EIGRP – Default Routing Using EIGRP

EIGRP supports numerous ways to dynamically advertise the gateway or network of last resort to other routers within the routing domain. These methods are as follows:

• Using the ip default-network command
• Using the network command to advertise network 0.0.0.0/0
• Redistributing the default static route
• Using the ip summary-address eigrp [asn] [network] [mask] command

The use to the ip default-network command is considered a legacy method of advertising the default route dynamically using EIGRP. However, because it is still supported in current IOS software versions, it is worth mentioning.

The ip default-network configuration command flags a network as the default network by inserting an asterisk (*) next to the network in the routing table. Traffic for destinations to which there is no specific routing table entry is then forwarded by the router to this network. The implementation of this feature is illustrated referencing the EIGRP topology in Figure 2-12:

Fig. 2-12. EIGRP Default Routing

Referencing Figure 2-12, assume that the 200.10.10.0/24 subnet is connected to the Internet. This subnet resides off the FastEthernet0/0 interface of R1. R1 and R2 are in turn connected via a back-to-back Serial connection. Both routers reside in EIGRP AS 150. To flag the 200.10.10.0/24 network as the network of last resort, the following configuration is implemented on R1:

Based on this configuration, R2 receives 200.10.10.0/24 as the network of last resort as follows:

The network command can be used to advertise an existing static default route point to either a physical or logical interface, typically the Null0 interface.

NOTE: The Null0 interface is a virtual interface on the router that discards all traffic that is routed to it. It you have a static route pointing to Null0, all traffic destined for the network specified in the static route is simply discarded. Think of the Null0 interface as a black-hole: packets enter, but none ever leave. It is essentially a bit-bucket on the router.

Referencing the diagram in Figure 2-12, the use of the network command in conjunction with an existing default static route is illustrated in the following configuration on R1:

Based on this configuration, the IP routing table on R2 shows the following:

Although route redistribution will be covered in detail later in this guide, the third method of advertising a default route via EIGRP is to redistribute the existing static default route into EIGRP using the redistribute static metric [bandwidth] [delay] [reliability] [load] [MTU] router configuration command. The same network topology used in the first two examples will be used to illustrate the implementation of this method:

Fig. 2-13. EIGRP Default Routing Continued

Referencing Figure 2-13, which is the same as Figure 2-12, the following is performed on R1:

NOTE: The values used in the metric can be derived from the interface or you can specify any values that you want when using this command.

Based on this configuration, the routing table on R2 shows the following:

Because the route was redistributed into EIGRP on R1, it is an external EIGRP route as reflected in the output above. For external routes, the EIGRP Topology Table includes information such as the router that originated the route, the protocol the route was received for, and the metric of the external route, for example. This is illustrated in the following output:

From the information in bold, we can determine that the default route is a static route that was redistributed into EIGRP on R1. This route has a metric of 0. In addition to this, we can also determine that the EIGRP Router ID (RID) of R1 is 1.1.1.1.

The final method of advertising the default route is by using the ip summary-address eigrp [asn] [network] [mask] interface configuration command. EIGRP route summarization will be described in detail later in this chapter. For the moment, concentrate your focus on the use of this command to advertise the default route when using EIGRP.

Referencing the network topology diagram illustrated in Figure 2-12, R1 is configured with the ip summary-address eigrp [asn] [network] [mask] interface configuration command to advertise the default route to R2 as follows:

The primary advantage to using this command is that a default route or network does not need to exist in the routing table in order for EIGRP to advertise network 0.0.0.0/0 to its neighbor routers. When this command is issued, the local router generates a summary route to the Null0 interface and flags the entry as the candidate default route. This is illustrated below:

The summary route is received as an internal EIGRP route on R2 as illustrated below: