Route Summarization

Chapter 6 explains route summarization (which is also called route aggregation or supernetting). In route summarization, a single summary address in the routing table represents a set of routes. Summarization reduces the routing update traffic, the number of routes in the routing table, and the overall router overhead in the router receiving the routes.

The Benefits of Route Summarization

A large flat network is not scalable because routing traffic consumes considerable network resources. When a network change occurs, it is propagated throughout the network, which requires processing time for route recomputation and bandwidth to propagate routing updates.

A network hierarchy can reduce both routing traffic and unnecessary route recomputation. To accomplish this, the network must be divided into areas that enable route summarization. With summarization in place, a route flap (a route that goes down and up continuously) that occurs in one network area does not influence routing in other areas. Instabilities are isolated and convergence is improved, thereby reducing the amount of routing traffic, the size of the routing tables, and the required memory and processing power for routing. Summarization is configured manually, or occurs automatically at the major network boundary in some routing protocols.


Recall from Chapter 6 that being able to summarize requires a well-planned underlying IP addressing design.

Recommended Practice: Summarize at the Distribution Layer

It is a recommended practice to configure summarization in a large network from the distribution layers toward the core, as illustrated in Figure 7-18. The distribution layer should summarize all networks on all interfaces toward the Campus Core. WAN connectivity and remote access points should be summarized toward the core. For example, remote subnets could be summarized into major networks, and only those major networks would be advertised to the core.

Implementing summarization at the distribution layer optimizes the convergence process. For example, if a link to an access layer device goes down, return traffic to that device is dropped at the distribution layer until the routing protocol converges. Summarizing also limits the number of peers that an EIGRP router must query or the number of LSAs that an OSPF router must process, which also reduces the convergence time.

Core routers that receive two routes for a network install the more-specific route in the routing table. Therefore, summary routes for primary links must use a longer subnet mask.

Recommended Practice: Passive Interfaces for IGP at the Access Layer

Another recommended practice is to limit unnecessary peering across the access layer. In Figure 7-19, the distribution multilayer switches are directly connected to each other and are also interconnected with three access layer switches, each having four VLANs. By default, the distribution layer devices send routing updates and attempt to peer with the remote distribution layer devices across the links from the access switches on every VLAN. Having the distribution switches form neighbor relationships over these 12 access layer connections provides no benefit and wastes resources (including CPU processing time and memory). Therefore, the interfaces on the distribution layer devices toward the access layer devices are configured as passive interfaces under the routing protocol configuration. This suppresses the advertisement of routing updates for that routing protocol on those interfaces.

Figure 7-19 Limit Unnecessary Peering Across the Access Layer

Figure 7-19 Limit Unnecessary Peering Across the Access Layer

Configuring Passive Interfaces

In a network that has many VLAN interfaces on distribution layer routers, configuring each interface as a passive interface could result in many commands. To ease the configuration burden, Cisco IOS 12.0 introduced the passive interface default command, which makes all interfaces passive. This command can be used in conjunction with the no passive interface {interface} command to run the routing protocol on only the interfaces from the distribution layer devices to the core layer devices, minimizing the configuration required.

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    What layer do you summarize routes?
    3 months ago

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