Discussion

Table 9-1. Default routing-policy actions (continued)
Protocol Routing table Default import action Default export action
LDP inet.3 Accept all LDP routes. Accept and export active LDP routes.
MPLS inet.3 Accept all MPLS routes. Accept and export active MPLS routes.
OSPF inet.0 Accept all OSPF routes. Policy cannot be
modified because OSPF requires that all
routers in an area have the same linkstate
database to maintain a stable,
loop-free network.
Reject everything (OSPF uses LSAs to
advertise its routes).
PIM dense mode inet.1 Accept all PIM DM routes. Accept and export active PIM DM routes.
PIM sparse mode inet.1 Accept all PIM SM routes. Accept and export active PIM SM routes.
RIP inet.0 Accept all RIP routes learned from RIP Reject everything.
neighbors.
RIPng inet6.0 Accept all RIPng routes learned from Reject everything.
RIPng neighbors.
Direct and static
routes
inet.0 Accept all routes. Do not export.
What are some reasons to modify the default routing-policy behavior? This recipe
illustrates one reason, which is to redistribute routes learned from one protocol
(here, static routes) to another protocol (here, OSPF). This type of policy affects
which routes the protocol advertises (or exports) from the routing table, so you configure
it with a set export command. Another reason is to keep a route out of the
routing table so that it can never become the active route. Active routes are placed in
the forwarding table and are used to forward traffic. For these types of policy, you
use a set import command to control the routes placed into (imported into) the routing
table. Another reason specific to BGP is to change the BGP properties associated
with a route, such as the AS path and community, and to configure route flap damping.
Depending on your purposes, you use the set import or set export commands to
apply policies.
For the link-state IGPs (IS-IS and OSPF), you should never modify the default policy
behavior for incoming routes (with a set import command). These protocols use
link-state databases to keep track of their routes, and the databases on all routers in
an area must be identical for the protocol to work properly.
Now let’s look at this recipe to understand how the policy is configured. The policy,
being a simple one, has only one term, called 1. While you could name the term with
a text string that describes what the term does, it is common practice to use a number
to name the term, especially in simple policies. The term name is not referenced
by other parts of the configuration. It is, however, used in logfiles created when tracing
routing-policy operation (see Recipe 9.7), so, for more complex networks and
policies, identify each term with a meaningful name so you can identify them in the
logfiles.
286 | Chapter 9: Routing Policy and Firewall Filters
This is the Title of the Book, eMatter Edition
Copyright © 2008 O’Reilly & Associates, Inc. All rights reserved.