Router Flows Affected by Policies

• 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’s 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, it 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’s 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 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 process 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.

  Note: In the rest of this chapter, the term packets refers to both data and local packets unless explicitly stated otherwise.

Figure 1 illustrates the flows 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 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 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.)

You can also use routing policies to do the following:

• Change specific route characteristics, which allow you to control which route is selected as the active route to reach a destination. In general, the active route is also advertised to a router’s neighbors.
• Change to the default BGP route flap-damping values.
• Perform per-packet load balancing.
• Enable class of service (CoS).

Figure 2: Routing Policies to Control Routing Information Flow

You can configure firewall filters to control the following aspects of packet flow (see Figure 3):

• Which data packets are accepted on and transmitted from the physical interfaces. To control the flow of data packets, you apply firewall filters to the physical interfaces.
• Which local packets are transmitted from the physical interfaces and to the Routing Engine. To control local packets, you apply firewall filters on the loopback interface, which is the interface to the Routing Engine.

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 incidents such as denial-of-service attacks.

Figure 3: Firewall Filters to Control Packet Flow

Published: 2013-11-18