Chapter 6: EIGRP

Enhanced Interior Gateway Routing Protocol (EIGRP) is an advanced distance vector routing protocol developed by Cisco Systems. As the name suggests, EIGRP is an enhancement of another Cisco routing protocol IGRP (Interior Gateway Routing Protocol). IGRP is an older classful, distance vector routing protocol, now obsolete since IOS 12.3.

EIGRP includes features found in link-state routing protocols. EIGRP is suited for many different topologies and media. In a well-designed network, EIGRP can scale to include multiple topologies and can provide extremely quick convergence times with minimal network traffic.

This chapter introduces EIGRP and provides basic configuration commands to enable it on a Cisco IOS router. It also explores the operation of the routing protocol and provides more detail on how EIGRP determines best path.

1. Features of EIGRP

EIGRP was initially released in 1992 as a proprietary protocol available only on Cisco devices. However, in 2013, Cisco released a basic functionality of EIGRP as an open standard to the IETF, as an informational RFC. This means that other networking vendors can now implement EIGRP on their equipment to interoperate with both Cisco and non-Cisco routers running EIGRP. However, advanced features of EIGRP, such as EIGRP stub, needed for the Dynamic Multipoint Virtual Private Network (DMVPN) deployment, will not be released to the IETF. As an informational RFC, Cisco will continue to maintain control of EIGRP.

EIGRP includes features of both link-state and distance vector routing protocols. However, EIGRP is still based on the key distance vector routing protocol principle, in which information about the rest of the network is learned from directly connected neighbors.

EIGRP is an advanced distance vector routing protocol that includes features not found in other distance vector routing protocols like RIP and IGRP.

In Cisco IOS Release 15.0(1)M, Cisco introduced a new EIGRP configuration option called named EIGRP. Named EIGRP enables the configuration of EIGRP for both IPv4 and IPv6 under a single configuration mode. This helps eliminate configuration complexity that occurs when configuring EIGRP for both IPv4 and IPv6. Named EIGRP is beyond the scope of this course.

Features of EIGRP include:

• Diffusing Update Algorithm - As the computational engine that drives EIGRP, the Diffusing Update Algorithm (DUAL) resides at the center of the routing protocol. DUAL guarantees loop-free and backup paths throughout the routing domain. Using DUAL, EIGRP stores all available backup routes for destinations so that it can quickly adapt to alternate routes when necessary.

• Establishing Neighbor Adjacencies - EIGRP establishes relationships with directly connected routers that are also enabled for EIGRP. Neighbor adjacencies are used to track the status of these neighbors.

• Reliable Transport Protocol - The Reliable Transport Protocol (RTP) is unique to EIGRP and provides delivery of EIGRP packets to neighbors. RTP and the tracking of neighbor adjacencies set the stage for DUAL.

• Partial and Bounded Updates - EIGRP uses the terms partial and bounded when referring to its updates. Unlike RIP, EIGRP does not send periodic updates and route entries do not age out. The term partial means that the update only includes information about the route changes, such as a new link or a link becoming unavailable. The term bounded refers to the propagation of partial updates that are sent only to those routers that the changes affect. This minimizes the bandwidth that is required to send EIGRP updates.

• Equal and Unequal Cost Load Balancing - EIGRP supports equal cost load balancing and unequal cost load balancing, which allows administrators to better distribute traffic flow in their networks.

Note: The term “hybrid routing” protocol may be used in some older documentation to define EIGRP. However, this term is misleading because EIGRP is not a hybrid between distance vector and link-state routing protocols. EIGRP is solely a distance vector routing protocol; therefore, Cisco no longer uses this term to refer to it.

2. Protocol Dependent Modules

EIGRP has the capability for routing different protocols, including IPv4 and IPv6. EIGRP does so by using protocol-dependent modules (PDMs). PDMs were also used to support the now obsolete Novell IPX and Apple Computer’s AppleTalk network layer protocols.

