Impact of Incorrect Host-Specific Static Route on Recursive Lookup and Traffic Flow

Products

• VMware VeloCloud SD-WAN

Issue/Introduction

 

The customer has configured a static route for a subnet that is one hop away, using the next-hop IP address from a directly connected subnet.

The expectation is that the edge should not perform a recursive route lookup for the next-hop IP, as it is part of the directly connected subnet.

However, in the customer’s network, a recursive route lookup is occurring because the customer has mistakenly configured an additional static route for the next-hop IP address (a host-specific static route), which is unnecessary and incorrect.

Example:

• Destination subnet (one hop away): 10.1.1.0/24
• Directly connected subnet on edge interface Ge1-LAN: 10.1.2.0/24
• Edge Ge1 IP address: 10.1.2.1/24
• Next-hop IP address on Ge1: 10.1.2.2/24

Static Route Configuration:

• Static Route 1:
  Destination subnet 10.1.1.0/24 via next-hop 10.1.2.2, exit interface Ge1 (This is the correct static route)

• Static Route 2:
  Destination 10.1.2.2/32 via next-hop 10.1.2.2, exit interface Ge1 (This is the incorrect static route)

In this case, traffic destined for the 10.1.1.0/24 subnet will be affected. The edge will attempt a recursive route lookup for the next-hop IP 10.1.2.2, which leads to a loop due to the unnecessary static route #2. Eventually, this will result in a routing failure.

Environment

All SDWAN edge software versions 

Cause

Static route #2 (destination 10.1.2.2/32 via next-hop 10.1.2.2) is not required. The edge router is incorrectly performing a recursive lookup because of this redundant route, which disrupts traffic flow to the 10.1.1.0/24 subnet.

Resolution

Remove static route #2 to prevent the recursive lookup and ensure proper routing of traffic to the destination subnet.