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Routing and Forwarding Tables
The primary function of the JUNOS routing protocol process is maintaining routing
tables and using the information in them to determine active routes to network
destinations. It copies information about the active routes into the Routing Engine’s
forwarding t able, which the JUNOS kernel copies to the Packet Forwarding Engine.
By default, the routing protocol process maintains the following routing tables and
uses the information in each table to determine active routes to network destinations:
Unicast routing table—Stores routing information for all unicast protocols
running on the router, including BGP, IS-IS, OSPF, and RIP. You can also
configure additional 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 f unction 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 47.
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 p rotocol 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 p olicy 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.
Yo u can configure additional routing tables to meet your requirements, as
described in the JUNOS 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.
Routing Engine Software Components 47