Network Statements, Part 1

In this multi-part series, we’ll examine the effects of Cisco IOS “network” statements for various IP routing protocols. Let’s start with the IGPs (Interior Gateway Protocols). The IGPs for which Cisco IOS uses network statements are:

• RIP
• OSPF
• EIGRP

Note that Cisco IOS does not use network statements for IS-IS (another IP IGP). Also, BGP, which is an EGP (Exterior Gateway Protocol), uses its network statements differently, so we’ll discuss it later.

Refer to the example topology shown in Figure 1:

As you can see, we have multiple logical networks connected to a router, those being:

• 192.168.1.0/24
• 172.16.1.0/24
• 10.1.1.0/24
• 10.2.2.0/24

What we want to do is get RIP running on the Fa0/1, Fa0/2 and Fa0/3 interfaces, but not on Fa0/0. To do this, we use “network” statements under RIP, as follows:

router rip
   network 172.16.0.0
   network 10.0.0.0

Note that a “network” statement has two functions:

1. It tells the router on which interfaces to run the routing protocol. Since we have network statements that cover the 10.0.0.0 and 172.16.0.0 networks, the Fa0/1, Fa0/2 and Fa0/3 interfaces will run the routing protocol. The meaning of this varies, but in the case of RIP, it means start sending RIP updates on the interfaces, and listen for incoming RIP updates.
2. It tells the router to inject the logical networks of the interfaces into the routing protocol. In our example, those would be the 10.1.1.0/24, 10.2.2.0/24 and 172.16.1.0/24 prefixes.

Note that when it comes to advertising the prefixes to neighbor routers, the exact result takes into account the type of route summarization, if any, that is being performed by the routing protocol. Let’s look at this in more detail. In our case, the router will advertise the following prefixes on its interfaces:

• Fa0/0 – nothing (this interface is not running the protocol)
• Fa0/1 – 10.1.1.0 and 172.16.0.0
• Fa0/2 – 10.0.0.0
• Fa0/3 – 10.2.2.0 and 172.16.0.0

Recall that RIPv1 is a classful protocol, meaning that the updates do not contain subnet masks. Because of this, RIPv1 performs automatic route summarization at the boundary between classful networks, which is why the router is advertising the classful network 172.16.0.0 on Fa0/1 and Fa0/3, and likewise 10.0.0.0 on Fa0/2. Note that the network 10.0.0.0 subnets are advertised on the interfaces belonging to that network (10.1.1.0 on Fa0/1, and 10.2.2.0 on F0/3). Since classful protocols do not allow VLSM (Variable-Length Subnet Masks), the assumption is that any neighbor routers will be using the same subnet mask with that classful network.

Next time, we’ll look more closely at the actions of network statements under RIP.

Author: Al Friebe