Chapter 02: Configuring the Enhanced Interior Gateway Routing Protocol (Part02)

Planning EIGRP Routing Implementations

This section describes how to plan and document an EIGRP deployment.

When preparing to deploy EIGRP in a network, you first need to gather the requirements, determine the existing network state, and consider different deployment options. Considerations for EIGRP include the following:

• IP addressing plan— The IP addressing plan governs how EIGRP can be deployed and how well the EIGRP deployment will scale. A detailed IP subnet and addressing plan must be produced, and should be hierarchical to enable EIGRP summarization, allow the network to scale more easily, and to optimize EIGRP behavior.

• Network topology— The topology consists of the devices (routers, switches, and so on) and the links connecting them. A detailed network topology should be created to assess EIGRP scalability requirements and to determine which EIGRP features might be required (for example, EIGRP stub routing).

• EIGRP traffic engineering— By changing the interface metrics, EIGRP traffic engineering can be deployed to improve bandwidth utilization and enable the administrator to have control over traffic patterns.

After you have assessed the requirements, you can create the implementation plan. The information necessary to implement EIGRP routing includes the following:

• The IP addresses to be configured on individual router interfaces.

• The EIGRP autonomous system number, used to enable EIGRP. The autonomous system number must be the same on all the routers in the EIGRP domain.

• A list of routers on which EIGRP is to be enabled along with the connected networks that are to run EIGRP and that need to be advertised (per individual router).

• Metrics that need to be applied to specific interfaces, for EIGRP traffic engineering. The required metric and the interface where the metric needs to be applied should be specified.

In the implementation plan, the list of tasks for each router in the network must be defined. For EIGRP, the tasks include the following:

• Enabling the EIGRP routing protocol

• Configuring the proper network statements

• Optionally configuring the metric to appropriate interfaces

After implementing EIGRP, it should be verified to confirm proper deployment on each router. Verification tasks include the following:

• Verifying the EIGRP neighbor relationships

• Verifying that the EIGRP topology table is populated with the necessary information

• Verifying that IP routing table is populated with the necessary information

• Verifying that there is connectivity in the network between routers and to other devices

• Verifying that EIGRP behaves as expected in a case of a topology change, by testing link failure and router failure events

After a successful EIGRP deployment, document the solution, the verification process, and the results, for future reference. Documentation should include a topology map, the IP addressing plan, the autonomous system number used, the networks included in EIGRP on each router, and any special metrics configured.

Configuring and Verifying EIGRP

This section covers the commands used to configure and verify EIGRP features. The following topics are discussed:

• Planning and configuring basic EIGRP

• Verifying EIGRP operation

• Using the passive-interface command with EIGRP

• Propagating an EIGRP default route

• EIGRP route summarization

Planning and Configuring Basic EIGRP

This section describes how to plan and configure basic EIGRP, and provides some detailed examples.

Planning for Basic EIGRP

The example network in Figure 2-18 is to be configured for EIGRP. The implementation plan for this network should include the following steps:

Figure 2-18: Sample Network for Planning and Implementing Basic EIGRP.

Step 1	Define the network requirements.
Step 2	Gather the required parameters.
Step 3	Define the EIGRP routing parameters.
Step 4	Configure basic EIGRP.
Step 5	Verify the EIGRP configuration.

Requirements and Parameters

Figure 2-18 shows three routers. Routers R1 and R2 are in EIGRP autonomous system 110. Router R3 is in an external network that is not part of the EIGRP autonomous system. Requirements for basic EIGRP are as follows:

• IP addressing— IP addresses for all device interfaces. Figure 2-18 includes the IP addresses for this example.

• EIGRP routing protocol autonomous system number— Recall that routers in the same EIGRP domain must have the same autonomous system number because each EIGRP process must be started with the same autonomous system number. Autonomous system number 110 is used in this example.

• Interfaces for EIGRP neighborship and networks participating in EIGRP— The interfaces included in EIGRP are used to exchange routing updates and other EIGRP packets between neighbors. The interfaces on which EIGRP is to run and the network numbers that are to participate in EIGRP must be defined. Both Router R1 and R2 have one serial interface and one Fast Ethernet interface included in EIGRP process. EIGRP routers advertise their local networks to all neighbors. Both Router R1 and R2 will advertise their directly to connected networks that are part of the EIGRP domain.

• Interface bandwidth— Because interface bandwidth is part of the EIGRP metric, changing the bandwidth changes the metric of the link. To influence path selection, interface bandwidth can be defined. The actual bandwidth on the serial link between Router R1 and R2 is 512 kbps and the configuration should reflect this so that EIGRP can select the proper route.

Note

In earlier Cisco IOS releases, the default bandwidth on all serial ports was T1, or 1.544 megabits per second (Mbps). In the latest Cisco IOS releases, the default bandwidth varies with interface type.

Note

Similarly, the delay tens-of-microseconds interface configuration command can be configured if the default delay value on the interface does not reflect the delay that should be used in the metric calculation. The parameter specifies the delay in tens of microseconds. Use the show interfaces command to see the default delay.

Basic EIGRP Configuration

The implementation plan should define the following tasks to configure basic EIGRP:

Step 1

Define EIGRP as the routing protocol: To enable EIGRP and define the autonomous system, use the router eigrp autonomous-system-number global configuration command. In this command, the autonomous-system-number identifies the local autonomous system and is used to identify this router to the other EIGRP routers that belong within the internetwork. This value must match on all routers within the internetwork. This command puts the router into router configuration mode.

Step 2

Define the attached networks participating in EIGRP: To indicate which networks are part of the EIGRP autonomous system, use the network network-number [wildcard-mask] router configuration command. Table 2-1 summarizes the parameters of this command. This command defines the interfaces over which EIGRP will attempt to establish EIGRP neighbor relationships and the networks that will be advertised to the EIGRP neighbors.

Table 2-1: network Command Parameters
Open table as spreadsheet

Parameter	Description
network-number	This parameter can be a network, a subnet, or the address of a directly connected interface. It determines which links on the router to advertise to, which links to listen to advertisements on, and which networks are advertised. Network commands should be configured only for interfaces on which the router will send and receive updates.
wildcard-mask	(Optional) An inverse mask used to determine how to interpret the network-number. The mask has wildcard bits, where 0 is a match and 1 is do not care. For example, 0.0.255.255 indicates a match in the first 2 octets.

If you do not use the optional wildcard mask, the EIGRP process assumes that all directly connected networks that are part of the major network will participate in the EIGRP routing process, and EIGRP will attempt to establish EIGRP neighbor relationships from each interface that is part of the overall Class A, B, or C network. Use the optional wildcard mask to identify a specific IP address, subnet, or network. The router interprets the network number using the wildcard mask to determine which connected interfaces will participate in the EIGRP routing process. The router then attempts to establish neighbor relationships on those interfaces. If you want to specify an interface address, use the mask 0.0.0.0 to match all 4 octets of the address. An address and wildcard mask combination of 0.0.0.0 255.255.255.255 matches all interfaces on the router. There is no limit to the number of network commands that can be configured on the router. Note If you incorrectly use a subnet mask in the network command, you probably assume that the router would interpret the mask incorrectly. We assumed that. However, when we tested the configuration, surprisingly, the router “corrected” our mistake instead. For example, the following commands were entered on a router: RouterA(config)#router eigrp 100 RouterA(config-router)#network 10.1.0.0 255.255.0.0 The following shows the configuration on the router; notice how the mask has been "fixed". RouterA#sh run | begin router eigrp router eigrp 100 network 10.1.0.0 0.0.255.255 Because we could not find any documentation on this feature, we encourage you to check your mask configurations carefully.

