> Source: [sk181768](https://support.checkpoint.com/results/sk/sk181768)

# sk181768 - Anti-Spoofing drops packets that arrive at a Security Gateway through interfaces with Topology "External"

| Property | Value |
|----------|-------|
| Solution ID | sk181768 |
| Date Created | 2023-12-11 |
| Last Modified | 2025-11-27 |
| Technical Level | Advanced |
| Products | Security Gateway, Scalable Platforms |
| Versions | R82, R81.20, R81.10 (EOS), R81 (EOS), R82, R81.20, R81.10 (EOS), R81 (EOS) |
| OS | Gaia |

## Symptoms

- Anti-Spoofing drops packets that arrive at a Security Gateway through interfaces with Topology "External".

## Cause

To calculate the topology of external interfaces, the Anti-Spoofing protection takes all IP addresses configured on the Security Gateway interfaces and excludes the IP addresses assigned to interfaces with the topology other than "External".

If there are routes configured for internal interfaces that overlap with routes configured for external interfaces, then the Anti-Spoofing protection treats the external interfaces as internal.

Example for IPv4:

1. eth0 - an external interface that has a route for a "larger" network 10.0.0.0/8
2. eth2 - an internal interface that has a route for a "smaller" network 10.0.0.0/16

## Solution

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