Irule with elseif and pool selection

You're using an if/elseif structure inside a switch conditional. Try this:

when HTTP_REQUEST { 
    switch -glob [string tolower [HTTP::host]] {
        "abc*" { 
            pool ss-devsp3-pool
        }       
        "def*" { 
            pool ss-devsp3-pool-81
        }
        "ghi*" { 
            pool ss-devsp3-pool-82
        }  
    }   
}

The problem you have is you are starting with a switch and confusing the runtime with if statements.

If you want to use If's, then it would look more like this:

when HTTP_REQUEST { 
    set hostname [string tolower [HTTP::host]]
    if { $hostname starts_with "abc" } { 
        pool ss-devsp3-pool
    } elseif { $hostname starts_with "def" } { 
        pool ss-devsp3-pool-81
    } elseif { $hostname starts_with "ghi" } { 
        pool ss-devsp3-pool-82
    }
}

Using the switch statement, it would look more like this:

when HTTP_REQUEST { 
    switch -glob [string tolower [HTTP::host]] {
        abc*    {  pool ss-devsp3-pool }
        def*    {  pool ss-devsp3-pool-81 }
        ghi*    {  pool ss-devsp3-pool-82 }
    }
}

Thanks for the help. Let me test these out, and I'll let you know which one worked for me.

Thanks again everyone for your help. I ended up using the if statements that Robert gave.

Also similar question here what happens if the pool "ss-devsp3-pool" is down, will it reach default pool if configured . And shouldn't switch have a default pool ?? am I wrong in understanding this.