Compare and contrast distance vector and link-state routing

Compare and contrast distance vector and link-state routing
2.1.6 This page will compare distance vector and link-state routing protocols.

All distance vector protocols learn routes and then send these routes to directly connected neighbors. However, link-state routers advertise the states of their links to all other routers in the area so that each router can build a complete link-state database. These advertisements are called link-state advertisements or LSAs. Unlike distance vector routers, link-state routers can form special relationships with their neighbors and other link-state routers. This is to ensure that the LSA information is properly and efficiently exchanged.

The initial flood of LSAs provides routers with the information that they need to build a link-state database. Routing updates occur only when the network changes. If there are no changes, the routing updates occur after a specific interval. If the network changes, a partial update is sent immediately. The partial update only contains information about links that have changed. Network administrators concerned about WAN link utilization will find these partial and infrequent updates an efficient alternative to distance vector routing protocols, which send out a complete routing table every 30 seconds. When a change occurs, link-state routers are all notified simultaneously by the partial update. Distance vector routers wait for neighbors to note the change, implement the change, and then pass the update to the neighbor routers. 

The benefits of link-state over distance vector protocols include faster convergence and improved bandwidth utilization. Link-state protocols support CIDR and VLSM. This makes them a good choice for complex and scalable networks. In fact, link-state protocols generally outperform distance vector protocols on any size network. Link-state protocols are not implemented on every network because they require more memory and processor power than distance vector protocols and can overwhelm slower equipment. Another reason they are not more widely implemented is the fact that link-state protocols are quite complex. Link-state routing protocols require well-trained administrators to correctly configure and maintain them.

This page concludes this lesson. The next lesson will introduce a link-state routing protocol called OSPF. The first page will provide an overview. 

Advantages and disadvantages of link-state routing

Advantages and disadvantages of link-state routing
2.1.5 This page lists the advantages and disadvantages of link-state routing protocols. The following are advantages of link-state routing protocols: 

• Link-state protocols use cost metrics to choose paths through the network. The cost metric reflects the capacity of the links on those paths.
• Link-state protocols use triggered updates and LSA floods to immediately report changes in the network topology to all routers in the network. This leads to fast convergence times.
• Each router has a complete and synchronized picture of the network. Therefore, it is very difficult for routing loops to occur.
• Routers use the latest information to make the best routing decisions.
• The link-state database sizes can be minimized with careful network design. This leads to smaller Dijkstra calculations and faster convergence.
• Every router, at the very least, maps the topology of its own area of the network. This attribute helps to troubleshoot problems that can occur.
• Link-state protocols support CIDR and VLSM.

The following are some disadvantages of link-state routing protocols: 

• They require more memory and processor power than distance vector protocols. This makes it expensive to use for organizations with small budgets and legacy hardware.
• They require strict hierarchical network design, so that a network can be broken into smaller areas to reduce the size of the topology tables.
• They require an administrator who understands the protocols well.
• They flood the network with LSAs during the initial discovery process. This process can significantly decrease the capability of the network to transport data. It can noticeably degrade the network performance.

The next page will continue the comparison of link-state and distance vector protocols.e

Link-state routing algorithms

Link-state routing algorithms
2.1.4 Link-state routing algorithms maintain a complex database of the network topology by exchanging link-state advertisements (LSAs) with other routers in a network. This page describes the link-state routing algorithm.
Link-state routing algorithms have the following characteristics:

• They are known collectively as SPF protocols.
• They maintain a complex database of the network topology.
• They are based on the Dijkstra algorithm.

Link-state protocols develop and maintain full knowledge of the network routers and how they interconnect. This is achieved through the exchange of LSAs with other routers in the network.
Each router constructs a topological database from the LSAs that it receives. The SPF algorithm is then used to compute the reachability of destinations. This information is used to update the routing table. This process can discover changes in the network topology caused by component failure or network growth.
An LSA exchange is triggered by an event in the network instead of periodic updates. This speeds up the convergence process because there is no need to wait for a series of timers to expire before the routers can converge. If the network shown in Figure uses a link-state routing protocol, there is no concern about connectivity between routers A and D. Based on the protocol that is employed and the metrics that are selected, the routing protocol can discriminate between two paths to the same destination and use the best one. In Figure there are two routing entries in the table for the route from Router A to Router D. In this figure, the routes have equal costs so the link-state routing protocol records both routes. Some link-state protocols provide a way to assess the performance capabilities of the two routes and choose the best one. If the preferred route through Router C experiences operational difficulties such as congestion or component failure, the link-state routing protocol can detect this change and route packets through Router B.
The next page will describe some advantages of link-state protocols.