Step 3

Define the interface bandwidth: For serial links, the link’s bandwidth may be specified for the purposes of sending routing update traffic on the link. If you do not define the bandwidth value for these interfaces, EIGRP assumes that the bandwidth on the link is the default, which varies with interface type. Recall that EIGRP uses bandwidth as part of its metric calculation. If the link is actually slower than the default, the router might not be able to converge, or routing updates might become lost. (The percent of the interface’s bandwidth that EIGRP uses can also be limited, as described in the “EIGRP Link Utilization” section, later in this chapter.) To define the bandwidth, use the bandwidth kilobits interface configuration command. In this command, kilobits indicates the intended bandwidth in kbps. The bandwidth command sets an informational parameter only; you cannot adjust the actual bandwidth of an interface with this command. For some media, such as Ethernet, the bandwidth is fixed. For other media, such as serial lines, you can change the actual bandwidth by adjusting hardware parameters. For both classes of media, you can use the bandwidth configuration command to communicate the current bandwidth to the higher-level protocols. For generic serial interfaces such as PPP and HDLC, set the bandwidth to match the line speed. For Frame Relay point-to-point interfaces, set the bandwidth to the committed information rate (CIR). For Frame Relay multipoint connections, set the bandwidth to the sum of all CIRs, or if the permanent virtual circuits (PVCs) have different CIRs, set the bandwidth to the lowest CIR multiplied by the number of PVCs on the multipoint connection.

Note

The apparent bandwidth of the outgoing interface is also used for other purposes. For example, the TCP protocol adjusts the initial retransmission parameters based on this bandwidth.

Basic Configuration Example

The configuration for Router R1 in Figure 2-18 is shown in Example 2-2. On Router R1, EIGRP is enabled in autonomous system 110 using the router eigrp 110 command. The network 172.16.1.0 0.0.0.255 command starts EIGRP on the Fast Ethernet 0/0 interface and allows Router R1 to advertise this network. With the wildcard mask used, this command specifies that only interfaces on the 172.16.1.0/24 subnet will participate in EIGRP. (However, the full Class B network 172.16.0.0 will be advertised to Router R2 because EIGRP automatically summarizes routes on the major network boundary by default.) The network 192.168.1.0 command starts EIGRP on the Serial 0/0/1 interface and allows Router R1 to advertise this network.

Example 2-2: Configuration of Router R1 in Figure 2-18

---

interface FastEthernet0/0
ip address 172.16.1.1 255.255.255.0

interface Serial0/0/1
bandwidth 512
ip address 192.168.1.101 255.255.255.224

interface Serial0/0/1
ip address 172.16.5.1 255.255.255.0

router eigrp 110
network 172.16.1.0 0.0.0.255
network 192.168.1.0

---

Router R1 is connected to the Router R3 and subnet 172.16.5.0 is used on the link between the two routers. Because Router R1 is not configured to run EIGRP on 172.16.5.0, Router R1 does not try to form an adjacency with the Router R3. If the wildcard mask was not used on Router R1, it would run EIGRP on all interfaces in 172.16.0.0/16 and would send EIGRP packets to Router R3. This would waste bandwidth and CPU cycles and would provide unnecessary information to the external network. The wildcard mask tells EIGRP to establish a relationship with EIGRP routers from interfaces that are part of subnet 172.16.1.0/24 only.

The bandwidth 512 command on interface Serial 0/0/1 sets the bandwidth to 512 kbps.

The configuration on Router R2 would be similar to Router R1’s configuration.

Another Basic EIGRP Configuration Example

Figure 2-19 shows another sample network, including the configuration of Router A for EIGRP.

Figure 2-19: Basic EIGRP Configuration Sample Network.

All routers in the network are part of autonomous system 109, so they can establish a neighbor relationship.

Because the wildcard mask is not used in Router A’s configuration, all interfaces on Router A that are part of network 10.0.0.0/8 and network 172.16.0.0/16 participate in the EIGRP routing process. In this case, this includes all four interfaces. Note that network 192.168.1.0 is not configured in the EIGRP configuration on Router A, because Router A does not have any interfaces in that network. Instead, suppose that the configuration in Example 2-3 was entered onto Router A.

Example 2-3: Alternative Configuration of Router A in Figure 2-19

---

routerA(config)#router eigrp 109
routerA(config-router)#network 10.1.0.0
routerA(config-router)#network 10.4.0.0
routerA(config-router)#network 172.16.7.0
routerA(config-router)#network 172.16.2.0

---

Because no wildcard mask was specified in Example 2-3, Router A would automatically change the network commands to have classful networks, and the resulting configuration would be as shown in Example 2-4.

Example 2-4: Router A’s Interpretation of the Configuration in Example 2-3

---

router eigrp 109
network 10.0.0.0
network 172.16.0.0

---

Alternatively, consider what would happen if the configuration shown in Example 2-5 was entered for Router A.

Example 2-5: Another Alternative Configuration of Router A in Figure 2-19

---

routerA(config)#router eigrp 109
routerA(config-router)#network 10.1.0.0 0.0.255.255
routerA(config-router)#network 10.4.0.0 0.0.255.255
routerA(config-router)#network 172.16.2.0 0.0.0.255
routerA(config-router)#network 172.16.7.0 0.0.0.255

---

In this case, Router A uses the wildcard mask to determine which directly connected interfaces participate in the EIGRP routing process for autonomous system 109. All interfaces that are part of networks 10.1.0.0/16, 10.4.0.0/16, 172.16.2.0/24, and 172.16.7.0/24 participate in the EIGRP routing process for autonomous system 109. In this case, all four interfaces participate.

Verifying EIGRP Operation

This section discusses commands used to verify EIGRP operation.

Table 2-2 describes some show commands used to verify EIGRP operation. Other options might be available with these commands. Use the Cisco IOS integrated help feature to see the full command syntax.

Table 2-2: EIGRP show Commands
Open table as spreadsheet

Command	Description
show ip eigrp neighbors	Displays neighbors discovered by EIGRP.
show ip route	Displays the current entries in the IP routing table for all configured routing protocols.
show ip route eigrp	Displays the current EIGRP entries in the IP routing table.
show ip protocols	Displays the parameters and current state of the active routing protocol processes. For EIGRP, this command shows the EIGRP autonomous system number, filtering and redistribution numbers, and neighbors and distance information.
show ip eigrp interfaces	Displays information about interfaces configured for EIGRP.
show ip eigrp topology	Displays the EIGRP topology table. This command shows the topology table, the active or passive state of routes, the number of successors, and the FD to the destination. Note that only successor and feasible successor routes are displayed. Add the all-links keyword to display all routes, including those not eligible to be successor or feasible successor routes.
show ip eigrp traffic	Displays the number of EIGRP packets sent and received. This command displays statistics on hello packets, updates, queries, replies, and acknowledgments.

Table 2-3 describes debug commands used to verify EIGRP operation. Other options might be available with these commands. Use the Cisco IOS integrated help feature to see the full command syntax.

