CCNA online course

EIGRP Manual Route Summarization

Learn how to calculate, configure, verify, and troubleshoot manual IPv4 route summarization in Cisco EIGRP, including classic and named mode.

EIGRP (Enhanced Interior Gateway Routing Protocol) is a Cisco routing protocol that forms neighbor relationships and selects paths using a composite metric, primarily bandwidth and delay under the default K values. Manual route summarization lets you advertise one aggregate prefix instead of many more-specific routes.

This lesson covers IPv4 summary calculation, classic-mode and named-mode configuration, Null0 behavior, verification, design rules, and troubleshooting.

What Route Summarization Does

A component route is an individual, more-specific route such as 172.16.20.0/24. A summary route is an aggregate that represents several component routes. For example, four contiguous /24 networks can be advertised as one /22:

172.16.20.0/24
172.16.21.0/24
172.16.22.0/24
172.16.23.0/24

Summary: 172.16.20.0/22

The summary is a supernet: its prefix is shorter than the prefixes of the networks it aggregates. A router receiving the summary does not need a separate EIGRP route for every included /24 through that neighbor.

• Smaller routing tables: fewer entries consume less memory.
• Less update processing: changes to individual component routes do not always need to be advertised beyond the summary boundary.
• Smaller query scope: EIGRP queries can be contained at a summary boundary, improving scalability.
• Hierarchical design: distribution, branch, and core layers can exchange summarized reachability rather than every individual subnet.

Automatic Versus Manual Summarization

Automatic summarization is legacy EIGRP behavior that summarizes routes at major classful network boundaries. A classful boundary is the boundary of a major Class A, B, or C network. This behavior can produce unexpected advertisements when networks are discontiguous.

Modern Cisco IOS EIGRP implementations generally have automatic summarization disabled by default, but older configurations and inherited configurations should still be checked. The command no auto-summary explicitly prevents legacy classful summaries.

Manual summarization is classless and deliberate. It is configured on a selected interface and can use any valid prefix length, such as /22, /20, or /27, provided the address and mask are correctly aligned.

Calculating an IPv4 Summary

Step 1: Identify the component networks

List every route that should be represented by the aggregate. The networks should normally be contiguous and organized behind the same topology boundary.

For example:

172.16.20.0/24
172.16.21.0/24
172.16.22.0/24
172.16.23.0/24

Step 2: Find the longest common prefix

All four networks share the first two octets, which provide 16 bits. The changing third-octet values are 20 through 23:

20 = 00010100
21 = 00010101
22 = 00010110
23 = 00010111

The first six bits of the third octet are common. Therefore, the total common prefix is 16 + 6 = 22 bits.

Step 3: Derive the network address and mask

A /22 mask is 255.255.252.0. The third-octet block size is 256 - 252 = 4. Valid /22 network starts in the third octet are 0, 4, 8, 12, 16, 20, 24, and so on. Because 20 is aligned to a block boundary, the summary is:

172.16.20.0/22
Mask: 255.255.252.0
Range: 172.16.20.0 through 172.16.23.255

The summary covers exactly the four intended /24 networks.

Another binary example

The networks 10.50.8.0/24 through 10.50.11.0/24 use these third-octet values:

8  = 00001000
9  = 00001001
10 = 00001010
11 = 00001011

They share the first six bits of the third octet, so the result is 10.50.8.0/22, with mask 255.255.252.0. A summary such as 10.50.9.0/22 would be invalid as a network address because 9 is not aligned to a /22 block boundary.

Before configuring a summary, confirm that it covers every intended subnet, does not unintentionally include unrelated active networks, and leaves room for planned growth without becoming excessively broad.

Where Manual EIGRP Summaries Are Configured

Configure an EIGRP summary on the outbound interface toward the receiving neighbor or routing domain. The summary is advertised out that interface; it is not configured globally under the EIGRP process in classic mode.

Common locations include:

• A branch router interface facing the core.
• A distribution router interface facing a campus core.
• A hub router interface facing a group of spokes.
• An interface at a redistribution or routing-domain boundary.

A single router can advertise different summaries from different interfaces. For example, it could send a campus aggregate toward the core and a narrower, separately designed aggregate toward another routing domain.

Classic-Mode EIGRP Configuration

In classic mode, the EIGRP autonomous system number identifies the local EIGRP process. Neighboring routers must use a compatible process number, and the relevant interfaces must be activated for EIGRP.

Basic IPv4 lab context

router eigrp 100
 network 172.16.20.0 0.0.3.255
 no auto-summary

The wildcard mask 0.0.3.255 matches the four /24 networks from 172.16.20.0 through 172.16.23.255. Adapt the network statement to the actual interface addresses in the lab.

Apply a manual summary

interface GigabitEthernet0/0
 ip summary-address eigrp 100 172.16.20.0 255.255.252.0

This command applies the 172.16.20.0/22 summary to EIGRP process 100 on GigabitEthernet0/0. The interface must participate in that EIGRP process. The interface should be the path toward the neighbor that should receive the aggregate.

