Summarization

Summarization
We would like to revisit the topic of route summarization here. Route summarization is the process
of combining multiple network ranges into one network range. We have briefly discussed
the use of route summarization for most of the IGPs we’ve covered in this study guide and BGP.
We’d like to take a minute to go over those again and also show you how to use summarization
for routes being redistributed into a routing protocol. We will look at each of the routing protocols
individually.
Routing protocols such as RIPv1, RIPv2, IGRP, EIGRP, and BGP support automatic summarization.
Automatic summarization will occur when routing information crosses a classful
network boundary. Manual summarization is supported by RIPv2, EIGRP, IS-IS, OSPF, and
BGP. RIPv1 and IGRP do not support the concept of manual summarization, because they are
classful routing protocols. They do, however, automatically summarize on classful network
boundaries, leading to their lack of support for discontiguous subnets.
EIGRP
EIGRP uses summarization the same way, whether it’s for a route originating in EIGRP or a route
being redistributed into EIGRP. In order to summarize a route, you need to use the ip summaryaddress
eigrp AS# command on the interface you want to perform the summarization. The following
is an example of an EIGRP summary route being placed on interface Serial 0:
R1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#interface s0
354 Chapter 10  Route Optimization
R1(config-if)#ip summary-address eigrp 100 172.16.0.0 255.255.0.0
R1(config-if)#^Z
R1#
This configuration creates a summary address of 172.16.0.0 /16 for EIGRP AS 100 that will
be advertised to the neighbor on interface S0.
OSPF
The way you would configure a summary address for internal OSPF routes is different from the
way you would configure a summary address for a route being redistributed into OSPF.
To configure summary addresses for internal OSPF routes, you use area ranges. For instance,
Area 1 has the following network addresses in it:
 172.16.16.0 /24
 172.16.32.0 /24
 172.16.48.0 /24
We want to be able to send out one route for all of the routes contained in Area 1. We
would need to create a summary address of 172.16.0.0 /18. If you do not remember how to
summarize addresses, refer back to Chapter 2, “IP Addressing.” In order to implement this,
you would need to create an area range for this summary address on the ABR for Area 1.
Here’s the configuration required:
R1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#router ospf 1
R1(config-router)#area 1 range 172.16.0.0 255.255.192.0
R1(config-router)#^Z
R1#
If the same addresses in the preceding example that we needed to summarize were redistributed
into OSPF, we would need to use the command summary-address address mask on the
ASBR that was performing the redistribution. The following is the needed configuration:
R1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#router ospf 1
R1(config-router)#summary-address 172.16.0.0 255.255.192.0
R1(config-router)#^Z
R1#
It’s important to note that this command can be used only on an ASBR, and it must be used
for routes being redistributed into OSPF.
Summary 355
IS-IS
IS-IS summarizes internal routes and redistributed routes in the same way as in the previous section
about OSPF. In order to summarize routes for IS-IS, you must use the summary-address
command with one of the following keywords:
 level-1 will summarize routes redistributed into the Level 1 area.
 level-2 will summarize Level 1 routes that are going into the Level 2 backbone. It will also
summarize routes that are being redistributed into the Level 2 backbone.
 level-1-2 will perform both Level 1 and Level 2 summarization.
The following routes are contained in a Level 1 area:
 172.16.126.0 /24
 172.16.4.0 /24
 172.16.48.0 /24
In order to configure summarization, you would first need to figure out the summary address
for these routes. In this case, the summary address would be 172.16.0.0 /17. Then you would
configure the summary address on the Level 1/2 router attached to the area. Here’s the configuration
that needs to be entered on the Level 1/2 router:
R1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#router isis
R1(config-router)#summary-address 172.16.0.0 255.255.128.0 level-2
R1(config-router)#^Z
R1#
This configuration summarizes the three routes into one route, and the Level 2 backbone would
know of only this one route.
Summary
When all of these techniques are used together, they can help optimize a network’s performance.
In turn, they can help you better perform your job.
There are many methods to use for filtering routing information. These methods include
access groups, distribute lists, and route maps. Access groups are used to filter traffic crossing
a router, distribute lists are used to filter routing updates, and route maps are used to filter
routes being redistributed.
Policy-based routing allows for routing to occur in a method that differs from the rules established
by dynamic routing protocols. This is accomplished by creating a policy that will define
how certain traffic should be routed for certain destinations.
356 Chapter 10  Route Optimization
Route information is not shared between different routing domains, by default (except
between IGRP and EIGRP using the same AS number). In certain situations, you may want to
share route information between these different routing domains. When this is the case, you
need to configure redistribution.
The larger a routing table is, the more CPU cycles are required to route information. In an
effort to reduce the size of routing tables, you can use summarization. Routing protocols such
as RIPv1, RIPv2, IGRP, EIGRP, and BGP support automatic summarization. Automatic summarization
will occur when routing information crosses a classful network boundary. Manual
summarization is supported by RIPv2, EIGRP, IS-IS, OSPF, and BGP.
We’ve covered a tremendous amount of information in this part of the study guide. You
should feel a sense of accomplishment in making it this far. The topics covered here are intended
not only to help you pass the BSCI exam but to help you succeed in the networking field. Take
time to practice and solidify your understanding of these topics so that you may better perform
your job and take the next step up the ladder of networking knowledge.
Exam Essentials
Explain the use of filters. There are many methods to use for filtering routing information.
These methods include access groups, distribute lists, and route maps. Access groups are used
to filter traffic crossing a router, distribute lists are used to filter routing updates, and route
maps are used to filter routes being redistributed.
Explain why policy-based routing is used and when it should be used. Policy-based routing
allows routing to occur in a method that differs from the rules established by dynamic routing protocols.
This is accomplished by creating a policy that defines how certain traffic should be routed
for certain destinations.
Understand the concept of redistribution. Route information is not shared between different
routing domains by default (except between IGRP and EIGRP using the same AS number). In
certain situations, you may want to share route information between these different routing
domains. When this is the case, you need to configure redistribution.
Understand why summarization is needed. The larger a routing table is, the more CPU
cycles are required to route information. In an effort to reduce the size of routing tables, you
can use summarization. Routing protocols such as RIPv1, RIPv2, IGRP, EIGRP, and BGP
support automatic summarization. Automatic summarization will occur when routing
information crosses a classful boundary. Manual summarization is supported by RIPv2,
EIGRP, IS-IS, OSPF, and BGP.

398