How routing information is maintained

How routing information is maintained

2.1.3 This page will explain how link-state protocols use the following features:

• The LSAs
• A topological database
• The SPF algorithm
• The SPF tree
• A routing table of paths and ports to determine the best path for packets 

Link-state routing protocols were designed to overcome the limitations of distance vector routing protocols. For example, distance vector protocols only exchange routing updates with immediate neighbors while link-state routing protocols exchange routing information across a much larger area.
When a failure occurs in the network, such as a neighbor becomes unreachable, link-state protocols flood LSAs with a special multicast address throughout an area. This process sends information out all ports, except the port on which the information was received. Each link-state router takes a copy of the LSA and updates its link-state, or topological database. The link-state router then forwards the LSA to all neighbor devices. LSAs cause every router within the area to recalculate routes. For this reason, the number of link-state routers within an area should be limited.
A link is the same as an interface on a router. The state of the link is a description of an interface and the relationship to the neighbor routers. For example, a description of the interface would include the IP address of the interface, the subnet mask, the type of network that it is connected to, the routers connected to that network, and so on. The collection of link-states form a link-state database which is sometimes called a topological database. The link-state database is used to calculate the best paths through the network. Link-state routers apply the Dijkstra shortest path first algorithm against the link-state database. This builds the SPF tree with the local router as the root. The best paths are then selected from the SPF tree and placed in the routing table.
The next page will discuss the link-state routing algorithm.

Link-state routing protocol features

Link-state routing protocol features
2.1.1 This page will explain how link-state protocols route data.
Link-state routing protocols collect route information from all other routers in the network or within a defined area of the network. Once all of the information is collected, each router calculates the best paths to all destinations in the network. Since each router maintains its own view of the network, it is less likely to propagate incorrect information provided by any of its neighboring routers.
The following are some link-state routing protocol functions:

• Respond quickly to network changes
• Send triggered updates only when a network change has occurred
• Send periodic updates known as link-state refreshes
• Use a hello mechanism to determine the reachability of neighbors 

Each router multicasts hello packets to keep track of the state of the neighbor routers. Each router uses LSAs to keep track of all the routers in its area of the network. The hello packets contain information about the networks that are attached to the router. In Figure, P4 knows about its neighbors, P1 and P3, on the Perth3 network. The LSAs provide updates on the state of links that are interfaces on other routers in the network.
Routers that use link-state routing protocols have the following features:

• Use the hello information and LSAs received from other routers to build a database about the network
• Use the SPF algorithm to calculate the shortest route to each network
• Store the route information in the routing table

The next page will provide more information about link-state protocols.

Link-State Routing Protocol / Overview of link-state routing

Link-State Routing Protocol
Overview of link-state routing

2.1.1 Link-state routing protocols perform differently than distance vector protocols. This page will explain the differences between distance vector and link-state protocols. This information is vital for network administrators. One essential difference is that distance vector protocols use a simpler method to exchange route information. Ooutlines the characteristics of both distance vector and link-state routing protocols.
Link-state routing algorithms maintain a complex database of topology information. While the distance vector algorithm has nonspecific information about distant networks and no knowledge of distant routers, a link-state routing algorithm maintains full knowledge of distant routers and how they interconnect.
The Interactive Media Activity will help students identify the different features of link-state and distance vector protocols.
The next page will describe link-state routing protocols.