Table 2-3: EIGRP debug Commands
Open table as spreadsheet

Command	Description
debug eigrp packets	Displays the types of EIGRP packets sent and received. A maximum of 11 packet types can be selected for individual or group display.
debug ip eigrp	Displays packets that are sent and received on an interface. Because this command generates large amounts of output, use it only when traffic on the network is light.
debug ip eigrp summary	Displays IP EIGRP summary route processing.
debug eigrp neighbors	Displays neighbors discovered by EIGRP and the contents of the hello packets.

Caution

Use caution when executing debug commands, because they may consume a lot of router resources and could cause problems in a busy production network. Debugging output takes priority over other network traffic. Too much debug output may severely reduce the performance of the router or even render it unusable in the worst case.

The following sections provide sample output from some of these commands, using the network in Figure 2-20 to illustrate the configuration, verification, and troubleshooting of EIGRP. Example 2-6 shows the configuration of the R1 router.

Figure 2-20: Sample Network for EIGRP Verification.

Example 2-6: Configuration for Router R1 in Figure 2-20

---

R1#show running-config

interface FastEthernet0/0
 ip address 172.16.1.1 255.255.255.0


interface Serial0/0/1
bandwidth 64
ip address 192.168.1.101 255.255.255.224


router eigrp 100
network 172.16.1.0 0.0.0.255
network 192.168.1.0

---

On the R1 router, EIGRP is enabled in autonomous system 100. The network 172.16.1.0 0.0.0.255 command starts EIGRP on the Fast Ethernet 0/0 interface and allows Router R1 to advertise this network. With the wildcard mask used, this command specifies that only interfaces on the 172.16.1.0/24 subnet will participate in EIGRP. Note, however, the full Class B network 172.16.0.0 will be advertised, because EIGRP automatically summarizes routes on the major network boundary by default. The network 192.168.1.0 command starts EIGRP on the serial 0/0/1 interface, and allows Router R1 to advertise this network.

Example 2-7 shows the configuration of the R2 router.

Example 2-7: Configuration for Router R2 in Figure 2-20

---

R2#show running-config

interface FastEthernet0/0
 ip address 172.17.2.2 255.255.255.0


interface Serial0/0/1
bandwidth 64
ip address 192.168.1.102 255.255.255.224


router eigrp 100
network 172.17.2.0 0.0.0.255
network 192.168.1.0

---

EIGRP is also enabled in autonomous system 100 on the R2 router. The network 172.17.2.0 0.0.0.255 command starts EIGRP on the Fast Ethernet 0/0 interface and allows Router R2 to advertise this network. With the wildcard mask used, this command specifies that only interfaces on the 172.17.2.0/24 subnet will participate in EIGRP. Note, however, the full Class B network 172.17.0.0 will be advertised, because EIGRP automatically summarizes routes on the major network boundary by default. The network 192.168.1.0 command starts EIGRP on the serial 0/0/1 interface and allows Router R2 to advertise this network.

Verifying EIGRP Neighbors

The show ip eigrp neighbors command displays the contents of the IP EIGRP neighbor table. Example 2-8 provides R1’s IP EIGRP neighbor table.

Example 2-8: Sample Output for the show ip eigrp neighbors Command

---

R1#show ip eigrp neighbors
IP-EIGRP neighbors for process 100
H   Address        Interface   Hold  Uptime   SRTT   RTO  Q  Seq
                              (sec)          (ms)       Cnt Num
0   192.168.1.102  Se0/0/1     10    00:07:22   10   2280  0  5
R1#

---

Note

The “EIGRP Neighbors” section, earlier in this chapter, provides a full description of the output from the show ip eigrp neighbors command.

Detailed neighbor information can be examined with the show ip eigrp neighbor detail command, as shown in Example 2-9. The additional information provided in this command includes the number of items a packet has been retransmitted (two in this example), the number of times an attempt was made to retransmit a packet (two in this example), the packets that are currently waiting to be sent (R1 has three updates waiting to be sent in this example), and the neighboring router IOS version (12.4 in this example). The sequence number (seq in the example) increments each time a query, update, or reply packet is sent, whereas the serial number (ser in the example) increments each time the topology table changes.

Example 2-9: Sample Output for the show ip eigrp neighbors detail Command

---

R1#show ip eigrp neighbors detail
IP-EIGRP neighbors for process 110
H   Address        Interface   Hold Uptime   SRTT   RTO  Q   Seq
                               (sec)          (ms)        Cnt Num
0   192.168.1.102 Se0/0/1      14   00:17:55  0     4500 3   274
  Last startup serial 569
  Version 12.4/1.0, Retrans: 2, Retries: 2, Waiting for Init Ack
   UPDATE seq 307 ser 29-569 Sent 8924 Init Sequenced
   UPDATE seq 310 ser 570-573 Sequenced
   UPDATE seq 312 ser 574-578 Sequenced

---

Verifying EIGRP Routes

To verify that the router recognizes EIGRP routes for any neighbors, use the show ip route eigrp command, as shown in Example 2-10. Example 2-11 exhibits the show ip route command, which displays the full IP routing table, including the EIGRP routes.

Example 2-10: show ip route eigrp Command Output

---

R1#show ip route eigrp
D    172.17.0.0/16 [90/40514560] via 192.168.1.102, 00:07:01, Serial0/0/1
    172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
D       172.16.0.0/16 is a summary, 00:05:13, Null0
    192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
D       192.168.1.0/24 is a summary, 00:05:13, Null0

---

Example 2-11: show ip route Command Output

---

R1#show ip route

Gateway of last resort is not set
D    172.17.0.0/16 [90/40514560] via 192.168.1.102, 00:06:55, Serial0/0/1
    172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
D       172.16.0.0/16 is a summary, 00:05:07, Null0
C       172.16.1.0/24 is directly connected, FastEthernet0/0
    192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C       192.168.1.96/27 is directly connected, Serial0/0/1
D       192.168.1.0/24 is a summary, 00:05:07, Null0

---

Using the highlighted line in Example 2-10 as an example, the fields in the routing table are interpreted as follows:

• Internal EIGRP routes are identified with a D in the far left column. (External EIGRP routes, not shown in this example, are identified with a D EX in the far left column.)

• The next column is the network number (172.17.0.0/16 in this example).

• After each network number is a field in brackets (90/40514560 in this example). The second number in brackets is the EIGRP metric. As discussed in the “EIGRP Metric Calculation” section, earlier in this chapter, the default EIGRP metric is the least-cost bandwidth plus the accumulated delays. The EIGRP metric for a network is the same as its FD in the EIGRP topology table.

  The first number, 90 in this case, is the administrative distance. Recall from Chapter 1 that the administrative distance is used to select the best path when a router learns two or more routes to exactly the same destination from different routing sources. For example, consider that this router uses Routing Information Protocol (RIP) and EIGRP and that RIP has a route to network 172.17.0.0 that is three hops away. The router, without the administrative distance, cannot compare three hops to an EIGRP metric of 40,514,560. The router does not know the bandwidth associated with hops, and EIGRP does not use hop count as a metric. To correct this problem, Cisco established an administrative distance for each routing protocol: the lower the value, the more preferred the route is. By default, EIGRP internal routes have an administrative distance of 90, and RIP has an administrative distance of 120. Because EIGRP has a metric based on bandwidth and delay, it is preferred over RIP’s hop count metric. As a result, in this example, the EIGRP route would be installed in the routing table.

  Note

  Remember that routers use the administrative distance only if the two routes are to the exact same destination (address and mask). For example, a router will choose a RIP route over an EIGRP route if the RIP route is a more specific route than the EIGRP route.

