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Filter-Based Forwarding Overview

Firewall filters can be used to block specific packets. They can also be used to affect how specific packets are forwarded.

Filters That Classify Packets or Direct Them to Routing Instances

For IPv4 or IPv6 traffic only, you can use stateless firewall filters in conjunction with forwarding classes and routing instances to control how packets travel in a network. This is called filter-based forwarding (FBF).

You can define a filtering term that matches incoming packets based on source address and then classifies matching packets to a specified forwarding class. This type of filtering can be configured to grant certain types of traffic preferential treatment or to improve load balancing. To configure a stateless firewall filter to classify packets to a forwarding class, configure a term with the nonterminating action forwarding-class class-name.

You can also define a filtering term that directs matching packets to a specified routing instance. This type of filtering can be configured to route specific types of traffic through a firewall or other security device before the traffic continues on its path. To configure a stateless firewall filter to direct traffic to a routing instance, configure a term with the terminating action routing-instance routing-instance-name <topology topology-name> to specify the routing instance to which matching packets will be forwarded.


Unicast Reverse Path Forwarding (uRPF) check is compatible with FBF actions. uRPF check is processed for source address checking before any FBF actions are enabled for static and dynamic interfaces. This applies to both IPv4 and IPv6 families.

To forward traffic to the master routing instance, reference routing-instance default in the firewall configuration, as shown here:


Do not reference routing-instance master. This does not work.

Input Filtering to Classify and Forward Packets Within the Router or Switch

You can configure filters to classify packets based on source address and specify the forwarding path the packets take within the router or switch by configuring a filter on the ingress interface.

For example, you can use this filter for applications to differentiate traffic from two clients that have a common access layer (for example, a Layer 2 switch) but are connected to different Internet service providers (ISPs). When the filter is applied, the router or switch can differentiate the two traffic streams and direct each to the appropriate network. Depending on the media type the client is using, the filter can use the source IP address to forward the traffic to the corresponding network through a tunnel. You can also configure filters to classify packets based on IP protocol type or IP precedence bits.

Output Filtering to Forward Packets to Another Routing Table

You can also forward packets based on output filters by configuring a filter on the egress interfaces. In the case of port mirroring, it is useful for port-mirrored packets to be distributed to multiple monitoring PICs and collection PICs based on patterns in packet headers. FBF on the port-mirroring egress interface must be configured.

Packets forwarded to the output filter have been through at least one route lookup when an FBF filter is configured on the egress interface. After the packet is classified at the egress interface by the FBF filter, it is redirected to another routing table for further route lookup.

Restrictions for Applying Filter-Based Forwarding

An interface configured with filter-based forwarding does not support source-class usage (SCU) filter matching or source-class and destination-class usage (SCU/DCU) accounting.