[cisco-bba] VRFs, VPDN and Virtual Access Interfaces

Dermot Williams dermot.williams at irishbroadband.ie
Mon Aug 13 06:46:51 EDT 2007

> -----Original Message-----
> From: Oliver Boehmer (oboehmer) [mailto:oboehmer at cisco.com]
> Sent: 11 August 2007 09:36
> To: Dermot Williams; cisco-bba at puck.nether.net
> Subject: RE: [cisco-bba] VRFs, VPDN and Virtual Access Interfaces
> Dermot Williams <> wrote on Friday, August 10, 2007 1:48 PM:
> > I'm new to the list so I apologise in advance if any of the
> > that I am asking have been asked and answered ad nausea. In my
> > defence, I did search the archives first and have already found
> answers to
> > other questions that I had.
> >
> > I'm doing some testing with L2TP-based VPDNs and VRFs for a future
> > L3VPN product. My goal is to switch certain users into a VRF
> configured on
> > the LNS and to continue to allow other users access to the internet.
> I
> > have this working but I do have some questions about my
> implementation.
> >
> > My main issue is that all of the VRF users are given a Virtual
> > interface and not a Virtual Access sub-interface. I know why this is
> > happening - I am using RADIUS to send "lcp:interface-config= ... "
> > attributes back for the VRF users. However, I also know that Virtual
> > Access interfaces have a larger memory/CPU overhead than VA
> > sub-interfaces.
> this is correct, the impact, however, depends on the number of
> and also the session setup rate. So full VAI are no "bad" per se, it
> depends on the environment. Can you describe your environment a bit
> more?
[Dermot Williams] 

I have this working in a lab environment at the moment so I have
complete control over both the LNS and the LAC. In this lab scenario I
am working with just one 'subscriber', an 800 series router. Obviously,
in this case, I don't really have to worry too much about resource
consumption by VRF forwarded subscribers.

However, in our production it won't be so clear-cut. We resell DSL
connections using the local incumbent's wholesale DSL product so we
don't have any control over the LAC. On the production 7301 we have
approximately one thousand subscribers currently - these subscribers are
all internet-bound so we aren't doing anything fancy. Our intention is
to start offering L3VPN services over DSL so eventually there will be a
mix of subscribers who are destined for the internet and subscribers who
are dumped into a VRF.

> > To mitigate against this I can pre-clone the VAIs but my
> understanding
> > is that if there are pre-cloned VAIs, none of my users will be
> > allocated VA sub-interfaces and I would like to avoid this if
> possible. I don't
> > want to use VAIs unless absolutely necessary and my assumption is
> that
> > if I were to pre-clone 50 VAIs, only 50 users would be allowed
> > at a given time.
> No, this is not correct, if the system needs more VAIs, it allocates
> them as needed.
> > Instead of using RADIUS attributes to create a user-specific VAI, is
> > there anyway that I could use them to force the VPDN session to use
> > Virtual-Template interface that already has the correct config on
> Yes, this is one of the two options. But you would need the LAC to
> forward these sessions in a different tunnel (and you can then use
> "terminate-from hostname <foo>" to map those to a vpdn-group  (i.e.
> vpdn-group per vrf). This could be a problem if the LAC is not owned
> you, or if the LAC doesn't know which vrf the user belongs to.
[Dermot Williams] 

Unfortunately, as I say, we don't have control over the LAC.

> In latest 12.2SB, you have two additional options:
> 1) Use Cisco-avpair = "ip:vrf-id=<vrf-name>" and
> "ip:ip-unnumbered=Loopback<n>", which are subinterface-capable
> 2) Use Cisco-avpair = "lcp:allow-subinterface=yes" and use the
> "lcp:interface-config" commands you've mentioned. This is a workaround
> for those lcp:interface-config config statements which are actually
> subinterface-capable, which I think is the case for all the ones you
> sent (vrf, unnumbered, pool). Other, non sub-VAI compatible config
> commands will be ignored, as far as I know.
> Please try this in the lab first, haven't really worked with this for
> while now.
[Dermot Williams] 

This is all benched in the lab at the moment so I can chop and change as
I want for the moment. We're actually running 12.4(11)T so I'll have to
download a 12.2SB image to test these AVPairs. Does it matter what order
I send the 'lcp:allow-subinterface=yes' VSa in? I.e., should it be sent
before the other VSAs? Incidentally, is there anywhere that I can find a
list of AVPairs for each IOS version?

Thanks for your help,


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