• To configure the EIGRP default route, use the following global configuration command:

ip default-network network-number

• The network-number will be announced to other routers as the last-resort gateway.
• In order for the router – where this command is configured – can consider the network as a candidate default route, the network must be reachable by this router.
• In addition, the network number in the 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 could be:
  • An EIGRP-derived network in the routing table.
  • Generated with a static route and redistributed into EIGRP.

The following scenario is based on the example given in page 96 of the BSCI study guide.

Dynampis .net Config file:

# EIGRP ip-default network Command - page 96 Of BSCI study guide
autostart = False
ghostios = true
sparsemem = true

[localhost]

    [[7200]]
        image = C7200.BIN
        # On Linux / Unix use forward slashes:
        # image = /opt/7200-images/c7200-jk9o3s-mz.124-7a.image
        npe = npe-400
        ram = 160

    [[ROUTER A]]
        fa0/0 = B fa0/0
        s1/0 = C s1/0
        model = 7200
        console = 2001
        idlepc = #this is a system-specific setting

    [[router B]]
        model = 7200
        console = 2002
        idlepc = #this is a system-specific setting    

    [[router C]]
        model = 7200
        console = 2003
        idlepc = #this a system-specific setting

Router A Configuration:

!
interface FastEthernet0/0
 ip address 10.5.1.1 255.255.255.0
 duplex auto
 speed auto
!
interface Serial1/0
 ip address 172.31.5.1 255.255.255.252
 serial restart-delay 0
!
!
router eigrp 1
 network 10.0.0.0
 network 172.31.0.0
 auto-summary
!
ip classless
ip default-network 172.31.0.0
!

• The command ip default-network 172.31.0.0 is configured on Router A to allow 172.31.0.0 network as a candidate default network.
• The command network 172.31.0.0 passes the network 172.31.0.0 to Router B, so that router B can use it as its default network and set its gateway of last resort to this network.

Router B configuration:

interface FastEthernet0/0
 ip address 10.5.1.3 255.255.255.0
 duplex auto
 speed auto
!
router eigrp 1
 network 10.5.1.3 0.0.0.0
 auto-summary
!
ip classless
!

Router C configuration:

!
interface Serial1/0
 ip address 172.31.5.2 255.255.255.252
 serial restart-delay 0
!
router eigrp 1
  network 172.31.0.0
 auto-summary
!
ip classless

Router B: IP routing table:

B# sh ip route

Gateway of last resort is 10.5.1.1 to network 172.31.0.0

D*   172.31.0.0/16 [90/2172416] via 10.5.1.1, 00:10:38, FastEthernet0/0
     10.0.0.0/24 is subnetted, 1 subnets
C       10.5.1.0 is directly connected, FastEthernet0/0

• The EIGRP-learned 172.31.0.0 network is marked as a candiate default network indicated by the * in the routing table.
• The gateway of last resort is also set to 10.5.1.1 (Router A) to reach the default network 172.31.0.0.

Router A: IP routing table

A(config)#do sh ip route

Gateway of last resort is 0.0.0.0 to network 172.31.0.0

 *   172.31.0.0/16 is variably subnetted, 2 subnets, 2 masks
D*      172.31.0.0/16 is a summary, 00:12:27, Null0
C       172.31.5.0/30 is directly connected, Serial1/0
     10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
D       10.0.0.0/8 is a summary, 00:12:27, Null0
C       10.5.1.0/24 is directly connected, FastEthernet0/0

• In earlier versions of IOS, the router on which the ip default-network command was configured would not set the gateway of last resort.
• As highlighted above, it now sets the gateway of last resort to 0.0.0.0, to the network specified – 172.31.0.0.