Web OS 10.0 Application Guide172 n Chapter 7: Filtering 212777-A, February 2002Stacking FiltersStacking filters are assigned and enabled on a per-port basis. Each filter can be used by itself orin combination with any other filter on any given switch port. The filters are numbered 1through 2048 on Alteon 184 and Alteon AD4 Web switches, and 1 though 224 on other AlteonWeb switches. When multiple filters are stacked together on a port, the filter’s number deter-mines its order of precedence: the filter with the lowest number is checked first. When traffic isencountered at the switch port, if the filter matches, its configured action takes place and therest of the filters are ignored. If the filter criteria doesn’t match, the next filter is tried.As long as the filters do not overlap, you can improve filter performance by making sure thatthe most heavily utilized filters are applied first. For example, consider a filter system wherethe Internet is divided according to destination IP address:Figure 7-1 Assigning Filters According to Range of CoverageAssuming that traffic is distributed evenly across the Internet, the largest area would be themost utilized and is assigned to Filter 1. The smallest area is assigned to Filter 4.Overlapping FiltersFilters are permitted to overlap, although special care should be taken to ensure the properorder of precedence. When overlapping filters are present, the more specific filters (those thattarget fewer addresses or ports) should be applied before the generalized filters.Example:Figure 7-2 Assigning Filters to Overlapping RangesIn this example, Filter 2 must be processed prior to Filter 3. If Filter 3 was permitted to takeprecedence, Filter 2 could never be triggered.Allow Deny RedirectFiltering by Destination IP Address RangesDeny0.0.0.0 255.255.255.255Filter 1Filter 3Filter 4Filter 2AllowRedirectFiltering by Destination IP Address RangesDeny0.0.0.0 255.255.255.255Filter 1Filter 3 Filter 2
