You can set the fallback host in the http profile, which will redirect traffic to the destination you configure. If you want more granular control, you can set this in an iRule:
when LB_FAILED {
HTTP::fallback "http://siteunavailable.mysite.com/"
}
If you don't have another server to redirect to, you can always serve the page from the LTM itself. Here are two examples from the codeshare
Text-only:
http://devcentral.f5.com/wiki/default.aspx/iRules/LTMMaintenancePageLite.html Click here
Text with Images:
http://devcentral.f5.com/wiki/default.aspx/iRules/LTMMaintenancePage.html Click here
Nimbostratus
