Juniper Networks G10 CMTS manual Routing and Forwarding Tables

JUNOSg Internet Software Overview

Routing and Forwarding Tables	•
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A 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 destinations. 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.	•
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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 information to their	•
neighbors.	•
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Using the routing table, the routing protocol process uses the collected routing information to	•
determine active routes to network destinations. 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.	•
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Routing Policy	•
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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.	•
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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.	•
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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.	•
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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.	•
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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.	•
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System Architecture Overview 59