ZyXEL Communications ZYWALL USG ZLD 2.21 Support Notes page 17

b. Go to CONFIGURATION > Network > Interface > PPP, add a new PPP
interface which is based on WAN 2(ge3) interface:
Step 2. Go to CONFIGURATION > Network > Interface > Trunk. Add WAN
Trunks.
a. Add WAN trunk for VoIP traffic — Set WAN1 as Active mode, while
setting WAN2_ppp as Passive mode.
Choose WAN interface WAN1, and enable Use another interface when the
specified WAN interface is not available.
Please enable SIP ALG to let this policy route apply to all VoIP traffic including
both SIP signaling and RTP (voice data).
Go to Advanced > ALG, enable SIP ALG.
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– ZyWALL USG Support Notes
17