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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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Summary Routes Advertised by EBGP Flap Frequently Because Routing Protocols Are Configured with Improper Preferences

Summary Routes Advertised by EBGP Flap Frequently Because Routing Protocols Are Configured with Improper Preferences

Fault Symptom

As shown in Figure 19-18, Router C and Router D are two egress devices on an enterprise network and are connected to Router A and Router B on the MAN through EBGP. Route suppression is configured on devices on the MAN to suppress EBGP routes from egress devices on the enterprise network. On the enterprise network, Router C and Router D are connected to the attached devices through IS-IS and IBGP peer relationships are established between them. Some traffic traverses the egress devices on the enterprise network. Therefore, to prevent link faults from causing routing loops, Router C and Router D establish an IBGP peer relationship using interface addresses and advertise enterprise network routes to devices on the MAN through static blackhole routes imported with the network command.

After a fault occurs on a board or a link, the IBGP peer relationship between Router C and Router D alternates between Up and Down states frequently and therefore all enterprise network services are interrupted.

Figure 19-18  Summary routes advertised by EBGP peer flap frequently

Fault Analysis

The possible causes are:
  • Associated routing policies, including local and remote routing policies, are changed manually.
  • Routes (mainly advertised summary routes) are added and then deleted two consecutive times.
  • Static and dynamic routing protocols are configured with improper preferences. As a result, some BGP summary routes cannot be advertised through the blackhole route imported with the network command.

Checking device logs shows that no associated routing policies have been changed manually and no routes have been added and then deleted.

Egress devices on the enterprise network advertise routes through blackhole routes imported with the network command. Therefore, route addition or deletion is not involved. Checking summary routes shows that the lifetime of these routes is very long. This indicates that service interruption is unlikely to occur.

On Router C and Router D, the preference of the BGP protocol is set to 20, and the preference of static blackhole routes defaults to 60. Therefore, if both IBGP routes and static routes exist, IBGP routes are advertised as summary routes first. As a result, when the IBGP peer relationship between Router C and Router D alternates Up and Down, the advertised summary routes flap, causing enterprise network services to be interrupted.

To solve this problem, change the preferences of BGP routes and static routes so that IBGP routes have a lower preference than static routes.

Procedure

  1. Run the system-view command on Router C to enter the system view.
  2. Run the bgp as-number to enter the BGP view.
  3. Run the preference preference command to set the preference for the BGP protocol to a value greater than 60.

    After performing the preceding operations, perform the same operations on Router D to change the preference of the BGP protocol. In this manner, enterprise network service transmission recovers.

Summary

Theoretically, enterprise network routes, which are advertised through blackhole routes imported with the network command, do not flap. In this case, however, devices on the enterprise network do not advertise routes as required because routing protocols are configured with improper preferences.

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Updated: 2019-05-10

Document ID: EDOC1000079719

Views: 444022

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