Ciscos, BGP, L2TPV3 pseudowires and loopback IPs

Dave Temkin davet1 at
Wed Nov 10 15:34:31 CST 2010

David Freedman wrote:
> e.
>> 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...
That's not correct.

 From a VZ IP circuit that I have:

interface Loopback0
 ip address x.x.x.x  (actual assigned mask is

interface Serial0/0/0
 bandwidth 1536
 ip unnumbered Loopback0

ip route Serial0/0/0

Works great for me across ~50 sites.


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