PDMs are responsible for network layer protocol-specific tasks. An example is the EIGRP module that is responsible for sending and receiving EIGRP packets that are encapsulated in IPv4. This module is also responsible for parsing EIGRP packets and informing DUAL of the new information that is received. EIGRP asks DUAL to make routing decisions, but the results are stored in the IPv4 routing table.

PDMs are responsible for the specific routing tasks for each network layer protocol, including:

• Maintaining the neighbor and topology tables of EIGRP routers that belong to that protocol suite

• Building and translating protocol-specific packets for DUAL

• Interfacing DUAL to the protocol-specific routing table

• Computing the metric and passing this information to DUAL

• Implementing filtering and access lists

• Performing redistribution functions to and from other routing protocols

• Redistributing routes that are learned by other routing protocols

When a router discovers a new neighbor, it records the neighbor’s address and interface as an entry in the neighbor table. One neighbor table exists for each protocol-dependent module, such as IPv4. EIGRP also maintains a topology table. The topology table contains all destinations that are advertised by neighboring routers. There is also a separate topology table for each PDM.

3. Reliable Transport Protocol

EIGRP was designed as a network layer independent routing protocol. Because of this design, EIGRP cannot use the services of UDP or TCP. Instead, EIGRP uses the Reliable Transport Protocol (RTP) for the delivery and reception of EIGRP packets. This allows EIGRP to be flexible and can be used for protocols other than those from the TCP/IP protocol suite, such as the now obsolete IPX and AppleTalk protocols.

The figure conceptually shows how RTP operates.

Although “reliable” is part of its name, RTP includes both reliable delivery and unreliable delivery of EIGRP packets, similar to TCP and UDP, respectively. Reliable RTP requires an acknowledgment to be returned by the receiver to the sender. An unreliable RTP packet does not require an acknowledgment. For example, an EIGRP update packet is sent reliably over RTP and requires an acknowledgment. An EIGRP Hello packet is also sent over RTP, but unreliably. This means that EIGRP Hello packets do not require an acknowledgment.

RTP can send EIGRP packets as unicast or multicast.

• Multicast EIGRP packets for IPv4 use the reserved IPv4 multicast address 224.0.0.10.

• Multicast EIGRP packets for IPv6 are sent to the reserved IPv6 multicast address FF02::A.

4. EIGRP Packet Types

EIGRP uses five different packet types, some in pairs. EIGRP packets are sent using either RTP reliable or unreliable delivery and can be sent as a unicast, multicast, or sometimes both. EIGRP packet types are also called EIGRP packet formats or EIGRP messages.

As shown in Figure 1, the five EIGRP packet types include:

Hello packets - Used for neighbor discovery and to maintain neighbor adjacencies.

• Sent with unreliable delivery

• Multicast (on most network types)

Update packets - Propagates routing information to EIGRP neighbors.

• Sent with reliable delivery

• Unicast or multicast

Acknowledgment packets - Used to acknowledge the receipt of an EIGRP message that was sent using reliable delivery.

• Sent with unreliable delivery

• Unicast

Query packets - Used to query routes from neighbors.

• Sent with reliable delivery

• Unicast or multicast

Reply packets - Sent in response to an EIGRP query.

• Sent with reliable delivery

• Unicast

Figure 2 shows that EIGRP messages are typically encapsulated in IPv4 or IPv6 packets. EIGRP for IPv4 messages use IPv4 as the network layer protocol. The IPv4 protocol field uses 88 to indicate the data portion of the packet is an EIGRP for IPv4 message. EIGRP for IPv6 messages are encapsulated in IPv6 packets using the next header field of 88. Similar to the protocol field for IPv4, the IPv6 next header field indicates the type of data carried in the IPv6 packet.

5. EIGRP Hello Packets

EIGRP uses small Hello packets to discover other EIGRP-enabled routers on directly connected links. Hello packets are used by routers to form EIGRP neighbor adjacencies, also known as neighbor relationships.

EIGRP Hello packets are sent as IPv4 or IPv6 multicasts, and use RTP unreliable delivery. This means that the receiver does not reply with an acknowledgment packet.

