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BGP Load Balancing Between Two ASs Fails on an NE40E

Publication Date:  2019-04-24 Views:  132 Downloads:  0

Issue Description

At a site, an NE40E is configured with BGP load balancing between two ASs, and four equal-cost routes generated by four BGP peers are required. The network topology is as follows:

As shown in the preceding figure, the NE40E has four EBGP peers. NE1 and NE2 belong to AS A. NE3 and NE4 belong to AS B. Load balancing is configured between NE1 and NE2. The attributes of NE3 and NE4 are consistent, but equal-cost routes cannot be generated.

Handling Process

Obtain key configuration of BGP.

 ipv4-family unicast
  compare-different-as-med
  bestroute as-path-ignore
  network X.X.X.X 255.255.255.255 route-policy 1:4 
  maximum load-balancing 8

It is found that a maximum of eight equal-cost routes are configured for BGP load balancing. According to the description in the product document, the maximum load-balancing and bestroute as-path-ignore commands are mutually exclusive.
The bestroute configuration indicates that AS-Path attribute is not checked during BGP route selection. The load-balancing indicates that the AS-Path attribute is not matched during load balancing check.

Root Cause

The bestroute configuration indicates that AS-Path attribute is not checked during BGP route selection. The load-balancing indicates that the AS-Path attribute is not matched during load balancing check. Therefore, the maximum load-balancing and bestroute as-path-ignore commands are mutually exclusive.

Solution

Delete the bestroute as-path-ignore command.

Suggestions

In addition to the preceding precautions, pay attention to the conditions for BGP load balancing.

END