CCNP ROUTE: 14.EIGRP Default routes

There are two main options for EIGRP to advertise default routes: to define a static default route and advertise it with EIGRP and to flag an existing route to be used also as a default route.

1. Advertising static default routes into EIGRP:

Step 1: Create a static route default route using the "ip route 0.0.0.0 0.0.0.0 outgoing-interface" command.
Step 2: Inject this route into the EIGRP topology database, either using the network 0.0.0.0 command or by redistributing the static route.

R7(config)#ip route 0.0.0.0 0.0.0.0 fastEthernet 0/1
R7(config)#do show ip route
...
S*   0.0.0.0/0 is directly connected, FastEthernet0/1
R7(config-router)#network 0.0.0.0

On R7: Gateway of last resort is 0.0.0.0 to network 0.0.0.0

R6#show ip route
...
Gateway of last resort is 172.16.0.10 to network 0.0.0.0
...
D*   0.0.0.0/0 [90/2195456] via 172.16.0.10, 00:00:34, Serial0/1
R5#show ip route
...
Gateway of last resort is 172.16.0.5 to network 0.0.0.0
...
D*   0.0.0.0/0 [90/2221056] via 172.16.0.5, 00:06:28, FastEthernet0/0
R4#show ip route
...
Gateway of last resort is 172.16.0.1 to network 0.0.0.0
...
D*   0.0.0.0/0 [90/2707456] via 172.16.0.1, 00:07:13, Serial0/0

or, using static redistribution:

R7(config)#ip route 0.0.0.0 0.0.0.0 fastEthernet 0/1
R7(config)#do show ip route
...
S*   0.0.0.0/0 is directly connected, FastEthernet0/1
R7(config)#router eigrp 1
R7(config-router)#redistribute static

R6#show ip route
...
Gateway of last resort is 172.16.0.10 to network 0.0.0.0
...
D*EX 0.0.0.0/0 [170/2195456] via 172.16.0.10, 00:00:11, Serial0/1
R5#show ip route
...
Gateway of last resort is 172.16.0.5 to network 0.0.0.0
...
D*EX 0.0.0.0/0 [170/2221056] via 172.16.0.5, 00:01:30, FastEthernet0/0
R4#show ip route
...
Gateway of last resort is 172.16.0.1 to network 0.0.0.0
...
D*EX 0.0.0.0/0 [170/2707456] via 172.16.0.1, 00:02:12, Serial0/0

2. Configuring a Default Network:

Step 1: On the router to which all traffic should be directed, identify a classful network that can be advertised into the EIGRP domain, and ensure that network is being advertised into EIGRP (typically using the EIGRP network command).
Step 2: Configure that network as a default network using the global command "ip default-network network-number".

By adding the ip default-network command to refer to one of these networks, EIGRP then flags this route as a candidate default route.
As a result, each EIGRP router treats their route for this particular network also as if it were a default route.

"Gateway of last resort" refers to the next-hop router of a router’s current default route.

R4#show ip protocols 
Routing Protocol is "eigrp 1"
  ...
  Routing for Networks:
    172.16.0.2/32
    172.16.0.12/30
    192.168.44.0
R4(config)#ip default-network 192.168.44.0
R4#show ip route
...
Gateway of last resort is not set
...
C*   192.168.44.0/24 is directly connected, Loopback44

Although R4 flags the route as a candidate default route, R4 itself does not use that route as its default, because R4 is actually the original advertiser of the default.

R5#show ip route
...
Gateway of last resort is 172.16.0.13 to network 192.168.44.0
...
D*   192.168.44.0/24 [90/2297856] via 172.16.0.13, 00:03:56, Serial0/1

R6#show ip route
...
Gateway of last resort is 172.16.0.2 to network 192.168.44.0
...
D*   192.168.44.0/24 [90/2297856] via 172.16.0.2, 00:04:31, Serial0/0

R7#show ip route
...
Gateway of last resort is 172.16.0.9 to network 192.168.44.0
...
D*   192.168.44.0/24 [90/2809856] via 172.16.0.9, 00:05:08, Serial0/0

If a router knows of multiple candidate default routes, it chooses the best route based on administrative distance and then metric, and uses that route as the basis for the gateway of last resort.

Setting Loopback 55 on R5 with the ip default-network command, R6 and R7 now have 2 default-routes in the routing table, with equal AD (EIGRP 90), but different costs.
R6 and R7 choose their gateway of last resort from the 2 default-networks based on metric (R5 will be the next hop for traffic destined outside the EIGRP domain).

R6#show ip route
...
Gateway of last resort is 172.16.0.6 to network 192.168.55.0
...
D*   192.168.44.0/24 [90/2297856] via 172.16.0.2, 00:09:30, Serial0/0
D*   192.168.55.0/24 [90/409600] via 172.16.0.6, 00:01:57, FastEthernet0/1

R7#show ip route
...
Gateway of last resort is 172.16.0.9 to network 192.168.55.0
...
D*   192.168.44.0/24 [90/2809856] via 172.16.0.9, 00:12:48, Serial0/0
D*   192.168.55.0/24 [90/2323456] via 172.16.0.9, 00:05:15, Serial0/0