• The next field, via 192.168.1.102 in this example, is the address of the next-hop router to which this router passes packets destined for 172.17.0.0/16. The next-hop address in the routing table is the same as the successor in the EIGRP topology table.

• The route also has a time associated with it (00:07:01 in this example). This is the length of time Because EIGRP last advertised this network to this router. EIGRP does not refresh routes periodically. It resends routing information only when neighbor adjacencies change.

• The interface, Serial 0/0/1 in this case, indicates the interface out which packets for 172.17.0.0/16 are sent.

Notice that the routing table includes routes, to null0, for the advertised (summarized) routes. Cisco IOS Software automatically puts these routes in the table when it autosummarizes. They are called summary routes. Null 0 is a directly connected, software-only interface. The use of the null0 interface prevents the router from trying to forward traffic to other routers in search of a more precise, longer match. For example, if the R1 router in Figure 2-20 receives a packet to an unknown subnet that is part of the summarized range—172.16.3.5 for example—the packet matches the summary route based on the longest match. The packet is forwarded to the null0 interface (in other words, it is dropped, or sent to the bit bucket), which prevents the router from forwarding the packet to a default route and possibly creating a routing loop.

Verifying EIGRP Operations

This section describes commands used to verify EIGRP operation.

show ip protocols Example

Use the show ip protocols command to provide information about any and all dynamic routing protocols running on the router.

As shown in Example 2-12, the command output displays that EIGRP process (autonomous system) 100 is running, and indicates any route filtering occurring on EIGRP outbound or inbound updates. It also identifies whether EIGRP is generating a default network or receiving a default network in EIGRP updates and provides information about additional settings for EIGRP, such as K values, hop count, and variance.

Example 2-12: show ip protocols Command Output

---

R1#show ip protocols
Routing Protocol is "eigrp 100"
 Outgoing update filter list for all interfaces is not set
 Incoming update filter list for all interfaces is not set
 Default networks flagged in outgoing updates
 Default networks accepted from incoming updates
 EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
 EIGRP maximum hopcount 100
 EIGRP maximum metric variance 1
 Redistributing: eigrp 100
 EIGRP NSF-aware route hold timer is 240s
 Automatic network summarization is in effect
 Automatic address summarization:
   192.168.1.0/24 for FastEthernet0/0
     Summarizing with metric 40512000
   172.16.0.0/16 for Serial0/0/1
     Summarizing with metric 28160
 Maximum path: 4
 Routing for Networks:
   172.16.1.0/24
   192.168.1.0
 Routing Information Sources:
   Gateway          Distance      Last Update
   (this router)          90      00:09:38
   Gateway          Distance      Last Update
   192.168.1.102          90      00:09:40
Distance: internal 90 external 170

---

Note

Two routers must have identical K values for EIGRP to establish an adjacency. The show ip protocols command is helpful in determining the current K value settings before an adjacency is attempted.

The output in Example 2-12 also indicates that automatic summarization is enabled and that the router is allowed to load balance over a maximum of four paths. Cisco IOS Software allows configuration of up to 16 paths for equal-cost load balancing, using the maximum-paths router configuration command.

The networks for which the router is routing are also displayed. As shown in Example 2-12, the format of the output varies, depending on the use of the wildcard mask in the network command. If a wildcard mask is used, the network address is displayed with a prefix length. If a wildcard mask is not used, the Class A, B, or C major network is displayed.

The routing information source portion of this command output identifies all other routers that have an EIGRP neighbor relationship with this router. The show ip eigrp neighbors command provides a detailed display of EIGRP neighbors.

The show ip protocols command output also provides the two administrative distances for EIGRP. An administrative distance of 90 applies to networks from other routers inside the same autonomous system. These are considered internal networks. An administrative distance of 170 applies to networks introduced to this EIGRP autonomous system through redistribution. These are called external networks.

show ip eigrp interfaces Example

The show ip eigrp interfaces command displays information about interfaces configured for EIGRP. Example 2-13 demonstrates show ip eigrp interfaces command output. This output includes the following key elements:

• Interface— Interface over which EIGRP is configured

• Peers— Number of directly connected EIGRP neighbors

• Xmit Queue Un/Reliable— Number of packets remaining in the Unreliable and Reliable retransmit queues

• Mean SRTT— Mean SRTT interval, in milliseconds

• Pacing Time Un/Reliable— Pacing time used to determine when EIGRP packets should be sent out the interface (for unreliable and reliable packets)

• Multicast Flow Timer— Maximum number of seconds that the router will wait for an ACK packet after sending a multicast EIGRP packet, before switching from multicast to unicast

• Pending Routes— Number of routes in the packets in the retransmit queue waiting to be sent

Example 2-13: show ip eigrp interfaces Command Output

---

R1#show ip eigrp interfaces
IP-EIGRP interfaces for process 100
                         Xmit Queue   Mean   Pacing Time   Multicast    Pending
Interface         Peers  Un/Reliable  SRTT   Un/Reliable   Flow Timer   Routes
Fa0/0                0        0/0         0        0/10           0           0
Se0/0/1              1        0/0        10       10/380        424           0

---

show ip eigrp topology Example

Another command used to verify EIGRP operations is the show ip eigrp topology command. This command displays the DUAL states and helps troubleshoot possible DUAL issues. Example 2-14 demonstrates output generated from this command.

Example 2-14: show ip eigrp topology Command Output

---

R1#show ip eigrp topology
IP-EIGRP Topology Table for AS(100)/ID(192.168.1.101)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
      r - reply Status, s - sia Status
P 192.168.1.96/27, 1 successors, FD is 40512000
       via Connected, Serial0/0/1
P 192.168.1.0/24, 1 successors, FD is 40512000
       via Summary (40512000/0), Null0
P 172.16.0.0/16, 1 successors, FD is 28160
       via Summary (28160/0), Null0
P 172.16.1.0/24, 1 successors, FD is 28160
       via Connected, FastEthernet0/0
P 172.17.0.0/16, 1 successors, FD is 40514560
       via 192.168.1.102 (40514560/28160), Serial0/0/1

---

The command output illustrates that Router R1 has a router ID of 192.168.1.101 and is in autonomous system 100. The EIGRP router ID is chosen as the highest IP address on an active interface on the router, unless loopback interfaces are configured, in which case it is the highest IP address assigned to a loopback interface. Alternatively, if the eigrp router-id ip-address router configuration command is used, it will override the use of the address of a physical or loopback interface as the router ID.

The command output also lists the networks known by this router through the EIGRP routing process. The codes used in the first column of this output indicate the state of the entry. Passive and Active refer to the EIGRP state with respect to this destination. Update, Query, and Reply refer to the type of packet being sent. The codes are as follows:

• Passive (P)— This network is available, and installation can occur in the routing table. Passive is the correct state for a stable network, indicating that no EIGRP computations are being performed for this route.

• Active (A)— This network is currently unavailable, and installation cannot occur in the routing table. Being active means that outstanding queries exist for this network, indicating that EIGRP computations are being performed for this route.

• Update (U)— This network is being updated (indicating that an update packet is being sent). This code also applies if the router is waiting for an acknowledgment for this update packet.

• Query (Q)— There is an outstanding query packet for this network, indicating that a query packet was sent. This code also applies if the router is waiting for an acknowledgment for a query packet.

