NSX-T Tier-0 expected Prefix Advertisement Behavior when connected to peers with same AS.

Products

• VMware NSX

Issue/Introduction

When multiple routers acting as BGP neighbors with same AS number , advertise the same IP prefixes to a Tier-0 Gateway, an administrator might observe the following:

• The Tier-0 Gateway does not advertise a specific prefix back to one of the advertising entities, even if that entity originated the prefix.
  
  
• Another advertising entity might receive the prefix from the Tier-0, but subsequently ignore it.
  
  
• This behaviour can lead to confusion regarding route propagation and reachability in a multi-path BGP setup involving the Tier-0 Gateway.

Environment

• VMware NSX

Cause

• This behaviour is an expected function of Border Gateway Protocol (BGP) to prevent routing loops and ensure optimal path selection.
  
  
• When the Tier-0 Gateway receives the same prefix from multiple BGP neighbours, it applies BGP's path selection algorithm (tie-breaker rules) to determine the "best" path for that prefix.

Resolution

• This is an expected behavior of BGP and does not typically require a resolution.