How to do Route Summarization in EIGRP

How to do Route Summarization in EIGRP

This article is about How to do Route Summarization in EIGRP on Cisco Router. Route Summarization decreases the number of entries in routing updates and reduces the number of entries in local routing tables. Automatic EIGRP summarization for IPv4 is disabled by default as of versions 15.0 (1) M and 12.2 (33) of Cisco IOS. To enable automatic EIGRP summarization, use the auto-summary command in router configuration mode. Use the show ip protocols command to verify the status of automatic summarization. Examine the routing table to verify that automatic summarization works. EIGRP automatically includes summary routes to Null0 to avoid routing loops to destinations that are included in the summary, but do not really exist in the routing table.
EIGRP is a versatile routing protocol that can be adjusted in many ways. Two of the most important adjustment capabilities are to summarize routes and to implement load balancing. Other capabilities are the propagation of a predetermined route, the setting of timers and the implementation of authentication between EIGRP neighbors to increase security. In this article, these additional tuning features and configuration mode commands are discussed to implement them for IPv4 and IPv6.

EIGRP Auto Summary

Before adjusting the EIGRP features, start with a basic EIGRP implementation. In Image, the network topology used in this chapter is shown.

In Images of R1, R2 and R3, the IPv4 interface configuration and EIGRP implementations in R1, R2 and R3, respectively, are shown.

The types of serial interfaces and their associated bandwidths may not necessarily reflect the most frequent types of connections found in networks today. The bandwidths of the serial links used in this topology help explain the calculation of the routing protocol metrics and the best route selection process.
Note: that the bandwidth commands were used on the serial interfaces to modify the default bandwidth of 1544 kb / s. In this chapter, the ISP router is used as the gateway of the Internet routing domain. The three routers run the Cisco IOS, version 15.2.

EIGRP AUTOMATIC SUMMARIZATION

One of the most common EIGRP adjustment methods is to enable and disable automatic route summarization . Route summarization allows a router to group networks and announce them as a large group through a single summarized route. The ability to summarize routes is necessary due to the rapid growth of networks. A border router is a router that is located on the edge of a network. This router must be able to advertise all known networks within its route table to a network router or ISP router connector. Potentially, this convergence can result in very large route tables.
Summarization decreases the number of entries in routing updates and reduces the number of entries in local routing tables. It also reduces the use of bandwidth for routing updates and speeds up searches in routing tables. To limit the number of routing ads and the size of the routing tables, routing protocols, such as EIGRP, use automatic summarization at classy boundaries . This means that EIGRP recognizes the subnets as a single class A, B or C network and creates only one entry in the routing table for the summarized route. As a result, all traffic destined for subnets travels along that route.

How EIGRP AUTO Summary Works

The illustration shows an example of how automatic summarization works.

• Routers R1 and R2 are configured with EIGRP for IPv4, with automatic summarization. R1 has three subnets in the routing table: 172.16.1.0/24, 172.16.2.0/24 and 172.16.3.0/24.
• In the class network addressing architecture, all these subnets are considered part of a larger class B network: 172.16.0.0/16.
• Because EIGRP on router R1 is configured for automatic summarization, when it sends the routing update to R2, it summarizes the three subnets / 24 as a single 172.16.0.0/16 network. This reduces the amount of routing updates that are sent and the number of entries in the IPv4 routing table of R2.
• All traffic destined for the three subnets travels through the only route. R2 does not maintain routes to individual subnets and no subnet information is discovered.

In an enterprise network, the route chosen to reach the summary route may not be the best choice for the traffic that the individual subnet is trying to reach.The only way that all routers can find the best routes for each individual subnet is for neighbors to send information about the subnets. In this situation, automatic summarization must be disabled. When automatic summarization is disabled, updates include subnet information.

EIGRP AUTO Summary Configuration

Note : Automatic EIGRP summarization for IPv4 is disabled by default starting with versions 15.0 (1) M and 12.2 (33) of the Cisco IOS. In Image, the result of the show ip protocols command on R1 indicates that automatic EIGRP summarization is disabled.

This router runs IOS 15.2; therefore, automatic EIGRP summarization is disabled by default. In following image, the current routing table of R3 is shown. Note that the R3 IPv4 routing table contains all networks and subnets within the EIGRP routing domain.

To enable automatic EIGRP summarization, use the auto-summary command in router configuration mode, as shown in Image 8:

R1 (config) # router eigrp  as-number
R1 (config-router) # auto-summary
The no form of this command is used to disable automatic summarization.

Verification Of EIGRP AUTO Summary Configuration

SHOW IP PROTOCOLS COMMAND

In Image (above), note that the EIGRP routing domain has three classy networks:

• Class B network 172.16.0.0/16, consisting of subnets 172.16.1.0/24, 172.16.2.0/24 and 172.16.3.0/30.
• Class C network 192.168.10.0/24, consisting of subnets 192.168.10.4/30 and 192.168.10.8/30.
• Class C network 192.168.1.0/24, which is not subnetted.

The result of the show ip protocols command on R1, which appears in Image 9, shows that automatic summarization is now enabled. The result also indicates which networks are summarized and in which interfaces.

