Routing Policy - Juniper M40E Hardware Manual

Copyright © 2010, Juniper Networks, Inc.
routes, such as static routes, for inclusion in the routing table. The unicast routing
protocols use the routes in this table when advertising routing information to their
neighbors.
In the unicast routing table, the routing protocol process designates routes with the
lowest preference values as active. By default, a 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 by setting routing policies and configuring other software
parameters. See "Routing Policy" on page 55.
Multicast routing table (cache)—Stores routing information for all multicast protocols
running on the router, including DVMRP and PIM. You can configure additional routes
for inclusion in the routing table.
In the multicast routing table, the routing protocol process uses traffic flow and other
parameters specified by the multicast routing protocol algorithms to select active
routes.
MPLS routing table—Stores MPLS label information.
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.
You can configure additional routing tables to meet your requirements, as described in
the Junos OS Routing Protocols Configuration Guide.

Routing Policy

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, IGPs (IS-IS, OSPF, and RIP) export the direct
(interface) routes for the interfaces on which the protocol is explicitly configured.
For each routing table, you can affect the routes that a protocol places into the table
and the routes from the table that the protocol advertises 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 called 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 called 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.
Chapter 3: Junos OS Overview
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