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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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Traffic Traverses the Egress Device of an AS Because BGP Delivers Default Routes with Different MEDs

Traffic Traverses the Egress Device of an AS Because BGP Delivers Default Routes with Different MEDs

Fault Symptom

In Figure 19-12, EBGP peer relationships are established between devices in AS 100 and AS 200. IBGP peer relationships are established between devices inside each AS. After Router A and Router B advertise BGP default routes, detailed information about BGP default routes on Router C shows that outgoing traffic from AS 200 is directed to Router D. That is, the next hop of BGP default routes is Router D. Consequently, outgoing traffic from AS 200 traverses Router C.

Figure 19-12  Outgoing traffic traversing the egress device of an AS

Fault Analysis

Run the display bgp routing-table 0.0.0.0 command on Router C to view detailed information about BGP default routes. The command output will show that different MEDs are set for Router A and Router B. As a result, outgoing traffic from AS 200 traverses Router C.

Procedure

  1. Run the system-view command on Router B or Router A to enter the system view.
  2. Run the bgp { as-number-plain | as-number-dot } command to enter the BGP view.
  3. Run the ipv4-family unicast command to enter the BGP-IPv4 unicast address family view.
  4. Run the default med med command to modify the default MED of the BGP routes, and make sure the MED of BGP routes on Router A matches the MED of BGP routes on Router B.

    After performing the preceding operations, run the display bgp routing-table 0.0.0.0 command on Router C to view detailed information about BGP default routes. The command output will show that outgoing traffic from AS 200 is transmitted through Router C. This indicates that the fault has been cleared.

Summary

When there are multiple egress devices between two ASs, set the same MED for the advertised default routes. BGP prefers routes learned from EBGP peers because the delivered default routes have the same route attributes, such as the local-preference and MED.

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Updated: 2019-05-10

Document ID: EDOC1000079719

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