• Reply (R)— The router is generating a reply for this network, indicating that a reply packet was sent, or is waiting for an acknowledgment for the reply packet.

• Reply status (r)— Indicates the flag that is set after the software has sent a query and is waiting for a reply.

• Stuck-in-active (s)— There is an EIGRP convergence problem for this network. (The “Stuck-in-Active Connections in EIGRP” section, later in this chapter, describes this problem and how it can be prevented.)

The number of successors available for a route is indicated in the command output. The number of successors corresponds to the number of best routes with equal cost. All networks in Example 2-14 have one successor.

For each network, the FD is listed next, followed by an indication of how the route was learned, such as the next-hop address if the route was learned via another router. Next is a field in brackets. The first number in the brackets is the FD for that network through the next-hop router, and the second number in the brackets is the AD from the next-hop router to the destination network.

show ip eigrp traffic Example

To display the number of various EIGRP packets sent and received, use the show ip eigrp traffic command, as illustrated in Example 2-15. For example, in this network, Router R1 has sent 429 hello messages and received 192 hello messages, has sent 4 updates and received 4 updates, has sent 1 query and received 0 queries, has sent 0 replies and received 1 reply, and has sent 4 ACKs and received 3 ACKs.

Example 2-15: show ip eigrp traffic Command Output

---

R1#show ip eigrp traffic
IP-EIGRP Traffic Statistics for AS 100
 Hellos sent/received: 429/192
 Updates sent/received: 4/4
 Queries sent/received: 1/0
 Replies sent/received: 0/1
 Acks sent/received: 4/3
 Input queue high water mark 1, 0 drops
 SIA-Queries sent/received: 0/0
 SIA-Replies sent/received: 0/0
 Hello Process ID: 113
PDM Process ID: 73

---

debug eigrp packets Examples

You can use the debug eigrp packets command to verify EIGRP connectivity. This command displays the types of EIGRP packets sent and received by the router on which this command is executed. Different packet types can be selected for individual or group display. Example 2-16 shows some output from this command on R2, when an interface on R1 comes up.

Example 2-16: debug eigrp packets Command Output on R2 When a Neighbor’s Interface Comes Up

---

R2#debug eigrp packets
EIGRP Packets debugging is on
   (UPDATE, REQUEST, QUERY, REPLY, HELLO, IPXSAP, PROBE, ACK, STUB, SIAQUERY,
SIAREPLY)
*May 11 04:02:55.821: EIGRP: Sending HELLO on Serial0/0/1
*May 11 04:02:55.821:   AS 100, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/0
R2#
*May 11 04:02:58.309: EIGRP: Received HELLO on Serial0/0/1 nbr 192.168.1.101
*May 11 04:02:58.309:   AS 100, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/0
peerQ un/rely 0/0
*May 11 04:02:58.585: EIGRP: Sending HELLO on FastEthernet0/0
*May 11 04:02:58.585:   AS 100, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/0
*May 11 04:02:59.093: EIGRP: Received UPDATE on Serial0/0/1 nbr 192.168.1.101
*May 11 04:02:59.093:   AS 100, Flags 0x0, Seq 5/4 idbQ 0/0 iidbQ un/rely 0/0
peerQ un/rely 0/0
*May 11 04:02:59.093: EIGRP: Enqueueing ACK on Serial0/0/1 nbr 192.168.1.101
*May 11 04:02:59.093:   Ack seq 5 iidbQ un/rely 0/0 peerQ un/rely 1/0
*May 11 04:02:59.097: EIGRP: Sending ACK on Serial0/0/1 nbr 192.168.1.101
*May 11 04:02:59.097:   AS 100, Flags 0x0, Seq 0/5 idbQ 0/0 iidbQ un/rely 0/0
peerQ un/rely 1/0
*May 11 04:02:59.109: EIGRP: Enqueueing UPDATE on Serial0/0/1 iidbQ un/rely 0/1
serno 9-9
*May 11 04:02:59.113: EIGRP: Enqueueing UPDATE on Serial0/0/1 nbr 192.168.1.101
iidbQ un/rely 0/0 peerQ un/rely 0/0 serno 9-9
*May 11 04:02:59.113: EIGRP: Sending UPDATE on Serial0/0/1 nbr 192.168.1.101
*May 11 04:02:59.113:   AS 100, Flags 0x0, Seq 5/5 idbQ 0/0 iidbQ un/rely 0/0
peerQ un/rely 0/1 serno 9-9
*May 11 04:02:59.133: EIGRP: Received ACK on Serial0/0/1 nbr 192.168.1.101
*May 11 04:02:59.133:   AS 100, Flags 0x0, Seq 0/5 idbQ 0/0 iidbQ un/rely 0/0
peerQ un/rely 0/1
*May 11 04:02:59.133: EIGRP: Serial0/0/1 multicast flow blocking cleared
R2#
*May 11 04:03:00.441: EIGRP: Sending HELLO on Serial0/0/1
*May 11 04:03:00.441:   AS 100, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/0
R2#
*May 11 04:03:03.209: EIGRP: Received HELLO on Serial0/0/1 nbr 192.168.1.101
*May 11 04:03:03.209:   AS 100, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/0
peerQ un/rely 0/0

---

The debug eigrp packets command traces transmission and receipt of EIGRP packets. The output in Example 2-16 shows normal transmission and receipt of EIGRP packets. The serial link is an HDLC point-to-point link. Therefore, the default hello time interval is 5 seconds. Hello packets are sent unreliably, so the sequence number (Seq) does not increment for hello packets.

In this sample output, when R2 receives an update from R1, values appear in the sequence number field. Seq 5/4 indicates that 192.168.1.101 (R1) is sending this packet as sequence number 5 to R2 and that sequence number 4 has been received from R2 by neighbor R1. R1 is expecting to receive sequence number 5 in the next reliable packet from R2.

R2 returns an ACK packet with Seq 0/5. The acknowledgment is sent as an unreliable packet. The neighbor unreliable/reliable flag (un/rel 1/0) is set, which means that the acknowledgment was sent in response to a reliable packet.

The serial number (serno 9-9) reflects the number of changes that the two neighbors register in their EIGRP topology tables. The sequence number increments each time a query, update, or reply packet is sent, whereas the serial number increments each time the topology table changes. A single update can contain more than 100 networks, for example if they all become unavailable Therefore, if the topology table has more than 100 changes, the serial number increases substantially, but the sequence number may only increase by 1.

When an interface on R1 is shut down, the resulting output on R2 is shown in Example 2-17. R1 sends a query packet to R2 to determine whether R2 knows a path to the lost network. R2 responds with an ACK packet to acknowledge the query packet—a reliable packet must be explicitly acknowledged with an ACK packet. R2 also responds to the query with a reply packet. The serial number reference (10-12) represents the number of changes to the topology table since the start of the neighbor relationship between these two EIGRP neighbors.

Example 2-17: debug eigrp packets Command Output on R2 When a Neighbor’s Interface Is Shut Down

---

R2#debug eigrp packets
*May 11 04:20:43.361: EIGRP: Received QUERY on Serial0/0/1 nbr 192.168.1.101
*May 11 04:20:43.361:   AS 100, Flags 0x0, Seq 6/5 idbQ 0/0 iidbQ un/rely 0/0
peerQ un/rely 0/0
*May 11 04:20:43.361: EIGRP: Enqueueing ACK on Serial0/0/1 nbr 192.168.1.101
*May 11 04:20:43.361:   Ack seq 6 iidbQ un/rely 0/0 peerQ un/rely 1/0
*May 11 04:20:43.365: EIGRP: Sending ACK on Serial0/0/1 nbr 192.168.1.101

*May 11 04:20:43.365:   AS 100, Flags 0x0, Seq 0/6 idbQ 0/0 iidbQ un/rely 0/0
peerQ un/rely 1/0
*May 11 04:20:43.373: EIGRP: Enqueueing REPLY on Serial0/0/1 nbr 192.168.1.101
iidbQ un/rely 0/1 peerQ un/rely 0/0 serno 10-12
*May 11 04:20:43.377: EIGRP: Requeued unicast on Serial0/0/1
R2#
*May 11 04:20:43.381: EIGRP: Sending REPLY on Serial0/0/1 nbr 192.168.1.101
*May 11 04:20:43.381:   AS 100, Flags 0x0, Seq 6/6 idbQ 0/0 iidbQ un/rely 0/0
peerQ un/rely 0/1 serno 10-12
*May 11 04:20:43.405: EIGRP: Received ACK on Serial0/0/1 nbr 192.168.1.101
*May 11 04:20:43.405:   AS 100, Flags 0x0, Seq 0/6 idbQ 0/0 iidbQ un/rely 0/0
peerQ un/rely 0/1

---

debug ip eigrp Examples

You can use the debug ip eigrp command to verify EIGRP operation. This command displays IP EIGRP packets that this router sends and receives. Example 2-18 shows the contents of the updates that are reported when the debug ip eigrp command is used on R2 to monitor when an interface on R1 comes up.

Example 2-18: debug ip eigrp Command Output on R2 When a Neighbor’s Interface Comes Up

---

R2#debug ip eigrp
IP-EIGRP Route Events debugging is on
R2#
*May 11 04:24:05.261: IP-EIGRP(Default-IP-Routing-Table:100): Processing incoming
UPDATE packet
*May 11 04:24:05.261: IP-EIGRP(Default-IP-Routing-Table:100): Int 192.168.1.0/24
M 4294967295 - 40000000 4294967295 SM 4294967295 - 40000000 4294967295
*May 11 04:24:05.261: IP-EIGRP(Default-IP-Routing-Table:100): Int 172.16.0.0/16
M 40514560 - 40000000 514560 SM 28160 - 25600 2560
*May 11 04:24:05.261: IP-EIGRP(Default-IP-Routing-Table:100): route installed
for 172.16.0.0  ()
*May 11 04:24:05.277: IP-EIGRP(Default-IP-Routing-Table:100): Int 172.16.0.0/16
metric 40514560 - 40000000 514560

---

In this example, an internal route (indicated by Int) for 172.16.0.0/16 is advertised to R2.

Recall that by default the EIGRP metric is equal to the bandwidth plus the delay. The EIGRP process uses the source metric (SM) information in the update to calculate the AD and place it in the EIGRP topology table. In this example, the SM information is SM 28160 − 25600 2560, which means the source metric (the AD) = 28160 = 25600 (the bandwidth) + 2560 (the delay).

The EIGRP metric calculation for the total delay uses the metric (M) information in the update. In this example, the M information is M 40514560 − 40000000 514560, which means the metric (the FD) = 40514560 = 40000000 (the bandwidth) + 514560 (the delay).

The EIGRP metric for this route is equal to the FD and, therefore, is 40,514,560.

Example 2-19 illustrates what occurs when R2 processes an incoming query packet for network 172.16.0.0/16 when the interface on the neighboring router (R1) that leads to that network is shut down. Note that comments (preceded by an exclamation point [!]) have been added to this output for easier understanding.

Example 2-19: debug ip eigrp Command Output on R2 When a Neighbor’s Interface Is Shut Down

---

R2#debug ip eigrp
IP-EIGRP Route Events debugging is on
R2#
! An interface on EIGRP neighbor R1 was shutdown
! R2 receives a query looking for a lost path from R1
*May 11 04:35:44.281: IP-EIGRP(Default-IP-Routing-Table:100): Processing incoming
QUERY packet
*May 11 04:35:44.281: IP-EIGRP(Default-IP-Routing-Table:100): Int 172.16.1.0/24
M 4294967295 - 0 4294967295 SM 4294967295 - 0 4294967295
*May 11 04:35:44.281: IP-EIGRP(Default-IP-Routing-Table:100): Int 192.168.1.0/24
M 4294967295 - 0 4294967295 SM 4294967295 - 0 4294967295
*May 11 04:35:44.281: IP-EIGRP(Default-IP-Routing-Table:100): Int 172.16.0.0/16
M 4294967295 - 0 4294967295 SM 4294967295 - 0 4294967295
! R2 realizes that if it cannot use R1 for this network then
! it does not have an entry in the routing table for this network
continues
*May 11 04:35:44.281: IP-EIGRP(Default-IP-Routing-Table:100): 172.16.0.0/16 rout-
ing table not updated thru 192.168.1.101
R2#
*May 11 04:35:44.301: IP-EIGRP(Default-IP-Routing-Table:100): 172.16.1.0/24 - not
in IP routing table
*May 11 04:35:44.301: IP-EIGRP(Default-IP-Routing-Table:100): Int 172.16.1.0/24
metric 4294967295 - 0 4294967295
*May 11 04:35:44.301: IP-EIGRP(Default-IP-Routing-Table:100): 192.168.1.0/24 -
poison advertise out Serial0/0/1
*May 11 04:35:44.301: IP-EIGRP(Default-IP-Routing-Table:100): Int 192.168.1.0/24
metric 40512000 - 40000000 512000
*May 11 04:35:44.301: IP-EIGRP(Default-IP-Routing-Table:100): 172.16.0.0/16 - not
in IP routing table
! R2 sends an update to R1 saying it does not know how to reach that network either
*May 11 04:35:44.301: IP-EIGRP(Default-IP-Routing-Table:100): Int 172.16.0.0/16
metric 4294967295 - 40000000 4294967295
R2#

---

The neighbor previously advertised 172.16.0.0/16 to this router. The query performs the following two functions:

• R2 discovers that its neighbor no longer knows how to get to network 172.16.0.0/16. The metric value (4,294,967,295) is the highest possible value for a 32-bit number—it indicates that the route is unreachable. R2 removes this entry from the EIGRP topology table and looks for alternative EIGRP routes.

• The debug output indicates that the routing table is not updated, which means that EIGRP did not find an alternative route to the network. The next statement verifies that the EIGRP process has removed the old route and that the route is not in the IP routing table. R2 then informs the neighbor that it does not have a path to this network either.

Using the passive-interface Command with EIGRP

There are times when you must or want to include a subnet in a routing protocols’ network command, although you do not want the interface on which the subnet is connected to participate in the routing protocol.

The passive-interface {type number} | default router configuration command prevents a routing protocol’s routing updates from being sent through the specified router interface. This command is used to set either a particular interface or all router interfaces to passive; use the default option to set all router interfaces to passive.

Table 2-4 describes the parameters of this command.

Table 2-4: passive-interface Command
Open table as spreadsheet

Parameter	Description
type number	Specifies the type of interface and interface number that will not send routing updates (or establish neighbor relationships for link-state routing protocols and EIGRP)
default	Sets all interfaces on the router as passive by default

For EIGRP, the passive-interface command does the following:

• It prevents a neighbor relationship from being established over a passive interface.

