Tier-0 gateway not advertising routes received from one BGP peer to the other.

Products

• VMware NSX

Issue/Introduction

• VMware NSX Tier-0 Logical Router (LR) receives routes from two different BGP neighbors.
  
  
• When running get bgp 10.XX.XX.20/XX , it is seen in the command output,that the prefix is ​​advertised to both peers from which it was received from.
  
  

  edge-01(tier0_sr[2])> get bgp 10.XX.XX.0/XX
BGP routing table entry for 10.XX.XX.0/XX
Prefix advertised to: XX.XX.255.9 XX.XX.255.13 XX.XX.3.2 XX.XX.3.10
2 Paths available:
Aspath: 4XXXXXXXX9 4XXXXXXXX9 4XXXXXXXX9 4XXXXXXXX9 4XXXXXXXX9 4XXXXXXXX1, path-type as-sequence
Origin incomplete, Metric 0, LocalPref 100, Weight 0, best, valid
Peer is 1X.X.3.10 with router id 1XX.X4X.X.254

  Aspath: 4XXXXXX999 4xxxxxxxx9  4XXXXXX9 4XXXXXX9 42XXXXXX9 4XXXXXX1, path-type as-sequence
Origin incomplete, Metric 0, LocalPref 100, Weight 0, best, valid
Peer is 1X.X.3.2 with router id 1XX.X4X.X.254

  
  
  
• When checking the advertised routes for the Tier-0 , it is observed that routes learned from one neighbour are actually not advertised back to the other neighbour, even though the configuration appears to allow it.

Environment

• VMware NSX

Cause

This behaviour is due to the BGP Split Horizon rule, which prevents a BGP speaker from advertising a route back to the peer from which it was originally learned , when both of them have the same router ID.

Policy-wise, it should be possible to send these updates to both peers , hence the output of the command get bgp 10.XX.XX.20/XX , shows route being advertised to both neighbours.

At the time when the update needs to be sent, there is a split-horizon mechanism that detects that traffic is being sent  to the router ID of the origin of the prefix. Hence the route is not advertised.

Resolution

• This is expected BGP behaviour observed when multiple external routers share the same BGP Router ID.
  
  
• To ensure routes are advertised between the two external peers via the Tier-0 router, each external BGP router must be configured with a unique BGP Router ID.
  
  
• Once the external routers are configured with distinct Router IDs, the NSX Tier-0 router will correctly identify the origin of each route and advertise the best path to the other neighbour, as the Split Horizon rule will no longer incorrectly trigger.