> Source: [sk162437](https://support.checkpoint.com/results/sk/sk162437)

# sk162437 - Security Gateway drops packets with the log: "simi_ip_forwarding_checks"

| Property | Value |
|----------|-------|
| Solution ID | sk162437 |
| Date Created | 2019-10-23 |
| Last Modified | 2023-09-29 |
| Technical Level | Advanced |
| Products | Security Gateway |
| Versions | R81.20, R81.10 (EOS), R81 (EOS) |
| OS | Gaia |

## Symptoms

- Security Gateway drops packets with the following log, while running "fw ctl zdebug drop" command:

```

[SIM-206443808];do_cut_through: failed, dropping packet, simi_ip_forwarding_checks() failed, conn: ;
[SIM-206443808];sim_pkt_send_drop_notification: (0,0) received drop, reason: general reason, conn: ;
[SIM-206443808];sim_pkt_send_drop_notification: no track is needed for this drop - not sending a notificaion, conn: ;
[SIM-206443808];do_packet_finish: SIMPKT_IN_DROP vsid=0, conn:;
```

## Cause

The drops are caused by an invalid checksum or an invalid TTL.

Topology example for which the scenario can happen:

**Client** --\> (eth2) **Gateway** (eth3) --\> **Router** --\> **Server**

However, the server network does not actually exist, so the router sends the packets back to the Gateway.

Eventually, a client sends packets towards eth2 of the FW, the FW sends it out over eth3 to the router, the router sends back the **same** packets to the FW on eth3.

As the same packet, with the same TTL, returns to the Gateway, it detects this as invalid, and drops it.

This could be checked by collecting pcap files with tcpdumps.

## Solution

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