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Cisco 100-101 Exam
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Cisco Cisco CCENT Exams

100-101 - CCNA Interconnecting Cisco Networking Devices 1 (ICND1)

640-822 - Interconnecting Cisco Networking Devices Part 1

Defining Summarization

Introduction to Summarization

Summarization refers to the ability of a router to group a number of subnetworks and then summarize these subnetworks as one summary address. Summarization allows a number of networks to be represented as a single summary address. Summarized networks are called also called supernets or aggregated routes. Summarization can be used within a variety of networks. You can summarize subnetworks using different types of routers and routing protocols.

Routing protocols can summarize routes when they support Classless Inter-domain Routing (CIDR). Summarizing routes have the advantage of reducing the size of the routing table. It also results in less routing updates or traffic on the network, and less CPU and memory requirements. Using route summarization and incremental updates assist in reducing the number of router processing because the entries in the IP routing table are reduced. Route summarization takes place at major network boundaries. All routes advertised are summarized into a single entry.

Only classless routing protocols support summarization:

  • Routing Information Protocol version 2 (RIPv2)

  • Open Shortest Path First (OSPF)

  • Intermediate System to Intermediate System (IS-IS)

  • Enhanced Interior Gateway Routing Protocol (EIGRP)

  • Border Gateway Protocol (BGP)

The benefits of summarization include:

  • Reduction in the size of the routing table: When the routing table is reduced, the updates are smaller. This in turn leads to a number of other advantages:

  • A lower memory requirement in the router.

  • Less CPU in the routing process.

  • A lower bandwidth requirement from the network.

  • Enables better network growth: This is mainly due to network overhead being able to scale.

  • Simpler recalculation process of the network. This is also basically due to the reduction in the size of the routing table.

  • Network changes can be hidden: When a routing table holds a summary of the networks located under it, changes made in the network are hidden.

If you cannot implement summarization, a few alternative solutions are noted below:

  • You can not use summarization but understand the limitations imposed on the network.

  • You can readdress the network, if justified.

Creating Summary Addresses

Each specific routing protocol handles summarization in a different way.

The manner in which summarization takes place is dependent on the following factors:

  • The routing protocol being used

  • The configuration of summarization.

Each of the routing protocols has some degree of summarization. The initial routing protocols such as RIP and IGRP automatically summarize at the NIC or natural class boundary. The reason for this being that subnet mask information is not sent in their routing updates. This form of summarization is known as automatic summarization.

The later routing protocols, such as Enhanced IGRP (EIGRP) and Open Shortest Path First (OSPF) send subnet mask information in their routing updates. This basically means that both variable length subnet masks (VLSMs) and summarization can be performed. In this case, when a routing update is received, the mask is assigned to the specific subnet. When a routing protocol does a lookup in the routing table, it searches the entire database.

The routing protocol acts on the longest match, which in turn enables the following:

  • Summarization

  • Hierarchical design granularity

  • Discontiguous networks

A discontiguous network refers to a network wherein a different NIC segregates instances of the same NIC number. This basically means that the same NIC is found twice, but in different locations. If you have discontiguous networks, you should disable summarization. This is due to summarization not necessarily providing the required information to the routing table on the other end of the NIC. When using the EIGRP routing protocol and you have discontiguous networks, you should definitively turn off summarization. This is because EIGRP automatically summarizes at the NIC boundary.

Before you can create a summary address, you need to collect the networks that you want to summarize into a single entry and then express them in the binary format. After this, you have to create a network number and define the mask.

Both RIP and IGRP include the auto-summarization feature. You cannot disable auto-summarization. You can however disable auto-summarization for RIPv1. You should disable auto-summarization when running both versions of the RIP routing protocol. RFC 1723 for RIPv2 actually recommends that you disable auto-summarization.

Table 1 illustrates the differences between RIP and IGRP auto-summarization and route aggregation.

TABLE 1: Comparing RIP and IGRP auto-summarization features

Routing Protocol

Auto-summarization Enabled

Auto-summarization Disabled

Route Aggregation


Enabled by default

Cannot be disabled

Not allowed


Enabled by default

Can be disabled



Enabled by default

Cannot be disabled

Not allowed

To disable auto-summarization, use the following command:

  • no auto-summary command

The most popular protocols used to advertise a summary network is EIGRP and OSPF. RIP and IGRP automatically summarize on classful boundaries. EIGRP also automatically summarizes on classful boundaries.

If you want to manually configure a summary route through the EIGRP protocol, you can use the following interface command

  • ip summary-address command, followed by the summary network.

Summarization Design Considerations

The design considerations for summarization are listed here:

  • Route summarization works well where hierarchical addressing is implemented. With hierarchical addressing, all the IP subnets at the bottom of the tree are subsets of those subnets at the top of the tree:

  • Subnets at the bottom of the tree are those subnets that have the longer subnet masks.

  • Subnets at the top of the tree are those subnets that have the shorter subnet masks.

  • The classless routing protocols support route summarization:

  • Routing Information Protocol version 2 (RIPv2)

  • Open Shortest Path First (OSPF)

  • Intermediate System to Intermediate System (IS-IS)

  • Enhanced Interior Gateway Routing Protocol (EIGRP)

  • Border Gateway Protocol (BGP)

  • A noncontiguous range of networks results in an inefficient summarization entry.

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