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

Dermot Williams dermot.williams at irishbroadband.ie
Fri Aug 10 07:48:22 EDT 2007


I'm new to the list so I apologise in advance if any of the questions
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 Access
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

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 online at
a given time. Obviously the answer is to pre-clone as many VAIs as
possible but again, I'm concerned about the overhead of doing this.

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 a
Virtual-Template interface that already has the correct config on it?

The router is a 7301 with 1GB memory. My RADIUS attributes for the VRF
users are:

Cisco-AVPair := lcp:interface-config=ip vrf forwarding customer_alpha
Cisco-AVPair += lcp:interface-config=peer default ip address pool
ALPHA_POOL Cisco-AVPair += lcp:interface-config=ip unnumbered Loopback

My VPDN config looks like this:

vpdn-group TEST
! Default L2TP VPDN group
  protocol l2tp
  virtual-template 1
 source-ip a.b.c.d
 lcp renegotiation always
 no l2tp tunnel authentication
interface Loopback10
 description "The Internet"
 ip address
interface Loopback100
 description "Customer Alpha"
 ip vrf forwarding customer_alpha
 ip address secondary  ip address !
interface Virtual-Template1
 ip unnumbered Loopback10
 ip mtu 1492
 peer default ip address pool ADDR
 ppp authentication chap

I would appreciate any pointers or information that the list can give


Dermot Williams

Senior Network Engineer
Irish Broadband Internet Services

Mobile: 	+353 86 3887961
DDI: 	+353 1 4818481

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