration is enabled by default. Ensure that route iteration will not cause exceptions on a network.

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Updated: 2019-05-10

Document ID: EDOC1000079719

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