Chapter 8. Routing Policy and Firewall Filters

Policy Terminology

Comparison of Routing Policies and Firewall Filters

Routing Policy Framework

Configuring Routing Policy

Configuring Firewall Filters

Configuring Traffic Sampling and Forwarding

The JUNOS Internet software provides a policy framework, which is a collection of JUNOS policies that allows you to control flows of routing information and packets. The policy framework is composed of routing policy, which allows you to control the routing information or change between the routing protocols and the routing tables and between the routing tables and the forwarding table and firewall filter policy, which allows you to control packets transiting the router to a network destination and packets destined for and sent by the router.

NOTE

The term firewall filter policy is used here to emphasize that a firewall filter is a policy and shares some fundamental similarities with a routing policy. However, when referring to a firewall filter policy in the remainder of this book, the term firewall filter is used.

The JUNOS policies affect the following router flows:

• Flow of routing information between the routing protocols and the routing tables and between the routing tables and the forwarding table. The Routing Engine handles this flow. Routing information is the information about routes learned by the routing protocols from a router's neighbors. This information is stored in routing tables and is subsequently advertised by the routing protocols to the router's neighbors. Routing policies allow you to control the flow of this information.

• Flow of data packets in and out of the router physical interfaces. The Packet Forwarding Engine handles this flow. Data packets are chunks of data that transit the router as they are being forwarded from a source to a destination. When a router receives a data packet on an interface, the router determines where to forward the packet by looking in the forwarding table for the best route to a destination. The router then forwards the data packet toward its destination through the appropriate interface. Firewall filters allow you to control the flow of these data packets.

• Flow of local packets from the router physical interfaces and to the Routing Engine. The Routing Engine handles this flow. Local packets are chunks of data that are destined for or sent by the router. Local packets usually contain routing protocol data, data for IP services such as telnet or secure shell (ssh), and data for administrative protocols such as the Internet Control Message Protocol (ICMP). When the Routing Engine receives a local packet, it forwards the packet to the appropriate daemon or to the kernel, which are both part of the Routing Engine, or to the Packet Forwarding Engine. Firewall filters allow you to control the flow of these local packets.

Figure 8.1 illustrates the flows of routing information and packets through the router. Although the flows are very different from each other, they are also interdependent. Routing policies determine which routes are placed in the forwarding table. The forwarding table in turn has an integral role in determining the appropriate physical interface through which to forward a packet.

Figure 8.1. Flows of Routing Information and Packets

You can configure routing policies to control which routes the routing protocols place in the routing tables and to control which routes the routing protocols advertise from the routing tables (see Figure 8.2). The routing protocols advertise active routes only from the routing tables. (An active route is a route that is chosen from all routes in the routing table to reach a destination.)

Figure 8.2. Routing Policies to Control Routing Information Flow

For more information about the active route selection process, see "How the Active Route Is Determined," on page 376.

You can also use routing policies to change specific route characteristics, which allow you to control which route is selected as the active route to reach a destination, to effect changes to the default BGP route flap- damping values, to perform per-packet load balancing, and to enable class of service (CoS).

Firewall filters provide a means of protecting your router from excessive traffic transiting the router to a network destination or destined for the Routing Engine. Firewall filters that control local packets can also protect your router from external aggressions such as denial-of-service (DoS) attacks. You can configure firewall filters to control which data packets are accepted on and transmitted from the physical interfaces and which local packets are transmitted from the physical interfaces to the Routing Engine (see Figure 8.3).

Figure 8.3. Firewall Filters to Control Packet Flow