• It stops routing updates from being processed or sent over passive interface.

• It allows a subnet on a passive interface to be announced in an EIGRP process.

When you use the passive-interface command with EIGRP, hello messages are not sent out of the specified interface. Neighboring router relationships do not form with other routers that can be reached through that interface (because the hello protocol is used to verify bidirectional communication between routers). Because no neighbors are found on an interface, no other EIGRP traffic is sent.

Recall that the network command defines the interfaces over which EIGRP will attempt to establish neighbor relationships and the networks that will be advertised to EIGRP neighbors. Configuring an interface as passive only disables the neighbor relationship establishment. The router will still advertise the network to its EIGRP neighbors.

In the example network shown in Figure 2-20 used earlier, Routers R1 and R2 have no EIGRP neighbors available over their Fast Ethernet 0/0 interfaces, so there is no need to try to establish adjacencies over those interfaces. If EIGRP packets are sent on these interfaces, they would be ignored, but would still consume bandwidth and CPU resources.

In the original configurations of Router R1 and R2 shown in Example 2-6 and Example 2-7, network commands were configured for the Fast Ethernet 0/0 interfaces to allow the router to advertise those subnets to its neighbor. Example 2-20 provides alternate EIGRP configurations for both routers.

Example 2-20: Passive-Interface Configurations for Routers in Figure 2-20

---

R1#
router eigrp 110
passive-interface FastEthernet0/0
network 172.16.1.0 0.0.0.255
network 192.168.1.0
R2#
router eigrp 110
passive-interface FastEthernet0/0
network 172.17.2.0 0.0.0.255
network 192.168.1.0

---

Note

EIGRP will not bring up adjacencies on a passive interface even if a neighbor command is configured over that interface. The neighbor command is described in the “EIGRP Unicast Neighbors” section, later in this chapter.

In Internet service providers (ISPs) and large enterprise networks, many distribution routers have more than 100 interfaces. Before the passive-interface default command was introduced in Cisco IOS Software Release 12.0, network administrators would configure the routing protocol on all interfaces and then manually set the passive-interface command on the interfaces on which they did not require adjacencies to be established. However, this solution meant entering many passive-interface commands. A single passive-interface default command can now be used to set all interfaces to passive by default. To enable routing on individual interfaces where you require adjacencies to be established, use the no passive-interface command.

Example 2-21 provides an alternate configuration for Router R1, using the passive-interface default command to make all interfaces passive. The no passive-interface serial0/0/1 command allows EIGRP adjacencies to be established over Serial 0/0/1.

Example 2-21: Alternate Passive-Interface Configurations for Router R1 in Figure 2-20

---

R1#
router eigrp 110
passive-interface default
no passive-interface Serial0/0/1
network 172.16.1.0 0.0.0.255
network 192.168.1.0

---

To verify EIGRP operation when using passive interfaces, the following commands are used:

• The show ip eigrp neighbors command, to verify that all neighbor relationships are established.

• The show ip protocols command, to see which interfaces are configures as passive interface. The output in Example 2-22 for Router R1 shows that interface Fast Ethernet 0/0 is defined as a passive interface.

Example 2-22: show ip protocols Command Output on Router R1 in Figure 2-20

---

R1#show ip protocols
Routing Protocol is "eigrp 110"
 

 Automatic network summarization is in effect
 Automatic address summarization:
     172.16.0.0/16 for Serial0/0/1
     Summarizing with metric 28160
 Maximum path: 4
 Routing for Networks:
   172.16.1.0/24
   192.168.1.0
 Passive Interface(s):
   FastEthernet0/0
 

---

Propagating an EIGRP Default Route

Default routes usually decrease the size of the routing tables in routers that receive them. For example, routers on stub networks or at the access layer do not typically need to know all the routes in the entire network. Instead, they can use a default route to send traffic to routers that have more detailed routing tables.

A statically configured default route can be created with the ip route 0.0.0.0 0.0.0.0 {next-hop | interface [next-hop]} global configuration command. The interface is an outgoing interface through which all packets with unknown destinations will be forwarded. The next-hop is the IP address to which packets with unknown destinations will be forwarded. (Notice that both the interface and next-hop address can be specified in this command.)

Alternatively, any major network residing in the local routing table can become an EIGRP default route when used in the ip default-network network-number global configuration command. A router configured with this command considers the network-number the last-resort gateway that it will announce to other routers with the exterior flag set. The network must be reachable by the router that uses this command before it announces it as a candidate default route to other EIGRP routers. The network number in this command must also be passed to other EIGRP routers so that those routers can use this network as their default network and set their gateway of last resort to this default network. This means that the network must either be an EIGRP-derived network in the routing table, or be generated with a static route and redistributed into EIGRP.

Note

In EIGRP default routes cannot be directly injected (as they can in OSPF with the default -information originate command).

Note

Any route residing in the routing table, including non-EIGRP routes, can be marked as a default candidate.

Multiple default networks can be configured. Downstream routers then use the EIGRP metric to determine the best default route. When the best default route is selected, the router sets the gateway of last resort to the next-hop address of the selected candidate, unless the best candidate route is one of the router’s directly connected routes.

For example, in Figure 2-21, Router A is directly attached to external network 172.31.0.0/16. Router A is configured with the 172.31.0.0 network as a candidate default network using the ip default-network 172.31.0.0 command. Router A also has that network listed in a network command under the EIGRP process and, therefore, passes it to Router B.

Figure 2-21: EIGRP ip default-network Sample Network.

To verify default network information, use the show ip route command to view the routing table. On Router B, the EIGRP-learned 172.31.0.0 network is flagged as a candidate default network (indicated by the * in the routing table). Router B also sets the gateway of last resort as 10.5.1.1 (Router A) to reach the default network of 172.31.0.0. Router A also has the 172.31.0.0 network flagged as a default network and has its gateway of last resort set.

Note

In earlier versions of the IOS software, the router on which the ip default-network command was configured would not set the gateway of last resort, so it appeared that this command did not benefit that router directly. Figure 2-21 illustrates that it now does set the gateway of last resort as 0.0.0.0 to the network specified in the ip default-network command.

Note

When you configure the ip default-network command and specify a subnet, a static route (the ip route command) is generated in the router’s configuration; however, the IOS does not display a message to indicate that this has been done. The entry appears as a static route in the routing table of the router where the command is configured. This can be confusing when you want to remove the default network. The configuration must be removed with the no ip route command, not with the no ip default-network command.

Note

EIGRP (and IGRP) behave differently than RIP when using the ip route 0.0.0.0 0.0.0.0 command. For example, EIGRP does not redistribute the 0.0.0.0 0.0.0.0 default route by default. However, if the network 0.0.0.0 command is added to the EIGRP configuration, it redistributes a default route as a result of the ip route 0.0.0.0 0.0.0.0 interface command (but not as a result of the ip route 0.0.0.0 0.0.0.0 address or ip default-network command). For example, the partial configuration shown in Example 2-23 results in the 0.0.0.0 route being passed to the router’s EIGRP neighbors.

