Zyxel ZyWALL USG 50 [8/185] Understand packet flow

ZyWALL USG Support Notes

1.1. Understand Packet Flow

Before start setting up Routing and NAT on your USG ZyWALL, understanding its

packet flow, Routing and NAT priority may help a lot for your correct setting up.

1.1.1. General Packet Flow

Below is the general packet flow in USG ZyWALL. It reflects how the USG

ZyWALL processes traffic from the time it enters ZyWALL from one interface, till it

leaves ZyWALL from another interface.

For example, USG ZyWALL receives VoIP packet from LAN interface.

1. The frames sent through network may be fragmented to meet the MTU settings on

each router through the path the traffic goes on network. When the USG receives

traffic, it will first defragment the frames.

2. The ALG (Application Layer Gateway) will check and alter the application layer

information, e.g. Contact information in the SIP message header and SDP

information in message body.