7.2 Default Routing
7.2.1 Gateway of last resort
The gateway of last resort is the term applied to a routing entry in the Cisco routing table that the router forwards packets to when it lacks a more specific route. The gateway of last resort can be learned from a route provided by another router that is tagged as candidate default by the advertising router. The ip default-network command is one way to make a router tag a route as a gateway of last resort.

The ip default-network xxx.xxx.xxx.xxx command causes a router to treat xxx.xxx.xxx.xxx as a gateway of last resort. A router can have multiple IP default networks entered.

The ip default-gateway command is used with routers that have IP routing disabled. It gives them an address to which they can forward packets whose destination IP addresses are not in their address space. Cisco 2500s in boot ROM mode are a good example of this situation. The version of Cisco IOSŪ software that runs from the Cisco 2500 boot ROM doesn't understand the default-network command.

Note: To upgrade a Cisco 2500 with a single Flash memory partition, you must reconfigure the configuration registers so that the router comes up in boot ROM mode the next time it is rebooted.

The local domain refers to networks a router has local knowledge (context) of because it has a direct connection to them. Using secondary addresses is a way to use the same interface on a router to connect to two or more subnets. A secondary address is sometimes used when all host addresses from one subnet have been assigned and there are still more hosts on the physical network that need an IP address.

If a LAN segment has a subnetted address space of 168.71.2.16 with a mask of 255.255.255.240, only 14 host addresses are available. If more than 14 hosts are attached to the network, another IP address space (subnet) is required. In this situation, hosts in different subnets attached to the same LAN must use the router to forward packets to one another.

In the main figure, if router C did not have an explicit route to subnet 168.72.6.0 or the major network 168.72.0.0, it would need a gateway of last resort to send packets to this subnet.

In the routing table from router C, you can see that router C has installed a gateway of last resort network 10.0.0.0 with a next hop of 168.71.9.1. You can also see that there is no route to 168.72.6.0 or 168.72.0.0. (see RouterC#show ip route command output).

The gateway of last resort is being advertised by router A as a candidate default route. It is up to the router receiving a candidate default route to determine whether it should install it as the gateway of last resort. In this scenario, having router A advertise 10.0.0.0 as the candidate default route also caused router B to send a route to router C for 10.0.0.0 that router B flagged as a candidate default route.

Lab Activity
  In this lab, you will learn how to configure OSPF with a passive-interface, static route and a default route.

The main figure shows that as far as router C is concerned, the link between router C and router A is the best path for reaching 10.0.0.0 because router B increases the metric for 10.0.0.0 when advertising it to router C. The router A metric is smaller than the router B metric. Interestingly, as far as reaching 168.71.6.0 is concerned, both of router C physical paths --- via router A or router B --- could be of equal cost if all links in the network were the same delay and bandwidth. However, because this scenario relies on using the gateway of last resort to 10.0.0.0 to reach 168.71.6.0, only one path is used, even if they are equal cost in theory.

The configuration from router A shows how to enable this function. (see RouterA#show running-config command output). 

The following is a step-by-step explanation of enabling a router to advertise a gateway of last resort. The steps do not have to be performed in this exact order, but all steps must be completed.

  1. Enter the appropriate ip default-network command. In this case, network 10.0.0.0 is used.
  2. Interestingly, this network does not have to actually exist anywhere in the network. Because IP packets are forwarded on a hop-by-hop basis, it is necessary only to convince a router to send a packet to the next hop to ensure that the packet is making forward progress to its destination. What this command is doing is telling other routers that somewhere there is a router advertising 10.0.0.0 and that the router knows how to reach every other network in the world. Therefore, a packet forwarded in the general direction of this router is on its way to reaching its ultimate destination.
  3. Configure a next-hop address to reach the default network. In this case, a host in the secondary address space on Ethernet0 (171.68.207.129) is used. This is a static (manually configured) route.
  4. Enable the redistribute static command for the routing protocol in use. In this case, IGRP is being used. If you fail to enter this command, the static route is not advertised.
  5. Configure a default metric. If you fail to do so, the router uses the unreachable metric (infinity) when advertising this route, preventing the route from being accepted by other routers.
  6. The default metric command takes the following form: bandwidth, delay, reliability, load, and maximum transmission unit (MTU). Remember that by default only bandwidth and delay are used by IGRP. The delay in a route is cumulative, and the minimum bandwidth is used for all links in the path.