M160 Internet Router Hardware Guide
In addition, IGPs (IS-IS, OSPF, and RIP) export the direct (interface) routes
for the interfaces on whicht heprotocol is explicitly configured.
For each routingtable, you can affect the routes that a protocolplaces into the table
and the routes from the tablethat the protocol advertises by defining one or more
routing policies and then applyingthem to the specific routing protocol.
Routing policies applied whent herouting 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 exportedfrom the routing table. In other words, the terms
import and export are used with respectto the routing table.
Routing policy enables you to control (filter) which routes are imported into the
routing table and which routesare exported from the routing table. Routing policy
also allowsyou to set the information associated with a route as it isbeing imported
into or exported from the routing table. Routing policies ap plied to imported routes
control the routes used todetermine active routes, whereas policies applied to
exported routes controlwhich routes a protocol advertises to its neighbors.
Youimplement routing policy by defining policies. A policy specifies the conditions
tousetomatcharouteandtheactiontoperformontheroutewhenamatch
occurs. For example, whena routing table imports routing information from a
routing protocol, a routingpolicy might modify the route’s preference, mark the
route with a color toidentify it for later manipulation, or prevent the routefrom
even being installedin a routingtable. When a routing tableexports routes to a
routing protocol,a policy might assign metric values, modify the BGP community
information, tag the route withadditional information, or prevent the route
from being exported altogether. You also can define policies for redistributing
the routes learned from one protocolinto another protocol.
VPNsThe JUNOS software supports several types of VPNs:
Layer 2 VPNs—A Layer2 VPN links a set of sites sharing common routing
information, and whose connectivityis controlled by a collection of policies. A
Layer 2 VPN is not awareof routes within a customer’s network. It simply
provides private links between a customer’s sites over the service provider’s
existing public Internetbackbone.
Layer3VPNs—ALayer3VPNlinksasetofsitesthatshare commonrouting
information, and whose connectivity i s controlled by a collection of policies.
A Layer 3 VPN is awareof routes within a customer’s network, requiring
more configuration on the part of the serviceprovider than a Layer 2 VPN.
The sites that make up a Layer 3 VPN are connected over a service provide r’s
existing public Internetbackbone.
Interprovider VPNs—An interprovider VPN supplies connectivity between t wo
VPNs in separate autonomous systems (ASs). This functionality could be used
48 Routing Engine Software Components