NAT to two ISPs - can it work?

[vcjones]

Trying to get a fully redundant ISP connection using low cost DSL. No problem detecting link down with SAA (ping based routing), but a real problem getting NAT to bahave.

When the active link goes down, the default route switches automagically, but it is only useable for new connections until a "clear ip nat trans *" is executed. I know how to reduce the timeout on translations, but that is not a generic solution because I cannot guarantee a time gap between attempts.

The problem appears to be that once a translation is assigned, the ip nat source route statements are ignored. That is, the "ip nat source route" statements are only checked if there is not already a translation assigned to the address. As a result, the classic trick of assigning the NAT based on the outbound interface only works for the initial assignment and unless the translations are manually cleared, will not switch the NAT to match the remaining interface.

Short of running a daemon on a local PC, is there any way to automatically force the NAT translations to be reassigned before they have timed out?

Vincent C Jones

www.networkingunlimited.com

[kmarrero]

The only way to get this to work is to use NAT with route-maps. Here is a URL that explains the configuration. http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080093fca.shtml

[vcjones]

Necessary, but not sufficient. The problem is not the route maps on the NAT statements, but the fact that once the route map on a NAT statement has been used to define a translation, that translation will remain in effect until either the translation times out, the translation is cleared, or the interface physically fails (and that only works on serial links, yet another bug).

Any other ideas?

Vincent C Jones