In the ISA Server management tool, expand the management nodes that are beneath your server name.
Expand the Access Policy branch to view the list of access policies.
Right-click IP Packet Filters, point to New, and then click Filter to start the New IP Packet Filter Wizard.
Type a descriptive name for the filter (such as SNTP Allow Filter), and then click Next.
Click Allow packet transmission as the Filter Mode, and then click Next.
Click Custom as the filter type, and then click Next.
On the Filter Settings page, click UDP for the IP protocol, click Send receive for the direction, All ports as the local port, click Fixed port as the remote port, use port 123 as the remote port number, and then click Next.
Keep the default settings by clicking Next through the next two wizard pages, and then click Finish at the Completing The New IP Packet Filter Wizard page.