The routing protocol process controls the routing protocols that run on the router. It starts all configured routing protocols and handles all routing messages. It maintains one or more routing tables, which consolidate the routing information learned from all routing protocols into common tables. From this routing information, the routing protocol process determines the active routes to network destinations and installs these routes into the Routing Engine's forwarding table. Finally, it implements routing policy, which allows you to control the routing information that is transferred between the routing protocols and the routing table. Using routing policy, you can filter routing information so that only some of it is transferred, and you also can set properties associated with the routes.
The JUNOS software implements full IP routing functionality, providing support for IP Version 4 (IPv4). The routing protocols are fully interoperable with existing IP routing protocols, and they have been developed to provide the scale and control necessary for the Internet core.
The software provides the following routing and MPLS applications protocols:
A major function of the JUNOS routing protocol process is to maintain the Routing Engine's routing tables and from these tables to determine the active routes to network destinations. The routing protocol process then installs these routes into the Routing Engine's forwarding table. The JUNOS kernel then copies this forwarding table to the Packet Forwarding Engine. Refer to Figure 1 for an illustration of the interrelationships between the routing and forwarding tables.
The routing protocol process maintains multiple routing tables. By default, it maintains three routing tables. You can configure additional routing tables to suit your requirements.
With each routing table, the routing protocol process uses the collected routing information to determine active routes to network destinations.
For unicast routes, the routing protocol process determines active routes by choosing the most preferred route, which is the route with the lowest preference value. By default, the route's preference value is simply a function of how the routing protocol process learned about the route. You can modify the default preference value using routing policy and with software configuration parameters.
For multicast traffic, the routing protocol process determines active routes based on traffic flow and other parameters specified by the multicast routing protocol algorithms. The routing protocol process then installs one or more active routes to each network destination into the Routing Engine's forwarding table.
By default, all routing protocols place their routes into the routing table. When advertising routes, the routing protocols by default advertise only a limited set of routes from the routing table. Specifically, each routing protocol exports only the active routes that were learned by that protocol. In addition, the IGPs (IS-IS, OSPF, and RIP) export the direct (interface) routes for the interfaces on which the protocol is explicitly configured.
You can control the routes that a protocol places into each table and the routes from that table that the protocol advertises. You do this by defining one or more routing policies and then applying them to the specific routing protocol.
Routing policies applied when the routing protocol places routes into the routing table are referred to as import policies because the routes are being imported into the routing table. Policies applied when the routing protocol is advertising routes that are in the routing table are referred to as export policies because the routes are being exported from the routing table. In other words, the terms import and export are used with respect to the routing table.
Routing policy allows you to control (filter) which routes a routing protocol imports into the routing table and which routes a routing protocol exports from the routing table. Routing policy also allows you to set the information associated with a route as it is being imported into or exported from the routing table. Filtering imported routes allows you to control the routes used to determine active routes. Filtering routes being exported from the routing table allows you to control the routes that a protocol advertises to its neighbors.
You implement routing policy by defining policies. A policy specifies the conditions to use to match a route and the action to perform on the route when a match occurs. For example, when a routing table imports routing information from a routing protocol, a routing policy might modify the route's preference, mark the route with a color to identify it and allow it to be manipulated at a later time, or prevent the route from even being installed in a routing table. When exporting routes from a routing table into a routing protocol, a policy might assign metric values, modify the BGP community information, tag the route with additional information, or prevent the route from being exported altogether. You also can define policies for redistributing the routes learned from one protocol into another protocol.