Configuring Ospf Areas; Configuring Ospf Summary Import; Configuring Ospf Redistribution Rules - Juniper NETWORK AND SECURITY MANAGER 2010.4 - CONFIGURING SCREENOS DEVICES GUIDE REV 01 Manual

Configuring ScreenOS Devices Guide

Configuring OSPF Areas

Configuring OSPF Summary Import

Configuring OSPF Redistribution Rules

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By default, all routers are grouped into a single "backbone" area called area 0 (usually
denoted as area 0.0.0.0). However, you might want to segment large geographically
dispersed networks into multiple areas for better scalability.
Using multiple areas reduces the amount of routing information passed throughout the
network because a router only maintains a link-state database for the area in which it
resides. The VR maintains link-state information for all connected areas, and does not
maintain link-state information for networks or routers outside the area.
AS external advertisements describe routes to destinations in other autonomous systems
and are flooded throughout an AS. To prevent AS external advertisements from flooding
an AS, configure the OSPF area as a stub area:
Stub area—An area that receives route summaries from the backbone area but does
not receive link-state advertisements from other areas for routes learned through
non-OSPF sources (BGP, for example). A stub area can be considered a totally stubby
area if no summary routes are allowed in the stub area.
Not So Stubby Area (NSSA)—Like a normal stub area, NSSAs cannot receive routes
from non-OSPF sources outside the current area. However, external routes learned
within the area can be learned and passed to other areas.
All areas must connect to area 0, which is defined by default on the virtual router when
you enable the OSPF routing instance on the virtual router. For areas that cannot be
physically connected to the backbone area, you must configure a virtual link to provide
the remote area with a logical path to the backbone through another area. For details
on virtual links, see "Configuring OSPF Virtual Links" on page 317.
In large internetworks where hundreds or even thousands of network addresses can exist,
routers can become overly congested with route information. After you have redistributed
a series of routes from an external protocol to the current OSPF routing instance, you
can bundle the routes into one generalized or summarized network route. By summarizing
multiple addresses, you enable a series of routes to be recognized as one route, simplifying
the process.
Using route summarization in a large, complex network can isolate topology changes
from other routers. An intermittently failing link in a domain does not affect the summary
route, so no router external to the domain needs to modify its routing table due to the
link failure. Route summarization also prevents LSAs from propagating to other areas
when a summarized network goes down or comes up.
You can summarize inter area routes or external routes.
Use route redistribution to exchange route information between routing protocols. You
can redistribute the following types of routes into the OSPF routing instance in the same
VR:
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