EIGRP Default Routing

The EIGRP default route can be created with the ip default-network {network-number} global configuration command. An EIGRP router which is configured with this command will consider the network-number as the gateway of last-resort and will announce to other routers. The network must be reachable by the router that is configured with this command before it is announced as a candidate default route to other routers; which means that the network must either be an EIGRP-derived network in the routing table or generated with a static route and redistributed into EIGRP. The network number will be propagated to other routers so that they can utilize this network as the default network and eventually the gateway of last-resort.

In a complex topology where multiple default networks are configured as candidate default, downstream routers will use the EIGRP metric to dynamically determine the best default route, rather than requiring manually configured static routes.

Network Setup for EIGRP Default Routing

EIGRP Default Network configuration on RT1:

!
router eigrp 100
 network 10.0.0.0
 network 172.16.0.0
 auto-summary
!
ip default-network 172.16.0.0
ip route 172.16.0.0 255.255.0.0 172.16.1.1
!
RT1#sh ip route

Gateway of last resort is 172.16.1.1 to network 172.16.0.0

 *   172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
S*      172.16.0.0/16 [1/0] via 172.16.1.1
C       172.16.1.0/24 is directly connected, Serial1/0
     10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C       10.10.10.0/24 is directly connected, FastEthernet0/0
D       10.0.0.0/8 is a summary, 00:06:04, Null0
RT1#

Note: The auto-summary EIGRP router subcommand is required for this sample to work.
Note: For EIGRP to propagate the route, the network specified by the ip default-network command must be known to EIGRP. This means that the network must be an EIGRP route in the routing table, or the static route that generates the route to the network must be redistributed into EIGRP, or advertised into EIGRP using the network command.

Below shows the routing table on RT2. The default network is advertised from RT1 to RT2.

RT2#sh ip route

Gateway of last resort is 10.10.10.1 to network 172.16.0.0

D*   172.16.0.0/16 [90/2172416] via 10.10.10.1, 00:05:12, FastEthernet0/0
     10.0.0.0/24 is subnetted, 1 subnets
C       10.10.10.0 is directly connected, FastEthernet0/0
RT2#
RT2#ping 11.11.11.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 11.11.11.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 60/79/100 ms
RT2#

RT1 is configured with the 172.16.0.0/16 network as a candidate default network using the ip default-network 172.16.0.0 global configuration command. RT1 also has that network configured with the network router subcommand, and therefore will advertise it to RT2. RT1 routing table does not set the gateway of last resort – the ip default-network command has no effect on RT1. On RT2, the 172.16.0.0/16 network learnt via EIGRP is flagged as a candidate default network as indicated by the * besides the candidate default route. RT2 also sets its gateway of last resort as 10.10.10.1 (RT1) to reach the default network 172.16.0.0.

The ip default-network command is classful. When the network specified with this command is a subnet of a major network (eg: 10.10.0.0, 172.16.1.0), the router will configure a static route to the major network number with the ip route command instead of configuring a default network with the ip default-network command. The static route is configured without the notice of the user, as no message will be displayed. Deleting this route requires the no ip default-network command instead of the no ip route command!

EIGRP propagates default route differently than RIP when using the ip route 0.0.0.0 0.0.0.0 command. 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 EIGRP router subcommand is configured, EIGRP will redistribute a default route as a result of the ip route 0.0.0.0 0.0.0.0 {outgoing-intf} global command, but not as a result of the ip route 0.0.0.0 0.0.0.0 {next-hop-addr} or ip default-network command. Below shows the alternative configuration on RT1 for it to propagate the 0.0.0.0 default route information to RT2. RT1 and RT2 are able to ping 11.11.11.1.

RT1 configuration:
!
router eigrp 100
 network 0.0.0.0
 auto-summary | no auto-summary
!
ip route 0.0.0.0 0.0.0.0 Serial1/0
!
----------------------------------------------------------------------
RT2#sh ip route | in Gateway|D[*]
Gateway of last resort is 10.10.10.1 to network 0.0.0.0
D*   0.0.0.0/0 [90/2172416] via 10.10.10.1, 00:06:25, FastEthernet0/0
RT2#

The best solution is simply configure and redistribute a static default route into EIGRP on RT1. :-)