• The reserved EIGRP multicast address for IPv4 is 224.0.0.10.

• The reserved EIGRP multicast address for IPv6 is FF02::A.

EIGRP routers discover neighbors and establish adjacencies with neighbor routers using the Hello packet. On most modern networks, EIGRP Hello packets are sent as multicast packets every five seconds. However, on multipoint, non-broadcast multiple access (NBMA) networks with access links of T1 (1.544 Mb/s) or slower, Hello packets are sent as unicast packets every 60 seconds.

Note: NBMA networks using slower interfaces include legacy X.25, Frame Relay, and Asynchronous Transfer Mode (ATM).

EIGRP also uses Hello packets to maintain established adjacencies. An EIGRP router assumes that as long as it receives Hello packets from a neighbor, the neighbor and its routes remain viable.

EIGRP uses a Hold timer to determine the maximum time the router should wait to receive the next Hello before declaring that neighbor as unreachable. By default, the hold time is three times the Hello interval, or 15 seconds on most networks and 180 seconds on low-speed NBMA networks. If the hold time expires, EIGRP declares the route as down and DUAL searches for a new path by sending out queries.

6. EIGRP Update and Acknowledgment Packets

EIGRP Update Packets

EIGRP sends Update packets to propagate routing information. Update packets are sent only when necessary. EIGRP updates contain only the routing information needed and are sent only to those routers that require it.

Unlike the older distance vector routing protocol RIP, EIGRP does not send periodic updates and route entries do not age out. Instead, EIGRP sends incremental updates only when the state of a destination changes. This may include when a new network becomes available, an existing network becomes unavailable, or a change occurs in the routing metric for an existing network.

EIGRP uses the terms partial update and bounded update when referring to its updates. A partial update means that the update only includes information about route changes. A bounded update refers to the sending of partial updates only to the routers that are affected by the changes. Bounded updates help EIGRP minimize the bandwidth that is required to send EIGRP updates.

EIGRP Update packets use reliable delivery, which means the sending router requires an acknowledgment. Update packets are sent as a multicast when required by multiple routers, or as a unicast when required by only a single router. In the figure, the updates are sent as unicasts because the links are point-to-point.

EIGRP Acknowledgment Packets

EIGRP sends Acknowledgment (ACK) packets when reliable delivery is used. An EIGRP acknowledgment is an EIGRP Hello packet without any data. RTP uses reliable delivery for Update, Query, and Reply packets. EIGRP Acknowledgment packets are always sent as an unreliable unicast. Unreliable delivery makes sense; otherwise, there would be an endless loop of acknowledgments.

In the figure, R2 has lost connectivity to the LAN attached to its Gigabit Ethernet interface. R2 immediately sends an update to R1 and R3 noting the downed route. R1 and R3 respond with an acknowledgment to let R2 know that they have received the update.

Note: Some documentation refers to the Hello and acknowledgment as a single type of EIGRP packet.

7. EIGRP Query and Reply Packets

EIGRP Query Packets

DUAL uses Query and Reply packets when searching for networks and other tasks. Queries and replies use reliable delivery. Queries can use multicast or unicast, whereas replies are always sent as unicast.

In the figure, R2 has lost connectivity to the LAN and it sends out queries to all EIGRP neighbors searching for any possible routes to the LAN. Because queries use reliable delivery, the receiving router must return an EIGRP acknowledgment. The acknowledgment informs the sender of the query that it has received the query message. To keep this example simple, acknowledgments were omitted in the graphic.

EIGRP Reply Packets

All neighbors must send a reply, regardless of whether or not they have a route to the downed network. Because replies also use reliable delivery, routers, such as R2, must send an acknowledgment.

It may not be obvious why R2 would send out a query for a network it knows is down. Actually, only R2’s interface that is attached to the network is down. Another router could be attached to the same LAN and have an alternate path to this same network. Therefore, R2 queries for such a router before completely removing the network from its topology table.

8. Encapsulating EIGRP Messages

he data portion of an EIGRP message is encapsulated in a packet. This data field is called type, length, value (TLV). The types of TLVs relevant to this course are EIGRP parameters, IP internal routes, and IP external routes.

The EIGRP packet header is included with every EIGRP packet, regardless of its type. The EIGRP packet header and TLV are then encapsulated in an IPv4 packet. In the IPv4 packet header, the protocol field is set to 88 to indicate EIGRP, and the IPv4 destination address is set to the multicast 224.0.0.10. If the EIGRP packet is encapsulated in an Ethernet frame, the destination MAC address is also a multicast address, 01-00-5E-00-00-0A.

Figures 1 to 4 show the Data Link Ethernet Frame. EIGRP for IPv4 is encapsulated in an IPv4 packet. EIGRP for IPv6 would use a similar type of encapsulation. EIGRP for IPv6 is encapsulated using an IPv6 header. The IPv6 destination address would be the multicast address FF02::A and the next header field would be set to 88.

9. EIGRP Network Topology

Figure 1 displays the topology that is used in this chapter to configure EIGRP for IPv4.

The routers in the topology have a starting configuration that includes addresses on the interfaces. There is currently no static routing or dynamic routing configured on any of the routers.

Figures 2, 3, and 4 display the interface configurations for the three EIGRP routers in the topology. Only routers R1, R2, and R3 are part of the EIGRP routing domain. The ISP router is used as the routing domain’s gateway to the Internet.

10. Autonomous System Numbers

EIGRP uses the router eigrp autonomous-system command to enable the EIGRP process. The autonomous system number referred to in the EIGRP configuration is not associated with the Internet Assigned Numbers Authority (IANA) globally assigned autonomous system numbers used by external routing protocols.

So what is the difference between the IANA globally assigned autonomous system number and the EIGRP autonomous system number?

An IANA globally assigned autonomous system is a collection of networks under the administrative control of a single entity that presents a common routing policy to the Internet. In the figure, companies A, B, C, and D are all under the administrative control of ISP1. ISP1 presents a common routing policy for all of these companies when advertising routes to ISP2.

The guidelines for the creation, selection, and registration of an autonomous system are described in RFC 1930. Global autonomous system numbers are assigned by IANA, the same authority that assigns IP address space. The local regional Internet registry (RIR) is responsible for assigning an autonomous system number to an entity from its block of assigned autonomous system numbers. Prior to 2007, assigned autonomous system numbers were 16-bit numbers ranging from 0 to 65,535. Today, 32-bit autonomous system numbers are assigned thereby increasing the number of available autonomous system numbers to over 4 billion.

Usually, only Internet Service Providers (ISPs), Internet backbone providers, and large institutions connecting to other entities require an autonomous system number. These ISPs and large institutions use the exterior gateway routing protocol, Border Gateway Protocol (BGP), to propagate routing information. BGP is the only routing protocol that uses an actual autonomous system number in its configuration.

The vast majority of companies and institutions with IP networks do not need an autonomous system number, because they are controlled by a larger entity, such as an ISP. These companies use interior gateway protocols, such as RIP, EIGRP, OSPF, and IS-IS to route packets within their own networks. They are one of many independent and separate networks within the autonomous system of the ISP. The ISP is responsible for the routing of packets within its autonomous system and between other autonomous systems.

The autonomous system number used for EIGRP configuration is only significant to the EIGRP routing domain. It functions as a process ID to help routers keep track of multiple running instances of EIGRP. This is required because it is possible to have more than one instance of EIGRP running on a network. Each instance of EIGRP can be configured to support and exchange routing updates for different networks.

11. The router eigrp Command

The Cisco IOS includes the processes to enable and configure several different types of dynamic routing protocols. The router global configuration mode command is used to begin the configuration of any dynamic routing protocol. The topology shown in Figure 1 is used to demonstrate this command.

As shown in Figure 2, when followed by a question mark (?), the router global configuration mode command lists of all the available routing protocols supported by this specific IOS release running on the router.

