subinterface on ISDN port

Unanswered Question

Dear Friends,

I have a customer who is having an mpls-vpn connectivity in Hub-spoke topology .His HO is having seperate internet connectivity as well.Now one of his spoke site wanted to access Internet through Hub site(HO)using same last mile he is used for mpls-vpn.His last mile for mpls vpn is 128K isdn-bri.

Now I would like to know can we create virtual interface(sub interface) on ISDN port ?so that I can create 2 sub interfaces -one ofr VPN and one for internet.

what is the L-2 encapsulation used in case of ISDN

what would be the configuration.

Kindl assist with possible options if any


I have this problem too.
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Paolo Bevilacqua Wed, 08/25/2010 - 10:10

You can have sub-interface only if the encapsulation is frame-relay.

Since that is not your case, and there is no reason to use frame-relay, you shall not use sub-interface.

Peter Paluch Wed, 08/25/2010 - 10:14


Is the client using different telephone numbers to reach the HO and to reach the ISP? If yes then this task can be accomplished by creating two Dialer interfaces, both using the same BRI interface but each of them dialing a different number, utilizing one B channel from the BRI interface.

Generally, the BRI interfaces use either HDLC or PPP encapsulation to carry packets. Unfortunately, as far as I know, neither of these supports creating subinterfaces - both are implemented only for point-to-point communication and no further discrimination into subinterfaces is known to me.

Best regards,


Paolo Bevilacqua Wed, 08/25/2010 - 10:17


This is not a dialup connection. Likely the OP is in Germany, where leased-line ISDN was popular in the '90.

Paolo Bevilacqua Wed, 08/25/2010 - 10:38

Then I take back my wrong supposition about leased line. That does not change the substance of the matter: no subinterfaces.

Giuseppe Larosa Wed, 08/25/2010 - 13:45

Hello Vicky,

so the customer has a BRI that is made of 2 B 64 kbps channels and one D channel (16 kbit).

From your description RAS places the caller in an MPLS VPN during authentication phase using received AV from Radius server.

This can be called ISDN access to MPLS VPN and on cisco routers can be implemented with VRF aware dialer map on receiver side.

if you are able to do the following:

having two dialers that don't use PPP multilink (to avoid first dialer to use both B channels) you may be able to achieve a split of B channels use.

one dialer to be used for VPN access and one dialer to access the internet.

Second dialer has to use a different username/password that will be associated to global routing table or to an MPLS VPN with internet access.

There was also a way to use D channel to carry some data called always on ISDN.

Hope to help



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