IP Routing

Why Doesn't RIP or IGRP Support Discontiguous Networks?

Document ID: 13724

Updated: Aug 10, 2005



A discontiguous network comprises a major net that separates another major net. In Figure 1, a subnet of network separates network is a discontiguous network. This document describes why RIPv1 and IGRP do not support discontiguous networks and explains how you can work around this issue.

Figure 1 – Discontiguous Network




Cisco recommends that you have knowledge of these topics:

  • How to Configure RIPv1 and IGRP

  • Concepts that underlie IP Addresses and Subnets

Components Used

This document is not restricted to specific software and hardware versions.

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.


Refer to Cisco Technical Tips Conventions for more information on document conventions.

Background Information

RIP and IGRP are classful protocols. Whenever RIP advertises a network across a different major net boundary, RIP summarizes the advertised network at the major net boundary. In Figure 1, when Router 1 sends an update that contains to Router 2 across, the router converts into This process is called auto-summarization.

When Router 1 Sends Updates to Router 2

Use the topology in Figure 1 to identify what questions you need answer when Router 1 prepares to send an update to Router 2. Refer to Behavior of RIP and IGRP When Sending and Receiving Updates for more detailed information about this decision-making. Remember that the advertisement of network is of interest here. Here is the question you need to answer:

  • Is part of the same major network as, which is the network assigned to the interface that sources the update?

    • No: Router 1 summarizes and advertises the route The summarization is done to the major classful boundary. In this case, the address is a class B address, and so, the summary is 16 bits.

    • Yes: Although this is not the case in the example, if the answer to the question is yes, Router 1 would not summarize the network and would advertise the network with subnet information intact.

Use the debug ip rip command on Router 1 to see the update that Router 1 sends:

RIP: sending v1 update to via Serial3/0 (
RIP: build update entries
  network metric 1

Router 2 Receives Updates from Router 1

When Router 2 prepares to receive and update from Router 1, you need to identify the questions that need to be answered. Again, remember that the reception of network is of interest here. However, remember that when Router 1 sent the update the network was summarized to Here is the question you need to answer:

  • Is the network that receives updates ( part of the same major network of, which is the address assigned to the interface that received the update?

    • No: Do any subnets of this major network already exist in the routing table known from interfaces other than that which received the update?

    • Yes: Ignore the update.

Again, use the debug ip rip command on Router 2 to see the updates that came in from Router 1:

RIP: received v1 update from on Serial2/0 in 1 hops

However, the routing table of Router 2 indicates that the update was ignored. The only entry for any subnetwork or network on is the one directly connected to Ethernet0. The output of the show ip route command on Router 2 shows: is subnetted, 1 subnets
C is directly connected, Serial2/0 is subnetted, 1 subnets
C is directly connected, Ethernet0/0

The behavior of RIPv1 and IGRP is such that when Router 1 and Router 2 exchange updates, both Router 1 and Router 2 do not learn about the attached subnetworks of and As a result, devices on these two subnetworks are unable to communicate with each other.


In some situations, discontiguous networks are unavoidable. In these situations Cisco recommends that you do not use RIPv1 or IGRP. Routing protocols like EIGRP or OSPF are better suited for this situation.

Establish Connectivity

In the event that you use RIPv1 or IGRP with discontiguous networks, you must use static routes to establish connectivity between the discontiguous subnetworks. In this example these static routes establish this connectivity:

For Router 1:

ip route

For Router 2:

ip route

Related Information

Updated: Aug 10, 2005
Document ID: 13724