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Understanding Dynamic Firewall Filters

Firewall filters provide rules that define whether to accept or reject packets that are transiting an interface on a router. The subscriber management feature supports four categories of firewall filters:

  • Classic filters are static filters that are applied to an interface dynamically. They are compiled at commit time and then, when a service is activated, an interface-specific filter is created and attached to a logical interface. This dynamic application is performed by associating input or output filters with a dynamic profile. When triggered, a dynamic profile applies the filter to an interface. Because classic filters are static, they cannot contain subscriber-specific terms (also called rules).

  • Parameterized filters allow you to implement customized filters for each subscriber session. In parameterized filters, you use variables to define a filter. When services are activated for a subscriber, actual values such as policing rates, destination addresses, or ports are substituted for the variables and are used to create filters.

  • Ascend-Data-Filters allow you to create dynamic filters based on values received from the RADIUS server in the Ascend-Data-Filter attribute (RADIUS attribute 242). The filter is configured on the RADIUS server and contains rules that specifically match conditions for traffic and define an action for the router to perform. When services are activated for a subscriber, a filter is created based on the values in the RADIUS attribute. You can also use Ascend-Data-Filters to create static filters by configuring the Ascend-Data-Filter attribute in a dynamic profile.

  • Fast update filters are similar to classic filters. However, fast update filters support subscriber-specific, rather than interface-specific, filter values. Fast update filters also allow individual filter terms to be incrementally added or removed from filters without requiring that the entire filter be recompiled for each modification. Fast update filters are essential for networking environments in which multiple subscribers share the same logical interface.

You configure firewall filters to determine whether to accept or reject traffic before it enters or exits an interface to which the firewall filter is applied. An input (or ingress) firewall filter is applied to packets that are entering a network. An output (or egress) firewall filter is applied to packets that are exiting a network. You can configure firewall filters to subject packets to filtering or class-of-service (CoS) marking (grouping similar types of traffic together and treating each type of traffic as a class with its own level of service priority).