The following global configuration mode command is used to enter the router configuration mode for EIGRP and begin the configuration of the EIGRP process:

Router(config)# router eigrp autonomous-system

The autonomous-system argument can be assigned to any 16-bit value between the number 1 and 65,535. All routers within the EIGRP routing domain must use the same autonomous system number.

Figure 3 shows the configuration of the EIGRP process on routers R1, R2, and R3. Notice that the prompt changes from a global configuration mode prompt to router configuration mode.

In this example, 1 identifies this particular EIGRP process running on this router. To establish neighbor adjacencies, EIGRP requires all routers in the same routing domain to be configured with the same autonomous system number. In Figure 3, that same EIGRP is enabled on all three routers using the same autonomous system number of 1.

Note: EIGRP and OSPF can support multiple instances of the routing protocol. However, this multiple routing protocol implementation is not usually needed or recommended.

The router eigrp autonomous-system command does not start the EIGRP process itself. The router does not start sending updates. Rather, this command only provides access to configure the EIGRP settings.

To completely remove the EIGRP routing process from a device, use the no router eigrp autonomous-system global configuration mode command, which stops the EIGRP process and removes all existing EIGRP router configurations.

12. EIGRP Router ID

The EIGRP router ID is used to uniquely identify each router in the EIGRP routing domain.

The router ID is used in both EIGRP and OSPF routing protocols. However, the role of the router ID is more significant in OSPF. In EIGRP IPv4 implementations, the use of the router ID is not that apparent. EIGRP for IPv4 uses the 32-bit router ID to identify the originating router for redistribution of external routes. The need for a router ID becomes more evident in the discussion of EIGRP for IPv6. While the router ID is necessary for redistribution, the details of EIGRP redistribution are beyond the scope of this curriculum. For purposes of this curriculum, it is only necessary to understand what the router ID is and how it is determined.

To determine its router ID, a Cisco IOS router will use the following three criteria in order:

1. Use the address configured with the eigrp router-id ipv4-address router configuration mode command.

2. If the router ID is not configured, choose the highest IPv4 address of any of its loopback interfaces.

3. If no loopback interfaces are configured, choose the highest active IPv4 address of any of its physical interfaces.

If the network administrator does not explicitly configure a router ID using the eigrp router-id command, EIGRP generates its own router ID using either a loopback or physical IPv4 address. A loopback address is a virtual interface and is automatically in the up state when configured. The interface does not need to be enabled for EIGRP, meaning that it does not need to be included in one of the EIGRP network commands. However, the interface must be in the up/up state.

Using the criteria described above, the figure shows the default EIGRP router IDs that are determined by the routers’ highest active IPv4 address.

Note: The eigrp router-id command is used to configure the router ID for EIGRP. Some versions of IOS will accept the command router-id, without first specifying eigrp. The running-config, however, will display eigrp router-id regardless of which command is used.

13. Configuring the EIGRP Router ID

The eigrp router-id ipv4-address router configuration command is the preferred method used to configure the EIGRP router ID. This method takes precedence over any configured loopback or physical interface IPv4 addresses.

Note: The IPv4 address used to indicate the router ID is actually any 32-bit number displayed in dotted-decimal notation.

The ipv4-address router ID can be configured with any IPv4 address except 0.0.0.0 and 255.255.255.255. The router ID should be a unique 32-bit number in the EIGRP routing domain; otherwise, routing inconsistencies can occur.

Figure 1 shows the configuration of the EIGRP router ID for routers R1 and R2.

If a router ID is not explicitly configured, then the router would use its highest IPv4 address configured on a loopback interface. The advantage of using a loopback interface is that unlike physical interfaces, loopbacks cannot fail. There are no actual cables or adjacent devices on which the loopback interface depends for being in the up state. Therefore, using a loopback address for the router ID can provide a more consistent router ID than using an interface address.

If the eigrp router-id command is not used and loopback interfaces are configured, EIGRP chooses the highest IPv4 address of any of its loopback interfaces. The following commands are used to enable and configure a loopback interface:

Router(config)# interface loopbacknumber

Router(config-if)# ip address ipv4-address subnet-mask

Verifying the EIGRP Process

Figure 2 shows the show ip protocols output for R1, including its router ID. The show ip protocols command displays the parameters and current state of any active routing protocol processes, including both EIGRP and OSPF. The show ip protocols command displays different types of output specific to each routing protocol.

