RIP and Discontiguous Networks

Recall that discontiguous networks have subnets of the same major network, separated by a different major network.

RIP does not carry a prefix mask, and it summarizes at the natural classful length mask. This raises another issue: How can you support discontiguous networks? This type of network must communicate exact addresses across the entire network. Referring again to Figure 6-3, when the router must send updates about subnet 140.10.20.0 to router R1, it summarizes the update at the natural network boundary across the serial line because the interface is configured with a different major network. As mentioned earlier, when an update is sent across a network boundary with RIPV1, it is summarized at the natural class A, B, C mask. This is done because a RIP update does not carry a mask, so network boundaries should be defined. When R1 receives this update, it drops an update about network 140.10.0.0/16 because one of its own interfaces is connected to one of the subnets of network 140.10.0.0/16. In this case, the subnet is 140.10.10.0. The RIPV1 router will not accept an update about a route to which its own interface is connected because all subnets of its connected major network should fall behind a classful boundary. Therefore, from Figure 6-3, R1 expects all the subnets of 140.10.0.0/16 to stay behind the Ethernet of R1. This is because the serial interface is a different major network, so no part of network 140.10.0.0 should exist behind the serial interface.

When R1 sends updates about subnet 140.10.2.0 through the serial interface, it sends a 140.10.0.0/16 classful mask, because it is sending the update across a different major network. When R2 receives this update, it drops it because it has a direct connection to one of the subnets of the same major network.

Therefore, RIPV1 is able to support discontiguous networks. You can make this topology work, as shown in Figure 6-10, using a Cisco router. One method to accomplish this is to configure a secondary address, as shown in Figure 6-10. In this case, you must ensure that the secondary address belongs to the same major network to which you are trying to connect, and that the subnet mask is consistent. As Figure 6-10 illustrates, the secondary address of the serial link is the same as that of the two Ethernets of R1 and R2. Now, the network is no longer discontiguous because the secondary address matches the major network.

Figure 6-10. Secondary Address to Support Discontiguous Networks

Figure 6-10. Secondary Address to Support Discontiguous Networks

Consider the configuration of the serial interface of R1:

# interface serial 0

# ip address 140.10.11.1 255.255.255.0 secondary

The second method of dealing with the RIP topology is not as simple as the first. However, it will be successful when you do not have address space left to assign a secondary address for the network that you are making contiguous. As an example, this method would be helpful if there were no available space within network 140.10.0.0 to be assigned for the secondary address on the serial link. In that case, you could configure static routes on both routers for the destinations across the other end of the links.

For example, observe Figure 6-11. If the major network static is configured, then it must be configured on all the routers. So, for Figure 6-11, all routers should have a static route to 140.10.0.0. Obviously, this does not scale if there are multiple routers across both ends of the serial link of a discontiguous network. The most effective method to accomplish this is to create a static route that advertises the exact routes with the correct mask, instead of creating a major net route. In referring to Figure 6-11, a static route would be needed on R1 and R2.

Figure 6-11. Static Routes to Support Discontiguous Networks

Figure 6-11. Static Routes to Support Discontiguous Networks

Router R1

R1# config t ip route 140.10.20.0 255.255.255.0 serial 0 ip route 140.10.21.0

255.255.255.0

serial 0

ip route 140.10.21.0 255.255.255.0 serial 0

router rip network 140.10.0.0

network 131.108.0.0

redistribute static default-metric 2

The exact same configuration is required on router R2 for the links behind R1:

ip route 140.10.20.0 ip route 140.10.21.0 ip route 140.10.11.0 ip route 140.10.12.0 ip route 140.10.10.0 router rip

255,

.255.

.255.

0

serial

0

255,

.255.

.255.

0

serial

0

255.

.255.

.255.

0

serial

0

255.

.255.

.255.

0

serial

0

255.

.255.

.255.

0

serial

0

network 140.10.0.0 network 131.108.0.0 redistribute static default-metric 2

The solutions for the RIP and discontiguous networks explained here are not long-term, however. As a network administrator, you should use these as a workaround strategy and begin planning your network migration to scalable classless protocols such as OSPF, IS-IS, and Enhanced IGRP, which are discussed in Chapter 12.

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  • darcy
    How scale rip network?
    1 year ago

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