confused about demux interface

02.16.12   |  
‎02-16-2012 10:06 PM

I don't quite understand the usage of demux interface. For example, if I want to configure an interface to dynamically create the vlan sub-interfaces. The ip address for the user's gateway is fixed. does this needs demux interface?

I saw examples in documents:

this one uses the demux interface, while another example:

it just use the unnumberred loopback address.


Can someone explain me why there is a demux interface, and in what circumstances it will be used.


I attached the two examples



Re: confused about demux interface

02.17.12   |  
‎02-17-2012 03:05 AM



The dynamic VLAN interfaces and demux interface are used in BRAS/BNG/BSR environments.

Meaning that you have clients coming in using either DHCP or PPP sessions on VLANs, either single or stacked.

With the dynamic VLAN profiles you can automatically generate subinterfaces per VLAN based on variables.


When you have a deployment where every customer is using it's own stacked VLAN (C-VLAN model) you are done there. In case you have a single VLAN or Service VLAN model (S-VLAN) you have multiple clients sharing a single VLAN. THen you want to force another policy per customer. The DEMUX interface is then used to identify clients based on source IP address (configured using demux-source on vlan subinterface). Based on every clients source address a demux subinterface is generated using variables you defined.


So to summarize, the demux interface is used to identify clients based on IP address when they share a single VLAN.

Is this clear for you? 


Triple CCIE #21946 (R&S / Service Provider / Storage), JNCIE-SP #851, Technical Consultant at Telindus-ISIT