Use the Syntax Checker in Figure 3 to configure and verify the router ID for R3.

EIGRP router configuration mode allows for the configuration of the EIGRP routing protocol. Figure 1 shows that R1, R2, and R3 all have networks that should be included within a single EIGRP routing domain. To enable EIGRP routing on an interface, use the network ipv4-network-addressrouter configuration mode command. The ipv4-network-address is the classful network address for each directly connected network.

The network command has the same function as in all IGP routing protocols. The network command in EIGRP:

• Enables any interface on this router that matches the network address in the network router configuration mode command to send and receive EIGRP updates.

• The network of the interfaces is included in EIGRP routing updates.

Figure 2 shows the network commands required to configure EIGRP on R1. In the figure, a single classful network statement, network 172.16.0.0, is used on R1 to include both interfaces in subnets 172.16.1.0/24 and 172.16.3.0/30. Notice that only the classful network address is used.

Figure 3 shows the network command used to enable EIGRP on R2’s interfaces for subnets 172.16.1.0/24 and 172.16.2.0/24. When EIGRP is configured on R2’s S0/0/0 interface, DUAL sends a notification message to the console stating that a neighbor adjacency with another EIGRP router on that interface has been established. This new adjacency happens automatically because both R1 and R2 use the same autonomous system number (i.e., 1), and both routers now send updates on their interfaces in the 172.16.0.0 network.

DUAL automatically generates the notification message because the eigrp log-neighbor-changes router configuration mode command is enabled by default. Specifically, the command helps verify neighbor adjacencies during configuration of EIGRP and displays any changes in EIGRP neighbor adjacencies, such as when an EIGRP adjacency has been added or removed.

14. The network Command and Wildcard Mask

By default, when using the network command and an IPv4 network address, such as 172.16.0.0, all interfaces on the router that belong to that classful network address are enabled for EIGRP. However, there may be times when the network administrator does not want to include all interfaces within a network when enabling EIGRP. For example, in Figure 1, assume that an administrator wants to enable EIGRP on R2, but only for the subnet 192.168.10.8 255.255.255.252, on the S0/0/1 interface.

To configure EIGRP to advertise specific subnets only, use the wildcard-mask option with the network command:

Router(config-router)# networknetwork-address [wildcard-mask]

A wildcard mask is similar to the inverse of a subnet mask. In a subnet mask, binary 1s are significant while binary 0s are not. In a wildcard mask, binary 0s are significant, while binary 1s are not. For example, the inverse of subnet mask 255.255.255.252 is 0.0.0.3.

Calculating a wildcard mask may seem daunting at first but it’s actually pretty easy to do. To calculate the inverse of the subnet mask, subtract the subnet mask from 255.255.255.255 as follows:

   255.255.255.255

 - 255.255.255.252  

   ---------------

    0.  0.  0.  3   Wildcard mask

Figure 2 continues the EIGRP network configuration of R2. The network 192.168.10.8 0.0.0.3 command specifically enables EIGRP on the S0/0/1 interface, a member of the 192.168.10.8 255.255.255.252 subnet.

Configuring a wildcard mask is the official command syntax of the EIGRP network command. However, the Cisco IOS versions also accepts a subnet mask to be used instead. For example, Figure 3 configures the same S0/0/1 interface on R2, but this time using a subnet mask in thenetwork command. Notice in the output of theshow running-config command, the IOS converted the subnet mask command to its wildcard mask.

Use the Syntax Checker in Figure 4 to configure the EIGRP network commands for router R3.

  Passive Interface

As soon as a new interface is enabled within the EIGRP network, EIGRP attempts to form a neighbor adjacency with any neighboring routers to send and receive EIGRP updates.

