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We're using a 5505 where the primary connection is a DHCP IP (outside) from the provider and we have a backup ISP with a static IP (outsideBackup).

Can we create an SLA Monitor on the outside interface, even though there is no set gateway (say to check Google DNS or another very up ip address)?

Without a specific route command for the primary I am concerned this plan of action will not work for us.

What we have found is that the backup/failover we set in the ASDM seems to only take effect if the interface goes down, not the actual connection.

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up vote 2 down vote accepted

What I will generally do is ping a router a hop or two up from your default gateway. This way if they put you in a different subnet you're still likely to be able to reach this router fairly quickly. Continually pinging servers is generally frowned upon because that is not the server's intended use. If you're receiving your default route in the routing table from DHCP you're going to have to track it with the dhcp client route track <number> command instead of adding it to a static route. You can read more about this here

Cisco ASA 5500 Series Command Reference 8.2 - dhcp client route track

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Thank you. The command worked like a charm and was exactly what we needed. I've added the SLA and tests work just ask expected. – eric.s Feb 16 '12 at 1:56

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