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A router’s default route matches the destination of all packets that are not matched by any
other route in the IP routing table. In fact, a default route can be thought of as the ultimate
summary route–a route for the prefix that includes all IPv4 addresses, as represented
by prefix/length 0.0.0.0/0.
This section first examines the most common use of default routes inside an Enterprise: to
draw Internet traffic toward the Internet-connected routers without having to put routes
for all Internet destinations into the Enterprise routers’ routing tables. Following that, this
section examines two methods for EIGRP to advertise the default route.
Default Routing to the Internet Router
Consider an Enterprise network and its connection to the Internet, as shown in Figure 4-9.
For now, the design shows a single Internet-facing router (I1). As is often the case, the entire
Enterprise in this figure uses private IP addresses. In this case, all Enterprise subnets
are part of private class A network 10.0.0.0.
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.
First, examine the command listed for Step 1: ip route 0.0.0.0 0.0.0.0 S0/0/0. The prefix
and mask together represent all IPv4 addresses. The reasoning is that if a mask of
255.255.0.0 means “the last two octets can be any value,” and 255.0.0.0 means “the last
three octets can be any value,” then a subnet mask of 0.0.0.0 means that all four octets can
be any value. The outgoing interface, S0/0/0 in this case, tells I1 to send packets for otherwise
unknown destinations over the link to the Internet, as intended.
After Step 1, Router I1 has a route in its routing table, but EIGRP does not yet advertise
the route. I1 could be configured to perform route redistribution for this static route. (Refer
to Chapter 9 for more information on route redistribution.) The other option is to use
the network 0.0.0.0 EIGRP subcommand. Oddly enough, this command does not actually
match interface IP addresses of interfaces, but is a special case in which IOS thinks “if my
routing table has a default route in it, put a default route (0.0.0.0/0) into the EIGRP table.”
(If the route leaves the routing table, then the router will notify neighbors that the route
has failed.)
Configuring a Default Network
The second option for creating a default route is to flag a route for a classful network–for
a prefix that will be advertised into the EIGRP domain–as a route that can be used as a default
route. Then each router can use the forwarding details in that route–the outgoing interface
and next-hop router–as its default route.
Configuring this feature requires a couple of steps. The concepts require the most
thought, with the configuration commands that follow being relatively simple:
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.
Step 1 requires a class A, B, or C network, known in the routing table of the router that
will generate the default route (Router I1 in Figure 4-9). Most often, that route is either
created off a loopback interface for the purpose of making this process work, or an existing
route on the Internet side of the router is used.
Figure 4-10 shows two examples. First, class C network 198.133.219.0/24 exists off I1’s
S0/0/0 interface, so I1 has a connected route for this class C network in its routing table.
Alternatively, the engineer could configure a loopback interface, such as loopback 9, so
that I1 would have a connected route for 192.31.7.0/24. In both cases, the routes would
need to be advertised into EIGRP, by matching the address using the network command.
If the configuration stopped at Step 1, then the Enterprise routers simply know yet another
route. 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.
! lines omitted for brevity
C* 192.31.7.0/24 is directly connected, Loopback8
I1#show ip eigrp topology 192.31.7.0/24
IP-EIGRP (AS 1): Topology entry for 192.31.7.0/24
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 128256
Routing Descriptor Blocks:
0.0.0.0 (Loopback8), from Connected, Send flag is 0x0
Composite metric is (128256/0), Route is Internal
Vector metric:
Minimum bandwidth is 10000000 Kbit
Total delay is 5000 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1514
Hop count is 0
Exterior flag is set
The configuration has several results, as seen in the example:
■ A connected route for 192.31.7.0/24, a class C network
■ The advertisement of that network into EIGRP due to the network 192.31.7.0 command
■ The setting of the exterior flag on the route
Because of the ip default-network 192.31.7.0 command, the routing table lists the route
as a candidate default route, as denoted by an asterisk.
Interestingly, the router with the ip default-network command configured–I1 in this
case–does not use that route as a default route, as indicated by the highlighted phrase
“Gateway of last resort not set.” (Gateway of last resort refers to the next-hop router of a
router’s current default route.) Although I1 flags the route as a candidate default route, I1
itself does not use that route as its default, because I1 is actually the original advertiser of
the default.
Moving on to another Enterprise router, in this case B1, you can see in Example 4-8 that
not only does the remote router learn the candidate default route, but that the B1 uses this
same information as B1’s default route.
Example 4-8 Gateway of Last Resort on Router B1
B1#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
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Chapter 4: EIGRP Route Summarization and Filtering 131
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is 10.1.1.1 to network 192.31.7.0
10.0.0.0/8 is variably subnetted, 15 subnets, 3 masks
Lines omitted for brevity
D* 192.31.7.0/24 [90/2297856] via 10.1.1.1, 00:05:10, Serial0/0/0.1
In this case, B1 has indeed learned an EIGRP route for 192.31.7.0/24, a route flagged as exterior.
Because this happens to be the only candidate default route learned by B1 at this
point, it is the best default route. So, B1 sets its gateway of last resort to 10.1.1.1–the nexthop
IP address of B1’s route to 192.31.7.0/24. If B1 knew of multiple candidate default
routes, it would have chosen the best route based on administrative distance and then
metric, and used that route as the basis for the gateway of last resort.