At times it may be necessary, or advantageous, to include a directly connected network in the EIGRP routing update, but not allow any neighbor adjacencies off of that interface to form. Thepassive-interface command can be used to prevent the neighbor adjacencies. There are two primary reasons for enabling the passive-interface command:

• To suppress unnecessary update traffic, such as when an interface is a LAN interface, with no other routers connected

• To increase security controls, such as preventing unknown rogue routing devices from receiving EIGRP updates

Figure 1 shows R1, R2, and R3 do not have neighbors on their GigabitEthernet 0/0 interfaces.

The passive-interface router configuration mode command disables the transmission and receipt of EIGRP Hello packets on these interfaces.

Router(config)# router eigrp as-number

Router(config-router)# passive-interface interface-type interface-number

Figure 2 shows the passive-interface command configured to suppress Hello packets on the LANs for R1 and R3. R2 is configured using the Syntax Checker.

Without a neighbor adjacency, EIGRP cannot exchange routes with a neighbor. Therefore, thepassive-interface command prevents the exchange of routes on the interface. Although EIGRP does not send or receive routing updates on an interface configured with the passive-interface command, it still includes the address of the interface in routing updates sent out of other non-passive interfaces.

Note: To configure all interfaces as passive, use the passive-interface default command. To disable an interface as passive, use the no passive-interface interface-type interface-number command.

An example of using the passive interface to increase security controls is when a network must connect to a third-party organization, for which the local administrator has no control, such as when connecting to an ISP network. In this case, the local network administrator would need to advertise the interface link through their own network, but would not want the third-party organization to receive or send routing updates to the local routing device, as this is a security risk.

Verifying the Passive Interface

To verify whether any interface on a router is configured as passive, use the show ip protocols privileged EXEC mode command, as shown in Figure 3. Notice that although R3’s GigabitEthernet 0/0 interface is a passive interface, EIGRP still includes the interface’s network address of 192.168.1.0 in its routing updates.

Use the Syntax Checker in Figure 4 to configure R2 to suppress EIGRP Hello packets on its GigabitEthernet 0/0 interface.

15. Verifying EIGRP: Examining Neighbors

Before EIGRP can send or receive any updates, routers must establish adjacencies with their neighbors. EIGRP routers establish adjacencies with neighbor routers by exchanging EIGRP Hello packets.

Use the show ip eigrp neighbors command to view the neighbor table and verify that EIGRP has established an adjacency with its neighbors. For each router, you should be able to see the IPv4 address of the adjacent router and the interface that this router uses to reach that EIGRP neighbor. Using this topology, each router has two neighbors listed in the neighbor table.

The column headers in the show ip eigrp neighbors command output identify the following:

• H - Lists the neighbors in the order that they were learned.

• Address - IPv4 address of the neighbor.

• Interface - Local interface on which this Hello packet was received.

• Hold - Current hold time. When a Hello packet is received, this value is reset to the maximum hold time for that interface, and then counts down to zero. If zero is reached, the neighbor is considered down.

• Uptime - Amount of time since this neighbor was added to the neighbor table.

• Smooth Round Trip Timer (SRTT) and Retransmission Timeout (RTO) - Used by RTP to manage reliable EIGRP packets.

• Queue Count - Should always be zero. If more than zero, then EIGRP packets wait to be sent.

• Sequence Number - Used to track updates, queries, and reply packets.

The show ip eigrp neighbors command is very useful for verifying and troubleshooting EIGRP.

If a neighbor is not listed after adjacencies have been established with a router’s neighbors, check the local interface to ensure it is activated with theshow ip interface brief command. If the interface is active, try to ping the IPv4 address of the neighbor. If the ping fails, it means that the neighbor interface is down and must be activated. If the ping is successful and EIGRP still does not see the router as a neighbor, examine the following configurations:

• Are both routers configured with the same EIGRP autonomous system number?

• Is the directly connected network included in the EIGRP network statements?

16. Verifying EIGRP: show ip protocols Command

The show ip protocols command is useful to identify the parameters and other information about the current state of any active IPv4 routing protocol processes configured on the router. Theshow ip protocols command displays different types of output specific to each routing protocol.