Note that R1 summarizes two networks in EIGRP routing updates:

• 192.168.10.0/24 sent by the GigabitEthernet 0/0 and Serial 0/0/0 interfaces
• 172.16.0.0/16 sent by Serial interface 0/0/1

R1 has subnets 192.168.10.4/30 and 192.168.10.8/30 in the IPv4 routing table.

NETWORK SUMMARY

As indicated in Image 10, R1 summarizes subnets 192.168.10.4/30 and 192.168.10.8/30 and forwards summary address 192.168.10.0/24 to neighbors at Serial 0/0/0 and GigabitEthernet 0 / interfaces 0. Because R1 has no EIGRP neighbors on the GigabitEthernet 0/0 interface, only R2 receives the summary routing update.

As indicated in Image 11, R1 also has subnets 172.16.1.0/24, 172.16.2.0/24 and 172.16.3.0/30 in the IPv4 routing table.

R3 selects R1 as the successor at 172.16.0.0/16, because it has a smaller feasible distance. The S0 / 0/0 interface of R3 that connects to R1 uses a default bandwidth of 1544 kb / s. The link from R3 to R2 has a higher feasible distance, because the S0 / 0/1 interface of R3 was configured with a bandwidth of less than 1024 kb / s.
Note that the summary update of 172.16.0.0/16 is not sent by the GigabitEthernet 0/0 or Serial 0/0/0 interfaces of R1. This is because these two interfaces are members of the same class B network 172.16.0.0/16. R1 sends the routing update not summarized from 172.16.1.0/24 to R2.
Summary updates are only sent via interfaces on different classy main networks.

EIGRP TOPOLOGY TABLE

In Image, routers R1 and R2 send the R3 a summary EIGRP routing update of 172.16.0.0/16.

The routing tables of R1 and R2 have subnets of the 172.16.0.0/16 network, therefore, both routers send to the R3 summarized announcements through a different main network. The result of the show ip eigrp topology all-links command is shown below that was used to view the full EIGRP topology table of R3.
R3 # show ip eigrp topology all-links 
P 172.16.0.0/16 , 1 successors, FD is 2170112, serno 9
  via 192.168.10.5 (2170112/2816), Serial0 / 0/0
  via 192.168.10.9 0/0 (3012096/2816), Serial0 / 0/1
This verifies that R3 received summary route 172.16.0.0/16 from both R1 in 192.168.10.5 and R2 in 192.168.10.9. The first entry through 192.168.10.5 is the successor, and the second entry through 192.168.10.9 is the feasible successor. R1 is the successor because its 1544 kb / s link to R3 gives the latter a better EIGRP cost at 172.16.0.0/16 than that of R2, which uses a slower 1024 kb / s link.
The all-links option shows all updates received, regardless of whether the route qualifies as a feasible successor (FS) or not. In this case, R2 qualifies as FS. R2 is considered an FS because the reported distance (RD) of 2816 is less than the feasible distance (FD) of 2 170 112 through R1.

EIGRP ROUTING TABLE

Examine the routing table to verify that the summary route has been received. In Image, the R3 routing table is shown before automatic summarization.

Automatic summarization disabled

In Image, the R3 routing table is shown with automatic summarization enabled by means of the auto-summary command .

Automatic summarization enabled

Note that with automatic summarization enabled, the R3 routing table now only class B network address 172.16.0.0/16. The next hop successor or router is R1, through 192.168.10.5.
Note : automatic summarization is only an option with EIGRP for IPv4. Classy addressing does not exist in IPv6, therefore, automatic summarization is not necessary with EIGRP for IPv6.

THE NULL INTERFACE

When enabling automatic summarization, it is also necessary to understand the null interface (Null). In following image, the routing table of R1 is shown.

Note that an Null0 output interface is used in the two highlighted inputs. EIGRP automatically included a summary route to Null0 for two classy networks: 192.168.10.0/24 and 172.16.0.0/16.
The Null0 interface is a virtual interface of the IOS that constitutes a route to nowhere , commonly known as “the electronic limbo”. Packets that link a route with an exit interface Null0 are discarded.
EIGRP for IPv4 automatically includes a summary of Null0 routes when the following conditions occur:

• At least there is a subnet that was learned through EIGRP.
• There are two or more network commands of the EIGRP router configuration mode.
• Automatic summarization is enabled.

The purpose of the Null0 route summary is to avoid routing loops to destinations that are included in the summary, but that do not really exist in the routing table.

EIGRP Summary Route

The illustration shows a situation in which a routing loop could occur:

1. R1 has a default route 0.0.0.0/0 through the ISP router.
2. R1 sends a routing update to R2 with the default route.
3. R2 installs the default route of R1 in its IPv4 routing table.
4. The R2 routing table contains the 172.16.1.0/24, 172.16.2.0/24 and 172.16.3.0/24 subnets in its routing table.
5. R2 sends a summary update to R1 for network 172.16.0.0/16.
6. R1 installs the summary route for 172.16.0.0/16 using R2.
7. R1 receives a package for 172.16.4.10. Because R1 has a route for 172.16.0.0/16 through R2, it forwards the packet to R2.
8. R2 receives the packet with destination address 172.16.4.10 of R1. The packet does not match any specific route, so that, using the default route in its routing table, R2 forwards the packet back to R1.
9. The package for 172.16.4.10 comes and goes in a loop between R1 and R2 until the TTL expires and the package is discarded.

