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Ramazan Kornilov
Ramazan Kornilov

Cisco Manual Route Summarization Eigrp !FULL!

Route summarization is a method of representing multiple networks with a single summary address. It is often used in large networks with many subnets because it reduces the number of routes that the routers must maintain and minimizes routing updates. Two methods for summarizing routes exist: automatic summarization and manual summarization.

Cisco Manual Route Summarization Eigrp

The auto summary feature can cause problems with discontiguous networks. This is why this feature is usually turned off (it is much better to use manual summarization to control exactly where and how summarization affects the network).

One of the advantages of EIGRP over some other routing protocols (like OSPF) is that manual summarization can be done on any router within a network. A single route can be used to represent multiple routes, which reduces the size of routing tables in a network.

The manual summarization is a process of creating a summary route that will be used to represent multiple routes and can be used to reduce the sizes of routing tables in a network. EIGRP, unlike some other routing protocols such as OSPF, supports the manual summarization on any router within a network.

In the example above we have a network of three routers running EIGRP. R1 has two directly connected subnets: and Assuming that no automatic summarization is enabled, R1 will advertise these two networks to R2. Here is the routing table on R2:

Route summarization filters more specific routes and advertises a less specific summary route to neighboring routers, covering the range of the specific networks. Enhanced Interior Gateway Routing Protocol (EIGRP) can summarize both internal routes and external routes. Internal routes are those networks natively included in the EIGRP process by issuing the network ip-address [subnet-mask] command. External routes are networks redistributed into the EIGRP process by issuing the redistribute protocol command. Depending on the type of route summarized, the EIGRP process on a router can be configured for automatic or manual summarization.

EIGRP automatically summarizes networks at major network boundaries when the auto-summary command is issued in router configuration mode. For example, if a router has two interfaces belonging to and under EIGRP, these networks are summarized to their major network boundaries, and respectively, when the updates are advertised by EIGRP. Automatic summarization summarizes internal routes, but it does not summarize external routes.

Enhanced Interior Gateway Routing Protocol (EIGRP) can summarize both internal routes and external routes. The type of route you choose determines which commands you issue. When a router running EIGRP is configured for route summarization using manual or automatic summarization, a summary route pointing to null 0 interface is created in the routing table of the router.

A similar behavior is observed with BGP too. When Border Gateway Protocol (BGP) is configured for manual summarization, a summary route pointing to the null 0 interface is created in the routing table of the router.

command under a specific interface adds a route for the summary being configured to the routing table of the router pointing to interface null 0 . This is also true when the router is configured for automatic summarization by issuing the command: auto-summary command

To restore the default behavior of automatic summarization of subnet routes into network-level routes, use the auto-summary command in router configuration mode. To disable this function and send subprefix routing information across classful network boundaries, use the no form of this command.

The route to null 0 interface is added to avoid routing loops or sending packets to a black hole when a less specific route, like a default route, is also available on the router performing the summarization. The default route is used for reaching networks that are not specifically available in the routing table.

For example, router R1 is connected to few other routers within a private network and connects to a public network, such as the Internet, through the border router R2. R1 is learning specific networks from the routers in the private network through EIGRP, but it advertises only a summary route to R2, since R1 is the only way for R2 to reach all those specific networks. This is achieved by configuring the EIGRP process on R1 for either manual or automatic summarization. R1 also relies on a default route pointing to R2 to reach the routes available in the public network.

Similarly, when a router running BGP is configured for manual summarization using the aggregate-address command in the router configuration mode, a route pointing to interface null 0 for the summary being configured, is added to the routing table of the router. Here too, the addition of route pointing to null 0 is used to avoid routing loops and avoids forwarding packets to a black hole.

Summarization is extremely important in a well-designed EIGRP network.Summarization is one of the few weapons to prevent stuck in active problems.Most summarization problems are the result of a misconfiguration of the router.Figure 7-29 shows a flowchart for troubleshooting an EIGRP summarizationproblem.

In the configuration shown in Example 7-52, the summary route is configuredto be by using the command ip summary-addresseigrp 1 This summary route covers the networkaddress range from to From the routing table shown inExample 7-53, notice that no routes fit between the range of to172.16.95.255. Therefore, if no subnetworks of the configured summary route arepresent in the routing table, the router doesn't generate the summaryroute.

The solution to this problem is to configure an interface that falls in the172.16.80.0 range. You can configure a loopback interface withaddress to generate the summary route configured onEthernet 0. Example 7-54 shows the changed configuration in Router A that willfix this manual-summarization problem.

Since, by design, all those networks are very similar, I can summarize those routes. This will shrink our routing and topology table. To summarize routes you need to configure manual summarization on the router where those prefixes live. In my case this is R1. You need to configure this on the interface connected to the neighbor where you want to send the summary route to.

IPv6 EIGRP route summarization is similar to IPv4. The summarization is performed on interface-basis by command . After configuring the summarization command the router drops IPv6 EIGRP relationships to reestablish them again, this renew input events and make neighbors rebuild their topology tables and perform DUAL algorithm local computation again using the new advertisements from the router who reconfigured summarization.

In this demonstration, I will share with us on how to implement eigrp with manual route summarization on a Cisco router. Before we go into configuring enhanced interior gateway routing protocol (eigrp) for the network topology displayed below, here are a few things about eigrp you need to commit to memory:

* It is Cisco proprietary* It has a default administrative distance of 90* It supports variable length subnetmask (VLSM)* It supports classless addressing* It supports manual summarization* It is a hybrid protocol. ( It has the characteristics of both link state and distance vector protocols)* It supports authentication

Our objective in this lab is to configure eigrp for the network represented in the diagram above. The subnets are configured on the loopbacks on both routers. We will use eigrp to advertise the networks on both routers, making sure that all subnets are reachable. Furthermore, we will summarize the route advertisement on R2 as it is being advertised to R1.

R1(config)#router eigrp 25R1(config-router)#netw auto

The subnets advertised on R2 and being received individually. Now, lets summarize manually on R2 and then go back to R1 to see what the routing table will look like:To summarize in eigrp, we need to go to the interface connecting to R1 and enter the command below:R2(config-if)#ip summary-address eigrp 25

RIP Version 2 route summarization requires that the lowestmetric of the "best route" of an aggregated entry, or the lowestmetric of all current child routes, be advertised. The best metricfor aggregated summarized routes is calculated at routeinitialization or when there are metric modifications of specificroutes at advertisement time, and not at the time the aggregatedroutes are advertised.

The ip summary-address rip routerconfiguration commandcauses the router to summarize a given set of routes learned viaRIP Version 2 or redistributed into RIP Version 2. Host routes areespecially applicable for summarization.

The Cisco implementation of the Routing Information Protocol(RIP) Version 2 (RIPv2) supports authentication, key management,route summarization, classless interdomain routing (CIDR), andvariable-length subnet masks (VLSMs).

If you have disconnected subnets, disable automatic routesummarization to advertise the subnets. When route summarization isdisabled, the software sends subnet and host routing informationacross classful network boundaries. To disable automaticsummarization, use the no auto-summary command in router configurationmode.

Supernet advertisement (advertising any network prefix less thanits classful major network) is not allowed in RIP routesummarization, other than advertising a supernet learned in therouting tables. Supernets learned on any interface that is subjectto configuration are still learned. 350c69d7ab


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