The output in Figure 1 indicates several EIGRP parameters, including:

1. EIGRP is an active dynamic routing protocol on R1 configured with the autonomous system number 1.

2. The EIGRP router ID of R1 is 1.1.1.1.

3. The EIGRP administrative distances on R1 are internal AD of 90 and external of 170 (default values).

4. By default, EIGRP does not automatically summarize networks. Subnets are included in the routing updates.

5. The EIGRP neighbor adjacencies R1 has with other routers used to receive EIGRP routing updates.

Note: Prior to IOS 15, EIGRP automatic summarization was enabled by default.

The output from the show ip protocols command is useful in debugging routing operations. Information in the Routing Information Sources field can help identify a router suspected of delivering bad routing information. The field lists all the EIGRP routing sources the Cisco IOS software uses to build its IPv4 routing table. For each source, note the following:

• IPv4 address

• Administrative distance

• Time the last update was received from this source

As shown in Figure 2, EIGRP has a default AD of 90 for internal routes and 170 for routes imported from an external source, such as default routes. When compared to other IGPs, EIGRP is the most preferred by the Cisco IOS, because it has the lowest administrative distance. EIGRP has a third AD value of 5, for summary routes.

Verifying EIGRP: Examine the IPv4 routing table

Another way to verify that EIGRP and other functions of the router are configured properly is to examine the IPv4 routing tables with the show ip route command. As with any dynamic routing protocol, the network administrator must verify the information in the routing table to ensure that it is populated as expected, based on configurations entered. For this reason, it is important to have a good understanding of the routing protocol configuration commands, as well as the routing protocol operations and the processes used by the routing protocol to build the IP routing table.

Notice that the outputs used throughout this course are from Cisco IOS 15. Prior to IOS 15, EIGRP automatic summarization was enabled by default. The state of automatic summarization can make a difference in the information displayed in the IPv4 routing table. If a previous version of the IOS is used, automatic summarization can be disabled using the no auto-summary router configuration mode command:

Router(config-router)# no auto-summary

Figure 1 shows the topology for R1, R2, and R3.

In Figure 2, the IPv4 routing table is examined using the show ip route command. EIGRP routes are denoted in the routing table with a D. The letter D was used to represent EIGRP because the protocol is based upon the DUAL algorithm.

The show ip route command verifies that routes received by EIGRP neighbors are installed in the IPv4 routing table. The show ip route command displays the entire routing table, including remote networks learned dynamically, directly connected and static routes. For this reason, it is normally the first command used to check for convergence. After routing is correctly configured on all routers, the show ip route command reflects that each router has a full routing table, with a route to each network in the topology.

Notice that R1 has installed routes to three IPv4 remote networks in its IPv4 routing table:

• 172.16.2.0/24 network, received from router R2 on the Serial0/0/0 interface

• 192.168.1.0/24 network, received from router R2 on the Serial0/0/1 interface

• 192.168.10.8/30 network, received from both R2 on the Serial0/0/0 interface, and from R3 on the Serial0/0/1 interface

R1 has two paths to the 192.168.10.8/30 network, because its cost or metric to reach that network is the same or equal using both routers. These are known as equal cost routes. R1 uses both paths to reach this network, which is known as load balancing. The EIGRP metric is discussed later in this chapter.

Figure 3 displays the routing table for R2. Notice similar results are displayed including an equal cost route for the 192.168.10.4/30 network.

Figure 4 displays the routing table for R3. Similar to the results for R1 and R2, remote networks are learned using EIGRP, including an equal cost route for the 172.16.3.0/30 network.

Packet Tracer - Configuring Basic EIGRP with IPv4

Background/Scenario

In this activity, you will implement basic EIGRP configurations including network commands, passive interfaces, and disabling automatic summarization. You will then verify your EIGRP configuration using a variety of show commands and testing end-to-end connectivity.

Packet Tracer - Configuring Basic EIGRP with IPv4 Instructions

Packet Tracer - Configuring Basic EIGRP with IPv4 - PKA

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