Ciscos, BGP, L2TPV3 pseudowires and loopback IPs

David Freedman david.freedman at
Wed Nov 10 12:22:40 CST 2010

> 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.

I'm pretty sure this is just a recommendation based on good practise
(routeability to endpoints), I'm sure since you are not multihomed you
can just use "ip local interface WAN1" and be done with it, I seem to
remember doing something similar in an l2tpv3 pw class and it working.

> 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

No, if you were to do this you should get a new transfer network, you
can't have the same address on two interfaces (and in fact, you should
really be stealing an address from your internal /24 which doesn't
require any re-subnetting (if you are happy for this address to be
unreachable) and it should have a /32 mask...


David Freedman
Group Network Engineering
Claranet Group

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