NimbostratusAlias Address and Alias Service Port cause a monitor to test something other than the resource the monitor is assigned to but mark up or down the resource it is assigned to. For example, if you place a monitor with Alias Service Port set to 8443 on a pool member whose IP address:port is 10.200.19.73:443, the monitor will test the resource at 10.200.19.73:8443 but mark the pool member at 10.200.18.73:443 up or down based on the test results.
In response to your question then, no, if you want the monitor on the pool member at 10.200.18.73:80 to test that specific resource and mark it up or down based on the test results, it is not necessary to specify 80 in the Alias Service Port entry.
Alias Address and Alias Service Port are frequently used in monitors that are applied to resources with dependencies on other resources. E-Commerce applications provide a good example. Your application might have a pool of servers that handle non-encrypted (port 80) traffic and another pool of servers that handle encrypted traffic (port 443). Both services are provided by the same physical server. If a monitor test for a port 443 pool member fails, you may want to also mark down the port 80 pool member. In that case, you can apply a monitor to the port 80 pool member that tests the port 443 pool member but marks the port 80 pool member up or down based on the results.
Another example use of Alias Address is when you are load balancing traffic through ISP routers. If you apply a monitor to the router pool member, all you are testing is the path to the router, not the full path out to the Internet. So, instead of having the monitor test the actual pool member, you can actually have the monitor test a host that is further upstream from the router to make sure traffic can get to and, more importantly, through the pool member to something beyond. This requires the use of the Alias Address setting (and possibly the Alias Service Port setting) on the monitor assigned to the pool member.
