RIPv1 Topology Limitation


Static Routes and Null Interfaces
To configure the static supernet route on R2, the following command is used:
R2(config)#ip route 192.168.0.0 255.255.0.0 Null0
Remember that route summarization allows a single high-level route entry to represent many lower-level routes, thereby reducing the size of routing tables. The static route on R2 uses a /16 mask to summarize all 256 networks ranging from 192.168.0.0/24 to 192.168.255.0/24.
The address space represented by the static summary route 192.168.0.0/16 does not actually exist. In order to simulate this static route, we use a null interface as the exit interface. You do not need to enter any commands to create or configure the null interface. It is always up but does not forward or receive traffic. Traffic sent to the null interface is discarded. For our purposes, the null interface will serve as the exit interface for our static route. Remember from Chapter 2, "Static Routing," that a static route must have an active exit interface before it will be installed in the routing table. Using the null interface will allow R2 to advertise the static route in RIP even though networks belonging to the summary 192.168.0.0/16 do not actually exist.

Route Redistribution

The second command that needs to be entered is the redistribute static command:
R2(config-router)#redistribute static
Redistribution involves taking the routes from one routing source and sending those routes to another routing source. In our example topology, we want the RIP process on R2 to redistribute our static route (192.168.0.0/16) by importing the route into RIP and then sending it to R1 and R3 using the RIP process. We will see if this is indeed happening and if not, why not.

Links
"Configuring Logical Interfaces," http://www.cisco.com/en/US/products/sw/iosswrel/ps1828/products_configuration_guide_chapter09186a0080087da4.html

Verifying and Testing Connectivity

To test whether or not the topology has full connectivity, we first verify that both serial links on R2 are up using the show ip interface brief as shown in the figure for R2 Links. If a link is down, the Status field or the Protocol field (or both fields) will display down in the command output. If a link is up, both fields will display up, as shown here. R2 has direct connectivity to R1 and R3 across the serial links.
But can R2 ping LANs on R1 and R3? Are there any connectivity problems with a classful routing protocol and the discontiguous subnets of 172.30.0.0? Let's test the communications between the routers using ping.

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