Ciscos, BGP, L2TPV3 pseudowires and loopback IPs

James Smallacombe up at
Wed Nov 10 10:20:37 CST 2010

A simpler question(s) than it sounds:

Customer just brought up their first BGP session at a new location.  It is 
up fine with a full routing table, the second provider hookup is a few 
weeks away.

The provider allocated a /24 (x.x.1.0/24) for the network and a /30 for 
the PTP connection (x.x.129.172/30).  For the initial setup, I did not 
configure a loopback, I just put x.x.129.174 on the WAN interface and set 
up the neighbor as x.x.129.173.  It's working fine.

We will need to set up a L2TPV3 tunnel to their old location (single 
homed, no BGP on that side).  Upon initial reading of Cisco docs to do 
this, we will need a routable IP on a loopback interface for starters. 
Using one from the /24 LAN is out unless we subnet it, which we don't want 
to do.

So the question is, can I just "move" the PTP IP address x.x.129.174 from 
the WAN interface to the loopback like this?

  interface Loopback0
   ip address x.x.129.174  (that's the mask we're using on
             the WAN- Cisco's loopback examples show .255)

  interface WAN1 (actually a gigether)
   ip unnumbered loopback0  (or no ip addr?)

  neighbor x.x.128.173 update-source Loopback0

Does this look even close to right?  Or do we need another, single routabe 
IP from the provider for the loopback?  Also, I am assuming we don't need 
separate loopback interfaces for BGP as for the Bridge/Tunnel.  What about 
when the second provider comes up?  A second or third loopback to nail up 
their WAN IP?

***OR*** is there a way to put their WAN I/F IP on the loopback and take 
it off their LAN Ether...and then do IP unnum loop0 on the LAN?


James Smallacombe		      PlantageNet, Inc. CEO and Janitor
up at	

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