Summary of Module 1

Summary

This page summarizes the topics discussed in this module.

Variable-Length Subnet Masks (VLSM), often referred to as "subnetting a subnet", is used to maximize addressing efficiency. It is a feature that allows a single autonomous system to have networks with different subnet masks. The network administrator is able to use a long mask on networks with few hosts, and a short mask on subnets with many hosts.  

It is important to design an addressing scheme that allows for growth and does not involve wasting addresses. To apply VLSM to the addressing problem, large subnets are created for addressing LANs. Very small subnets are created for WAN links and other special cases.

VLSM helps to manage IP addresses. VLSM allows for the setting of a subnet mask that suits the link or the segment requirements. A subnet mask should satisfy the requirements of a LAN with one subnet mask and the requirements of a point-to-point WAN with another.

Addresses are assigned in a hierarchical fashion so that summarized addresses will share the same high-order bits. There are specific rules for a router. It must know in detail the subnet numbers attached to it and it does not need to tell other routers about each individual subnet if the router can send an aggregate route for a set of routers. A router using aggregate routes would have fewer entries in its routing tables.

If VLSM is the scheme chosen, it must then be calculated and configured correctly.

RIP v1 is considered an interior gateway protocol that is classful. RIP v1 is a distance vector protocol that broadcasts its entire routing table to each neighbor router at predetermined intervals. The default interval is 30 seconds. RIP uses hop count as a metric, with 15 as the maximum number of hops.

To enable a dynamic routing protocol, select a routing protocol, such as RIP v2, assign the IP network numbers without specifying the subnet values, and then assign the network or subnet addresses and the appropriate subnet mask to the interfaces. In RIP v2, the router command starts the routing process. The network command causes the implementation of three functions. The routing updates are multicast out an interface, the routing updates are processed if they enter that same interface, and the subnet that is directly connected to that interface is advertised. The version 2 command enables RIP v2.

The show ip protocols command displays values about routing protocols and routing protocol timer information associated with the router. Use the debug ip rip command to display RIP routing updates as they are sent and received. The no debug all or undebug all commands will turn off all debugging.

Default routes

Default routes
1.2.7 This page will describe default routes and explain how they are configured.
By default, routers learn paths to destinations three different ways:

• Static routes – The system administrator manually defines the static routes as the next hop to a destination. Static routes are useful for security and traffic reduction, as no other route is known.
• Default routes – The system administrator also manually defines default routes as the path to take when there is no known route to the destination. Default routes keep routing tables shorter. When an entry for a destination network does not exist in a routing table, the packet is sent to the default network.
• Dynamic routes – Dynamic routing means that the router learns of paths to destinations by receiving periodic updates from other routers.

In Figure , the static route is indicated by the following command:

Router(config)#ip route 172.16.1.0 255.255.255.0 17.16.2.1

The ip default-network command establishes a default route in networks using dynamic routing protocols: 

Router(config)#ip default-network 192.168.20.0

Generally after the routing table has been set to handle all the networks that must be configured, it is often useful to ensure that all other packets go to a specific location. This is called the default route for the router. One example is a router that connects to the Internet. All the packets that are not defined in the routing table will go to the nominated interface of the default router.
The ip default-network command is usually configured on the routers that connect to a router with a static default route. 
In Figure , Hong Kong 2 and Hong Kong 3 would use Hong Kong 4 as the default gateway. Hong Kong 4 would use interface 192.168.19.2 as its default gateway. Hong Kong 1 would route packets to the Internet for all internal hosts. To allow Hong Kong 1 to route these packets it is necessary to configure a default route as:

HongKong1(config)#ip route 0.0.0.0 0.0.0.0 s0/0

The zeros in the IP address and mask portions of the command represent any destination network with any mask. Default routes are referred to as quad zero routes. In the diagram, the only way Hong Kong 1 can go to the Internet is through interface s0/0.
This page concludes this lesson. The next page will summarize the main points from this module.

Troubleshooting RIP v2

Troubleshooting RIP v2
1.2.6 This page explains the use of the debug ip rip command.
Use the debug ip rip command to display RIP routing updates as they are sent and received. The no debug all or undebug all commands will turn off all debugging.
The example shows that the router being debugged has received updates from one router at source address 10.1.1.2. The router at source address 10.1.1.2 sent information about two destinations in the routing table update. The router being debugged also sent updates, in both cases to the multicast address 224.0.0.9 as the destination. The number in parentheses is the source address encapsulated into the IP header.
Other outputs sometimes seen from the debug ip rip command includes entries such as the following:

RIP: broadcasting general request on Ethernet0

RIP: broadcasting general request on Ethernet1

These outputs appear at startup or when an event occurs such as an interface transition or a user manually clears the routing table.
An entry, such as the following, is most likely caused by a malformed packet from the transmitter:

RIP: bad version 128 from 160.89.80.43

Examples of debug ip rip outputs and meanings are shown in Figure .
The Lab Activities will help students become more familiar with debug commands.
The next page will discuss default routes.