Remove a summary

interface GigabitEthernet0/0
 no ip summary-address eigrp 100 172.16.20.0 255.255.252.0

Named-Mode EIGRP Context

Named EIGRP mode uses a different configuration hierarchy from classic router eigrp <ASN> mode. The design principles remain the same: select the correct IPv4 address family, identify the outbound interface, and configure the summary in that interface context.

router eigrp CAMPUS
 address-family ipv4 unicast autonomous-system 100
  af-interface GigabitEthernet0/0
   summary-address 172.16.20.0 255.255.252.0

Exact named-mode syntax and feature support can vary by Cisco IOS platform and release. Verify the available commands with context-sensitive help and confirm the resulting running configuration.

EIGRP Behavior After Summarization

After the summary is configured, the receiving neighbor normally learns the aggregate instead of the selected component routes through that interface. The summarizing router can still retain the individual component routes in its own routing table; summarization changes what is advertised out the selected interface, not necessarily what the local router knows.

EIGRP also creates a local summary route pointing to Null0. Null0 is a Cisco IOS discard interface. It prevents a forwarding loop when traffic matches the aggregate but does not match any valid component route in the local routing table.

For example, if a router advertises 192.168.0.0/16 while it owns only 192.168.10.0/24 and 192.168.11.0/24, a packet for unused 192.168.200.0/24 may arrive because the remote router learned the /16. The local summary-to-Null0 route discards that packet instead of allowing it to loop or follow an incorrect default path.

The local routing output commonly identifies this as an EIGRP summary route and shows the summary with an EIGRP administrative distance. The exact display formatting varies by IOS release.

Metrics, Administrative Distance, and Route Selection

By default, EIGRP advertises a summary metric based on the best component metric represented by that summary. The manual summary command also supports an optional administrative-distance argument on platforms that provide it:

interface GigabitEthernet0/0
 ip summary-address eigrp 100 172.16.20.0 255.255.252.0 90

The final value changes the administrative distance of the locally installed summary route. Administrative distance is a trust value used when routes to the same prefix are learned from different sources. It is not the EIGRP metric.

Routing decisions should be considered in this order:

1. Longest-prefix match: among matching routes, the most-specific prefix wins. A /24 beats a /16 regardless of the /16's metric.
2. Administrative distance: if competing routes have the same prefix length, the route source with the lower administrative distance is preferred.
3. Metric: when routes come from the same protocol and have the same prefix length, EIGRP compares its composite metric.

Therefore, if one router advertises 10.30.0.0/16 and another path provides 10.30.50.0/24, traffic for 10.30.50.0/24 follows the /24 because it is more specific. Other destinations within the /16 can use the summary.

Verification

Verify the neighbor relationship before diagnosing route advertisement. A summary cannot be learned through an adjacency that is down.

On the summarizing router, inspect the routing table for the summary-to-Null0 entry. On the downstream router, confirm that the aggregate is present and that component routes are not being learned through the summarized interface. A separate path may legitimately advertise a component route, so compare the route source and next hop.

Design Rules and Limitations

• Place summaries at logical topology boundaries, not arbitrarily inside a flat network.
• Allocate contiguous, hierarchical address blocks so that safe aggregation is possible.
• Use the narrowest summary that represents the intended networks and planned growth.
• Avoid overlapping unrelated active networks unless the forwarding behavior has been explicitly designed and tested.
• Consider redundant paths. A summary can direct traffic toward a router that does not actually have every destination covered by the aggregate.
• Test failure conditions, including withdrawal of a component route and traffic to an unused address inside the summary.
• Distinguish internal EIGRP interface summarization from summarization performed while redistributing routes between protocols. They use different configuration and design considerations.

Troubleshooting Manual Summarization

Example: an overly broad summary

Suppose a router owns only 192.168.10.0/24 and 192.168.11.0/24 but advertises 192.168.0.0/16. A remote router can send traffic for 192.168.200.0/24 toward the summarizing router. Because no local component route matches that destination, the EIGRP summary route to Null0 discards it.

The correction is to advertise 192.168.10.0/23 if the two /24 networks are the intended contiguous block, or to use another design when the networks cannot be safely summarized.

Exam-Relevant Notes

• Manual EIGRP summarization is configured on an interface, not globally under the classic EIGRP process.
• The configured interface is the outbound interface toward the receiving neighbor.
• ip summary-address eigrp <ASN> <network> <mask> requires the EIGRP process number, summary address, and mask.
• EIGRP commonly installs a local summary route to Null0.
• Longest-prefix match is evaluated before administrative distance and routing-protocol metric.
• no auto-summary prevents legacy classful automatic summarization.
• A summary changes advertisements out an interface; it does not necessarily remove component routes from the local routing table.
• Always verify the EIGRP adjacency before investigating why a summary is not being learned.

Related Study

Review Configure OSPF to compare routing-protocol configuration concepts, and use the Computer Network Explained lesson for broader routing and addressing context.