Using the ip default-network Command
RIP and EIGRP can inject default routes by using the ip default-network command. To do so, the following must be true on the local router:
■ The local router must configure the ip default-network net-number command, with net number being a classful network number.
■ The classful network must be in the local router’s IP routing table, via any means.
■ For EIGRP only, the classful network must be advertised by the local router into EIGRP, again through any means.
■ This method is not supported by OSPF. When using the ip default-network command, RIP and EIGRP differ in how they advertise the default. RIP advertises a route to 0.0.0.0/0, but EIGRP flags its route to the classful network as acandi date default route. Because EIGRP flags these routes as candidates, EIGRP must then also be advertising those classful networks.
However, because RIP does not flag the classful network as a candidate default route, RIP does not actually have to advertise the classful network referenced in the ip default-network command.
Example 11-10 shows the key difference between RIP and EIGRP with regard to the ip default network command. In this case, R1 will advertise about classful network 10.0.0.0 using EIGRP due to the auto-summary command.
Using Route Summarization to Create Default Routes
Generally speaking, route summarization combines smaller address ranges into a small number of larger address ranges. From that perspective, 0.0.0.0/0 is the largest possible summary, because it includes all possible IPv4 addresses. And, as it turns out, EIGRP route summarization supports summarizing the 0.0.0.0/0 supernet, effectively creating a default route. Because route summarization causes a null route to be created for the summary, some Cisco documentation advises against using route summarization to create a default route. For example,
Example 11-10 Static Default Route with Route Redistribution
■ The local router must configure the ip default-network net-number command, with net number being a classful network number.
■ The classful network must be in the local router’s IP routing table, via any means.
■ For EIGRP only, the classful network must be advertised by the local router into EIGRP, again through any means.
■ This method is not supported by OSPF. When using the ip default-network command, RIP and EIGRP differ in how they advertise the default. RIP advertises a route to 0.0.0.0/0, but EIGRP flags its route to the classful network as acandi date default route. Because EIGRP flags these routes as candidates, EIGRP must then also be advertising those classful networks.
However, because RIP does not flag the classful network as a candidate default route, RIP does not actually have to advertise the classful network referenced in the ip default-network command.
Example 11-10 shows the key difference between RIP and EIGRP with regard to the ip default network command. In this case, R1 will advertise about classful network 10.0.0.0 using EIGRP due to the auto-summary command.
Using Route Summarization to Create Default Routes
Generally speaking, route summarization combines smaller address ranges into a small number of larger address ranges. From that perspective, 0.0.0.0/0 is the largest possible summary, because it includes all possible IPv4 addresses. And, as it turns out, EIGRP route summarization supports summarizing the 0.0.0.0/0 supernet, effectively creating a default route. Because route summarization causes a null route to be created for the summary, some Cisco documentation advises against using route summarization to create a default route. For example,
Example 11-10 Static Default Route with Route Redistribution
