Default Routes
Routers forward packets using a default route when there are no specific routes that match a
packet’s destination IP address in the IP routing table. Routing protocols can advertise default
routes, with each router choosing the best default route to list as that router’s gateway of last resort.
This section covers how a router can create a default route and then cause an IGP to advertise the
default route.
In addition to the advertisement of default routes, each router may use one of two options for how
the default route is used. As described in Chapter 6, “IP Forwarding (Routing),” each router’s
configuration includes either the (default) ip classless command or the no ip classless command.
With ip classless, if a packet’s destination does not match a specific route in the IP routing table,
the router uses the default route. With no ip classless, the router first checks to see if any part of
the destination address’s classful network is in the routing table. If so, that router will not use the
default route for forwarding that packet.
Cisco IOS supports five basic methods of advertising default routes with IGPs, four of which are
covered here. One method for advertising a default route is for one routing protocol to redistribute
another routing protocol’s default route. Because route redistribution has already been covered
heavily, this section of the chapter covers other methods. Of the other four methods, not all are
supported by all IGPs, as you can see in Table 10-10.
NOTE The topic of default routing requires discussion of the configuration on one router, plus
configuration of the other routers using the same IGP. For this section, I will call the router with
the default routing configuration the “local” router, and other routers using the same IGP “other”
routers.
330 Chapter 10: IGP Route Redistribution, Route Summarization, and Default Routing
Interestingly, when a router learns of multiple default routes, using any of these methods, it will
use the usual process for choosing the best route: administrative distance, route type (per Table 10-9,
earlier in this chapter), and lowest metric, in that order.
Figure 10-8 shows a sample network used with all the default route examples, in which R1 is the
local router that configures the default routing commands.
Figure 10-8 Sample Network for Default Route Examples
Table 10-10 Four Methods for Learning Default Routes
Feature RIP EIGRP OSPF
Static route to 0.0.0.0, with the redistribute static command Yes Yes No
The default-information originate command Yes No Yes
The ip default-network command Yes Yes No
Using summary routes No Yes No
NOTE Table 10-10 has details that may be difficult to memorize. To make it easier, you could
start by ignoring the use of summary static routes, because it is not recommended by Cisco.
Then, note that RIP supports the other three methods, whereas EIGRP supports two methods
and OSPF supports only one—with EIGRP and OSPF not supporting any of the same options.
R1
R9
R3 R5
13.1.2.3/24
13.1.1.0/24 15.1.1.0/24
17.1.1.0/24
Fa0/0
15.1.2.5/24
Fa0/0
17.1.2.7/24
Fa0/0
RID 1.1.1.1
RID 5.5.5.5
.3
0/0/0.1
.5
.1 .1 0/0.1
.1 .1
.7
R4
14.1.1.4/24 14.1.1.0/24
10.1.1.102/24
Fa0/0
Fa0/0
.4
0/0/0.1
R7
RIP
Network 13.0.0.0
OSPF 1 (Two Areas, with R5 as ABR)
Network 15.0.0.0
IS-IS (Both Routers Are L2)
Network 17.0.0.0
EIGRP 1
Network 14.0.0.0