HTTP monitoring from a shared server

You can do the same type of HOST header redirection on the BIG-IP that you're doing on the original (multi-site) server using a single external listener IP. This can be achieved with an HTTP class or an iRule, but easier to show what I mean with an iRule example:

1 VIP (1 IP and port)

3 DNS entries pointing to the same VIP IP

3 pools (one for each site) - application-specific monitor applied to each pool

1 iRule:

when HTTP_REQUEST {
     switch [string tolower [HTTP::host]] {
          "site1.domain.com" { pool site1_pool }
          "site2.domain.com" { pool site2_pool }
          "site3.domain.com" { pool site3_pool }
          default { drop }
     }
}