Verifying RIP v2

Verifying RIP v2

1.2.5 The show ip protocols and show ip route commands display information about routing protocols and the routing table. This page explains how show commands are used to verify a RIP configuration.
The show ip protocols command displays values about routing protocols and routing protocol timer information associated with the router. In the example, the router is configured with RIP and sends updated routing table information every 30 seconds. This interval is configurable. If a router running RIP does not receive an update from another router for 180 seconds or more, the first router marks the routes served by the non-updating router as being invalid. The holddown timer is set to 180 seconds. Therefore, an update to a route that was down and is now up could stay in the holddown state until the full 180 seconds have passed.
If there is still no update after 240 seconds the router removes the routing table entries. The router is injecting routes for the networks listed following the Routing for Networks line. The router is receiving routes from the neighboring RIP routers listed following the Routing Information Sources line. The distance default of 120 refers to the administrative distance for a RIP route.
The show ip interface brief command can also be used to list a summary of the information and status of an interface.
The show ip route command displays the contents of the IP routing table. The routing table contains entries for all known networks and subnetworks, and contains a code that indicates how that information was learned.
Examine the output to see if the routing table is populated with routing information. If entries are missing, routing information is not being exchanged. Use the show running-config or show ip protocols Privileged EXEC commands on the router to check for a possible misconfigured routing protocol.
The Lab Activity will teach students how to use show commands to verify RIP v2 configurations.
The next page will discuss the debug ip rip command.

Configuring RIP v2

Configuring RIP v2
1.2.4 This page will teach students how to configure RIP v2. RIP v2 is a dynamic routing protocol that is configured by naming the routing protocol RIP Version 2, and then assigning IP network numbers without specifying subnet values. This section describes the basic commands used to configure RIP v2 on a Cisco router. 
To enable a dynamic routing protocol, the following tasks must be completed:

• Select a routing protocol, such as RIP v2.
• Assign the IP network numbers without specifying the subnet values.
• Assign the network or subnet addresses and the appropriate subnet mask to the interfaces.

RIP v2 uses multicasts to communicate with other routers. The routing metric helps the routers find the best path to each network or subnet.
The router command starts the routing process. The network command causes the implementation of the following three functions:

• The routing updates are multicast out an interface.
• The routing updates are processed if they enter that same interface.
• The subnet that is directly connected to that interface is advertised.

The network command is required because it allows the routing process to determine which interfaces will participate in the sending and receiving of routing updates. The network command starts up the routing protocol on all interfaces that the router has in the specified network. The network command also allows the router to advertise that network.
The router rip and version 2 commands combined specify RIP v2 as the routing protocol, while the network command identifies a participating attached network. 
In this example, the configuration of Router A includes the following:

• router rip – Enables RIP as the routing protocol
• version 2 – Identifies version 2 as the version of RIP being used
• network 172.16.0.0 – Specifies a directly connected network
• network 10.0.0.0 – Specifies a directly connected network

The interfaces on Router A connected to networks 172.16.0.0 and 10.0.0.0, or their subnets, will send and receive RIP v2 updates. These routing updates allow the router to learn the network topology. Routers B and C have similar RIP configurations but with different network numbers specified.
Figure shows another example of a RIP v2 configuration.
The Lab Activities on this page will show students how to convert RIP v1 to RIP v2.
The next page will describe the commands that are used to verify RIP v2

Comparing RIP v1 and v2

Comparing RIP v1 and v2
1.2.3 This page will provide some more information about how RIP works. It will also describe the differences between RIP v1 and RIP v2. RIP uses distance vector algorithms to determine the direction and distance to any link in the internetwork. If there are multiple paths to a destination, RIP selects the path with the least number of hops. However, because hop count is the only routing metric used by RIP, it does not necessarily select the fastest path to a destination.
RIP v1 allows routers to update their routing tables at programmable intervals. The default interval is 30 seconds. The continual sending of routing updates by RIP v1 means that network traffic builds up quickly.  To prevent a packet from looping infinitely, RIP allows a maximum hop count of 15. If the destination network is more than 15 routers away, the network is considered unreachable and the packet is dropped. This situation creates a scalability issue when routing in large heterogeneous networks. RIP v1 uses split horizon to prevent loops. This means that RIP v1 advertises routes out an interface only if the routes were not learned from updates entering that interface. It uses holddown timers to prevent routing loops. Holddowns ignore any new information about a subnet indicating a poorer metric for a time equal to the holddown timer.
Figure summarizes the behavior of RIP v1 when used by a router.
RIP v2 is an improved version of RIP v1. It has many of the same features of RIP v1. RIP v2 is also a distance vector protocol that uses hop count, holddown timers, and split horizon. Figure compares and contrasts RIP v1 and RIP v2. The TTL field in the IP packet forces the packet to be dropped. When the hop count reaches 15 routers, the network is considered unreachable, and the packet is dropped because the router doesn't have a route to the destination network.
The first Lab Activity on this page will show students how to set up and configure RIP on routers. The second Lab Activity will review the basic configuration of routers. The Interactive Media Activity will help students understand the differences between RIP v1 and RIP v2.
The next page will explain how RIP v2 is configured.

RIP v2 feature

RIP v2 feature
1.2.2 This page will discuss RIP v2, which is an improved version of RIP v1. Both versions of RIP share the following features:

• It is a distance vector protocol that uses a hop count metric.
• It uses holddown timers to prevent routing loops – default is 180 seconds.
• It uses split horizon to prevent routing loops.
• It uses 16 hops as a metric for infinite distance.

RIP v2 provides prefix routing, which allows it to send out subnet mask information with the route update. Therefore, RIP v2 supports the use of classless routing in which different subnets within the same network can use different subnet masks, as in VLSM.
RIP v2 provides for authentication in its updates. A set of keys can be used on an interface as an authentication check. RIP v2 allows for a choice of the type of authentication to be used in RIP v2 packets. The choice can be either clear text or Message-Digest 5 (MD5) encryption. Clear text is the default. MD5 can be used to authenticate the source of a routing update. MD5 is typically used to encrypt enable secret passwords and it has no known reversal.
RIP v2 multicasts routing updates using the Class D address 224.0.0.9, which provides for better efficiency.
The next page will discuss RIP in greater detail.