[cisco-bba] l2tp failover and loadbalancing

Oliver Boehmer (oboehmer) oboehmer at cisco.com
Fri Jan 27 10:42:07 EST 2006

Raj Panchal <mailto:raj.panchal at vsnl.co.in> wrote on Friday, January 27,
2006 11:19 AM:

> Hi Oliver
> My first requirement is to redirect the user tunnel to secondary LNS
> if primary is not available.
> Second requirement is if both of the LNS are available then do load
> balancing in term of terminating users across LNS

well, then 

       Tunnel-Medium-Type = :1:IP,
       Tunnel-Server-Endpoint = :1:"",
       Tunnel-Preference = :1:"10",
       Tunnel-Type = :2:L2TP,
       Tunnel-Medium-Type = :2:IP,
       Tunnel-Server-Endpoint = :2:"",
       Tunnel-Preference = :2:"10",

would work for you.

> What I meant in my previous question was, what would be the condition
> resulting in Failover or Load balancing i.e
> If LAC find that Primary LNS is not available then what are the
> conditions when LNS is flagged as not available by LAC

Don't have time to go into details right now, but there are several ways
a LAC can detect the failure. Not getting a reply for its SCCRQ/ICRQ is
one of them (i.e. no connectivity or LNS died), but there are also other
failure codes like resource shortage causing a LAC to switch over to
alternate destinations..

> If LAC wants to do load balance the between the user tunnel setups ,
> what are the condition when both the LNS are alive but still user
> will be load balanced between different LNS by LAC ( how it finds that
> LNS is loaded and second is free to take up the load )

The LNS has ways (see above) to signal resource shortage, but I don't
know how the 3com balances the load under "normal" circumstances, i.e.
if it keeps track of the number of sessions within the tunnel or just
blindly does round-robin, this needs to be checked with 3com..


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