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Routing Engine Software Components
M40 Internet Router Hardware Guide
28
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 table, which the
JUNOS kernel copies to the Packet Forwarding Engine.
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 table, 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:
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.
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 28.
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 28.
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.
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.
parameters specified by the multicast routing protocol algorithms to select active routes.
MPLS routing table—Stores MPLS label information.
You can configure additional routing tables to meet your requirements, as described in the
JUNOS Internet Software Configuration Guide: Routing and Routing Protocols.
JUNOS Internet Software Configuration Guide: Routing and Routing Protocols.
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.
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.
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.
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.