Example 2-23: EIGRP Passes a Default Route Only if It Is Configured to Do So

---

Router#show run

interface serial 0/0/0
 ip address 10.1.1.1 255.255.255.0
!
ip route 0.0.0.0 0.0.0.0 serial 0/0/0
!
router eigrp 100
 network 0.0.0.0

---

EIGRP Route Summarization

Some EIGRP features, such as automatically summarizing routes at a major network boundary, have distance vector and classful characteristics. Traditional distance vector protocols, which are classful routing protocols, must summarize at network boundaries. They cannot presume the mask for networks that are not directly connected, because masks are not exchanged in the routing updates.

Summarizing routes at classful major network boundaries creates smaller routing tables. Smaller routing tables, in turn, make the routing update process less bandwidth intensive (because updates are smaller), and less CPU intensive (because there is less update information to process). Cisco distance vector routing protocols have autosummarization enabled by default. EIGRP automatic summarization on the major network boundary can be turned on or off.

The inability to create summary routes at arbitrary boundaries with a major network has been a drawback of distance vector protocols since their inception. EIGRP has added functionality to allow administrators to create one or more summary routes within a network on any bit boundary, on any router within the network, as long as a more specific route exists in the routing table. When the last specific route of the summary goes away, the summary route is deleted from the routing table.

It is important to note that the minimum metric of the specific routes is used as the metric of the summary route.

When summarization is configured on a router’s interface, a summary route is added to that router’s routing table, with the route’s next-hop interface set to null0—a directly connected, software-only interface. As described earlier for autosummarization, the use of the null0 interface prevents the router from trying to forward traffic to other routers in search of a more precise, longer match, thus preventing traffic from looping within the network. For example, if the summarizing router receives a packet to an unknown subnet that is part of the summarized range, the packet matches the summary route based on the longest match, and the packet is forwarded to the null0 interface and therefore is dropped.

For effective summarization, blocks of contiguous addresses (subnets) should funnel back to a common router so that a single summary route can be created and then advertised. In other words, the IP addressing plan for the network must have been created properly, with summarization in mind (it is unlikely that you would be lucky enough to be able to summarize randomly-assigned subnets). The number of subnets that can be represented by a summary route is calculated by the formula 2ⁿ, where n equals the difference in the number of bits between the summary and subnet masks. For example, if the summary mask contains 3 fewer bits than the subnet mask, eight (2³ = 8) subnets can be aggregated into one advertisement.

For example, if network 10.0.0.0 is divided into /24 subnets and some of these subnets are summarized to the summarization block 10.1.8.0/21, the difference between the /24 networks and the /21 summarizations is 3 bits. Therefore, 2³ = 8 subnets can be aggregated. The summarized subnets range from 10.1.8.0/24 through 10.1.15.0/24.

When creating summary routes, the administrator needs to specify the IP address of the summary route and the summary mask. The Cisco IOS handles the details of proper implementation, such as metrics, loop prevention, and removal of the summary route from the routing table if none of the more specific routes are valid.

Configuring Manual Route Summarization

In some cases you may want to turn off the EIGRP automatic summarization feature. For example, if you have discontiguous subnets, you need to disable autosummarization. Note that an EIGRP router does not perform automatic summarization of networks in which it does not participate.

To turn off automatic summarization, use the no auto-summary router configuration command. Use the ip summary-address eigrp as-number address mask [admin-distance] interface configuration command to manually create a summary route at an arbitrary bit boundary, as long as a more specific route exists in the routing table. Table 2-5 summarizes the parameters for this command.

Table 2-5: ip summary-address eigrp Command Parameters
Open table as spreadsheet

Parameter	Description
as-number	EIGRP autonomous system number.
address	The IP address being advertised as the summary address. This address does not need to be aligned on Class A, B, or C boundaries.
mask	The IP subnet mask used to create the summary address.
admin-distance	(Optional) Administrative distance. A value from 0 to 255.

For example, Figure 2-22 shows a discontiguous network 172.16.0.0. By default, both Routers A and B summarize routes at the classful boundary. As a result, Router C would have two equally good routes to network 172.16.0.0 and would perform load balancing between Router A and Router B. This would not be correct routing behavior.

Figure 2-22: Summarizing EIGRP Routes.

As shown in Example 2-24, you can disable the automatic route summarization on Router A. The same configuration would be done on Router B. With this configuration, Router C knows precisely that 172.16.1.0 is reached via Router A and that 172.16.2.0 is reached only via Router B. The routing tables of the routers in the 10.0.0.0 network, including Router C, now include these discontiguous subnets.

Example 2-24: Turning Off EIGRP Autosummarization on Router A (and Router B) in Figure 2-22

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RouterA(config)#router eigrp 1
RouterA(config-router)#network 10.0.0.0
RouterA(config-router)#network 172.16.0.0
RouterA(config-router)#no auto-summary

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An EIGRP router autosummarizes routes for only networks to which it is attached. If a network was not autosummarized at the major network boundary, as is the case in this example on Routers A and B because autosummarization is turned off, all the subnet routes are carried into Router C’s routing table. Router C will not autosummarize the 172.16.1.0 and 172.16.2.0 subnets because it does not own the 172.16.0.0 network. Therefore, Router C would send routing information about the 172.16.1.0 subnet and the 172.16.2.0 subnet to the WAN.

Forcing a summary route out Router C’s interface s0/0/0, as shown in Example 2-25, helps reduce route advertisements about network 172.16.0.0 to the WAN.

Example 2-25: Forcing Summarization on Router C in Figure 2-22

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RouterC#show run

router eigrp 1
 network 10.0.0.0
 network 192.168.4.0
!

int s0/0/0
 ip address 192.168.4.2 255.255.255.0
 ip summary-address eigrp 1 172.16.0.0 255.255.0.0

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Note

You can use the ip summary-address eigrp as-number 0.0.0.0 0.0.0.0 command to inject a default route to a neighbor, as an alternative to the methods described earlier in the “Propagating an EIGRP Default Route” section. However, the automatically generated route to null0 may cause problems in some topologies.

Verifying Manual Route Summarization

To verify manual route summarization, examine the IP routing tables. Example 2-26 illustrates Router C’s routing table. Router C has both 172.16.1.0 and 172.16.2.0, the discontiguous subnets, in its routing table, and the summary route to null0. Only the summary, network 172.16.0.0/16, is advertised out the Serial 0/0/0 interface.

Example 2-26: Routing Table of Router C in Figure 2-22

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RouterC#show ip route

Gateway of last resort is not set
    172.16.0.0/16 is variably subnetted, 3 subnets, 2 masks
D       172.16.0.0/16 is a summary, 00:00:04, Null0
D       172.16.1.0/24 [90/156160] via 10.1.1.2, 00:00:04, FastEthernet0/0
D       172.16.2.0/24 [90/20640000] via 10.2.2.2, 00:00:04, Serial0/0/1
C    192.168.4.0/24 is directly connected, Serial0/0/0
    10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
C       10.2.2.0/24 is directly connected, Serial0/0/1
C       10.1.1.0/24 is directly connected, FastEthernet0/0
D       10.0.0.0/8 is a summary, 00:00:05, Null0
RouterC#

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For manual summarization, the summary is advertised only if a component of the summary route (a more specific entry that is represented in the summary) is present in the routing table.

Note

IP EIGRP summary routes are given an administrative distance value of 5. Standard EIGRP routes receive an administrative distance of 90, and external EIGRP routes receive an administrative distance of 170. You will notice the EIGRP summary route with an administrative distance of 5 only on the local router that is performing the summarization (with the ip summary-address eigrp command), by using the show ip route network command, where network is the specified summarized route.