To enable fax passthrough in a H323 IOS gateway: (This can be given under a dial peer or globally under voice service voip mode)
Global mode:
1. modem passthrough nse [payload-type number] codec {g711ulaw|g711alaw} [redundancy] [maximum-sessions <value>]
* Payload-type default : 100 for IOS gateways (Set this to same for all other gateways)
* Codec should be g711ulaw for fax sent over a t1 trunk or g711alaw for fax sent over a e1 trunk
* redundancy – enables fax redundancy. Set this parameter to be same at both ends
* max sessions – defines how many fax calls can occur simultaneously. Default is 16. Valid values 1 – 26.
2. Disable Fax relay – Fax relay is enabled by default and takes precedence over Passthrough.
fax rate disable
3. If you gave the modem passthrough setting under voice service voip mode, then you need to give the following command in dial-peer mode.
dial-peer voice 1 voip
modem passthrough system
Dial-peer mode: (2 steps)
If you choose not to apply the modem passthrough settings in global mode, dial-peer voice 1 voip
fax rate disable
modem passthrough nse codec g711ulaw
To enable Fax passthrough in a Cisco IOS MGCP gateway
no ccm-manager fax protocol cisco
mgcp modem passthrough voip mode nse
mgcp modem passthrough voip codec g711ulaw|g711alaw.
CCIE Voice notes 