EIGRP – Summarization/Aggregation


Route summarization reduces the amount of information that routers must process, which allows for faster convergence within the network. Summarization also restricts the size of the area that is affected by network changes by hiding detailed topology information from certain areas within the network. Summarization is also used to define a Query boundary for EIGRP. EIGRP supports two types of route summarization;

  1. Automatic route summarization
  2. Manual route summarization

By default, automatic route summarization is in effect when EIGRP is enabled on the router. This is implemented using the auto-summary command. This command allows EIGRP to perform automatic route summarization at Classful boundaries. -> ->

With automatic summarization at the Classful boundary, EIGRP installs a route to the summary address into the EIGRP Topology Table and IP routing table.



In the routing table, the summary route is directly connected to the Null0 interface. The route has a default administrative distance value of 5. (EIGRP Internal routes AD 90, EIGRP External routes AD 170, EIGRP default routes AD 5. The AD for the internal and external routes can be changed under the router configuration mode with the command distance {internal} {external}). When EIGRP performs automatic summarization, the router advertises the summary route and suppresses the more specific routes. In other words, while the summary route is advertised, the more specific prefixes are suppressed in updates to EIGRP neighbors.

This default behavior works well in basic networks. However, it can have an adverse impact in discontiguous networks. A discontiguous network comprises a major network that separates another major network.


The summary route will be installed with a next hop interface of Null0. The Null0 interface is a ‘bit-bucket’.  Any packets sent to this interface are effectively discarded. It is important to know that EIGRP will not automatically summarize external networks unless there is an internal network that will be included in the summary. All external routes, EIGRP will not automatically summarize these prefixes if no internal prefix exists in the EIGRP routing table. So EIGRP will therefore not add a summary route to either the Topology Table or IP routing table for these entries.

Unlike EIGRP automatic summarization, EIGRP manual route summarization is configured and implemented at the interface level using the ip summary-address eigrp [ASN] [network] [mask] [distance] [leak-map <name>]interface configuration command. By default, an EIGRP summary address is assigned a default administrative distance value of 5.

By default, when manual route summarization is configured, EIGRP will not advertise the more specific route entries that fall within the summarized network entry. The leak-map <name> keyword can be configured to allow EIGRP route leaking wherein EIGRP allows specified specific route entries to be advertised in conjunction with the summary address. Those entries that are not specified in the leak map are still suppressed. When manually summarizing routes, it is important to be as specific as possible. Otherwise, the configuration might result in a black-holing of traffic.

By default, when a summary route is created for EIGRP, the router advertises the summary address with a metric equal to the minimum of all more specific routes. In other words, the summary address will have the same metric as the lowest most specific route included in the creation of the summary address.


A subnet address may be thought of as a summarization of a group of sub-subnets. And a major network address may be thought of as a summarization of a group of subnet addresses. In each case, the summarization is achieved by reducing the length of the address mask. Address aggregation takes summarization a step further by breaking the class limits of major network addresses. An aggregate address represents a numerically contiguous group of network addresses.

When designing a supernet, it is important that the member addresses should form a complete, contiguous set of the formerly masked bits.

11111111.11111111.11111111.00000000 = 24 bit mask

11000000.10101000.00010000.00000000 =

11000000.10101000.000100010.0000000 =

11000000.10101000.000100100.0000000 =

11000000.10101000.000100110.0000000 =

11000000.10101000.000101000.0000000 =

11000000.10101000.000101010.0000000 =

11000000.10101000.000101100.0000000 =

11000000.10101000.000101110.0000000 =

11000000.10101000.000110000.0000000 =

11000000.10101000.000110010.0000000 =

11000000.10101000.000110100.0000000 =

11000000.10101000.000110110.0000000 =

11000000.10101000.000111000.0000000 =

11000000.10101000.000111010.0000000 =

11000000.10101000.000111100.0000000 =

11000000.10101000.000111110.0000000 =

11000000.10101000.000100000.0000000 =

The 20-bit mask of the aggregate address is four bits less than the mask of the member addresses. Of the four “difference” bits, notice that they include every possible bit combination from 0000 to 1111. Failure to follow this design rule could make the addressing scheme confusing, could reduce the efficiency of aggregate routes, and might lead to routing loops and black holes.

The obvious advantage of summary addressing is the conservation of network resources. Bandwidth is conserved by advertising fewer routes, and CPU cycles are conserved by processing fewer routes. Most important, memory is conserved by reducing the size of the route tables.

Passive Interface

When EIGRP is enabled for a network, the router begins to send out Hello packets on all interfaces that fall within the specified network range. This allows EIGRP to dynamically discover neighbors and establish network relationships. This is desired on interfaces that are actually connected to physical media, e.g. Ethernet and Serial interfaces. However, this default behavior also results in an unnecessary waste of route resources on logical interfaces, such as Loopback interfaces, that will never have any other device connected to them that the router could ever establish an EIGRP neighbor relationship with.

To over come this waste of router resources we run a command under router  configuration called passive interface [name|default], name been the specific interface and default been all interfaces. EIGRP packets are not sent out on passive interfaces. No neighbor relationship will ever be established between passive interfaces.

If we say we had around 25 loopback interfaces that we wanted to advertise into EIGRP and also had 2 physical interface where we want to form EIGRP neighbor relationship. It would be easier to use the passive interface default command and then specific the interface you do want EIGRP packets to be sent out with the no passive interface [name] command.

To verify what interface have passive interface running on them use the show ip protocols command;