EIGRP MANUAL SUMMARIZATION

EIGRP can be configured to summarize routes, whether automatic summarization ( auto-summary ) is enabled or not. Because EIGRP is a classless routing protocol and includes the subnet mask in routing updates, manual summarization may include supernet routes. Remember, a super network is an aggregate of multiple addresses of classy main networks. In following image, two more networks are added to the R3 router with the loopback interfaces: 192.168.2.0/24 and 192.168.3.0/24. Although loopback interfaces are virtual interfaces, in this example they are used to represent physical networks.

The following are the commands to configure the two loopback interfaces and the configuration to enable both interfaces for EIGRP on R3.
R3 (config) # interface loopback 2 
R3 (config-if) # ip add 192.168.2.1 255.255.255.0 
R3 (config-if) # exit 
R3 (config) # interface loopback 3 
R3 (config-if) # ip add 192.168.3.1 255.255.255.0 
R3 (config-if) # exit 
R3 (config) # router eigrp 1 
R3 (config-router) # network 192.168.2.0 
R3 (config-router) # network 192.168.3.0
To verify that R3 sent the EIGRP update packets to R1 and R2, the routing tables of both routers are examined. Then, only the relevant routes are shown. These routing tables of R1 and R2 show these additional networks: 192.168.2.0/24 and 192.168.3.0/24.

R1 # show ip route
D 192.168.1.0/24 [90/2170112] via 192.168.10.6, 00: 47: 39, Serial0 / 0/1
D 192.168.2.0/24 [90/2297856] via 192.168.10.6, 00: 08: 09, Serial0 / 0/1
D 192.168.3.0/24 [90/2297856] via 192.168.10.6, 00: 08: 04, Serial0 / 0/1
R1 #

R2 # show ip route
D 192.168.1.0/24 [90/3012096] via 192.168.10.10, 00: 47: 58, Serial0 / 0/1
D 192.168.2.0/24 [90/3139840] via 192.168.10.10, 00: 08: 28, Serial0 / 0/1
D 192.168.3.0/24 [90/3139840] via 192.168.10.10, 00: 08: 23, Serial0 / 0/1
R2 #

Instead of sending three networks separately, R3 can summarize 192.168.1.0/24, 192.168.2.0/24 and 192.168.3.0/24 networks as a single route.

CONFIGURATION OF EIGRP MANUAL SUMMARY ROUTES

Following figure shows the two manual summary routes that are configured in R3. These summary routes are sent via the Serial 0/0/0 and Serial 0/0/1 interfaces to the EIGRP neighbors of R3.

To determine the summary of these three networks, the same method is used as for determining summary static routes, as shown in Image 20:

• Step 1 . Write the networks that will be summarized in binary format.
• Step 2 . To find the subnet mask for summarization, start with the far left bit.
• Step 3 . From left to right, find all the bits that match consecutively.
• Step 4 . When there is a column of bits that do not match, stop. This is the summary limit.
• Step 5 . Count the number of matching bits found on the far left. In the example, it is 22. This number is used to determine the subnet mask of the summarized route: / 22 or 255.255.252.0.
• Step 6 . To find the network address for the summary, copy the 22 matching bits and add all 0 bits at the end to get 32 ​​bits.

The result is the summarized network address and the mask for 192.168.0.0/22.

EIGRP MANUAL SUMMARIZATION CONFIGURATION

To set the EIGRP manual summarization on a specific EIGRP interface, use the following interface configuration mode command:
Router (config-if) # ip summary-address eigrp  as-number network-address subnet-mask
Below is the configuration to propagate a manual summary route in the Serial 0/0/0 interface of R3. Because the R3 has two EIGRP neighbors, the manual EIGRP summarization must be configured in both Serial 0/0/0 and Serial 0/0/1.

R3 (config) # interface serial 0/0/0
R3 (config-if) # ip summary-address eigrp 1 192.168.0.0
255.255.252.0
R3 (config-if) #

VERIFICATION OF MANUAL SUMMARY ROUTES

It is shown below that, after configuring the summarized route, the routing tables of R1 and R2 no longer include individual networks 192.168.1.0/24, 192.168.2.0/24 and 192.168.3.0/24.

R1 # show ip route
D 192.168.0.0/22 ​​[90/2170112] via 192.168.10.6, 01:53:19, 
Serial0 / 0/1
R1 #

R2 # show ip route
D 192.168.0.0/22 ​​[90/3012096] via 192.168.10.10, 01:53:33, 
Serial0 / 0/1
R2 #

Instead, they show a single summary route: 192.168.0.0/22. Summarized routes reduce the total number of routes in the routing tables, which makes the search process in those tables more efficient.
These routes also require less bandwidth utilization for routing updates, since a single route can be sent instead of several individual routes.