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System Architecture Overview
JUNOSg Internet Software Overview
59
Routing and Forwarding TablesA primary function of the JUNOSg routing protocol process is to maintain the Routing
Engine’s routing table and to determine the active routes to network destinatio ns. It then
installs these routes into the Routing Engine’s forwarding table. The JUNOSg kernel then
copies this forwarding table to the Packet Forwarding Engine.
The routing table stores routing information for all routing protocols running on the CMTS.
OSPF and RIP store their routing information in this common routing table, and you can
configure additional routes, such as static routes, to be included in this routing table. OSPF
and RIP use the routes in the routing table when advertising routing informa tion to their
neighbors.
Using the routing table, the routing protocol process uses the collected routing information to
determine active routes to network destinations. The routing protocol process deter mines
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.
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 (OSPF and RIP) export the direct (interface) routes
for the interfaces on which the protocol is explicitly configured.
For the 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 th e 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 becaus e 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 are imported into the routing table
and which routes are exported 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. Applying routing policy to imported routes allows you to control the routes used
to determine active routes. Applying routing policy to 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 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, 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.