Module 2: Single-Area OSPF (Overview)

Overview

The two main classes of IGPs are distance vector and link-state. Both types of routing protocols find routes through autonomous systems. Distance vector and link-state routing protocols use different methods to accomplish the same tasks.
Link-state routing algorithms, also known as shortest path first (SPF) algorithms, maintain a complex database of topology information. A link-state routing algorithm maintains full knowledge of distant routers and how they interconnect. In contrast, distance vector algorithms provide nonspecific information about distant networks and no knowledge of distant routers.
It is important to understand how link-state routing protocols operate in order to configure, verify, and troubleshoot them. This module explains how link-state routing protocols work, outlines their features, describes the algorithm they use, and points out the advantages and disadvantages of link-state routing.
Early routing protocols such as RIP v1 were all distance vector protocols. There are many distance vector routing protocols in use today such as RIP v2, IGRP, and the hybrid routing protocol EIGRP. As networks have grown larger and more complex, the limitations of distance vector routing protocols have become apparent. Routers that use a distance vector routing protocol learn about the network topology from the routing table updates of neighbor routers. Bandwidth usage is high because of the periodic exchange of routing updates, and network convergence is slow which results in poor routing decisions.
Link-state routing protocols differ from distance vector protocols. Link-state protocols flood route information, which allows every router to have a complete view of the network topology. Triggered updates allow efficient use of bandwidth and faster convergence. Changes in the state of a link are sent to all routers in the network as soon as the change occurs.
OSPF is one of the most important link-state protocols. OSPF is based on open standards, which means it can be developed and improved by multiple vendors. It is a complex protocol that is a challenge to implement in a large network. The basics of OSPF are covered in this module.
OSPF configuration on a Cisco router is similar to the configuration of other routing protocols. Similarly, OSPF must be enabled on a router and the networks that will be advertised by OSPF must be identified. OSPF has a number of features and configuration procedures that are unique. These features make OSPF a powerful choice for a routing protocol, but also make it a challenge to configure.
In large networks, OSPF can be configured to span many areas and several different area types. The ability to design and implement large OSPF networks begins with the ability to configure OSPF in a single area. This module also discusses the configuration of single-area OSPF.
This module covers some of the objectives for the CCNA 640-801 and ICND 640-811 exams. 
Students who complete this module should be able to perform the following tasks: 

• Identify key link-state routing protocol features
• Explain how link-state routing information is maintained
• Discuss the link-state routing algorithm
• Examine the advantages and disadvantages of link-state routing protocols
• Compare and contrast link-state routing protocols with distance vector routing protocols
• Enable OSPF on a router
• Configure a loopback address to set router priority
• Modify the cost metric to change OSPF route preference
• Configure OSPF authentication
• Change OSPF timers
• Describe the steps to create and propagate a default route
• Use show commands to verify OSPF operation
• Configure the OSPF routing process
• Define key OSPF terms
• Describe the OSPF network types
• Describe the OSPF Hello protocol

Identify the basics steps in the operation of OSPF

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.

RIP Version 2 (RIP history)

RIP Version 2 RIP history 1.2.1  This page will explain the functions and limitations of RIP. The Internet is a collection of autonomous systems (AS). Each AS is generally administered by a single entity. Each AS has a routing technology which can differ from other autonomous systems. The routing protocol used within an AS is referred to as an Interior Gateway Protocol (IGP). A separate protocol used to transfer routing information between autonomous systems is referred to as an Exterior Gateway Protocol (EGP). RIP is designed to work as an IGP in a moderate-sized AS. It is not intended for use in more complex environments. RIP v1 is considered a classful IGP. RIP v1 is a distance vector protocol that broadcasts the 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. If the router receives information about a network, and the receiving interface belongs to the same network but is on a different subnet, the router applies the one subnet mask that is configured on the receiving interface: For Class A addresses, the default classful mask is 255.0.0.0. For Class B addresses, the default classful mask is 255.255.0.0. For Class C addresses, the default classful mask is 255.255.255.0. RIP v1 is a popular routing protocol because virtually all IP routers support it. The popularity of RIP v1 is based on the simplicity and the universal compatibility it demonstrates. RIP v1 is capable of load balancing over as many as six equal-cost paths, with four paths as the default. RIP v1 has the following limitations: It does not send subnet mask information in its updates. It sends updates as broadcasts on 255.255.255.255. It does not support authentication. It is not able to support VLSM or classless interdomain routing (CIDR). RIP v1 is simple to configure, as shown in Figure . The next page will introduce RIP v2.

RIP Version 2 (

Configuring VLSM

Configuring VLSM
1.1.6 This page will teach students how to calculate and configure VLSM. If VLSM is the scheme chosen, it must then be calculated and configured correctly. 
The following are VLSM calculations for the LAN connections in Figure :

• Network address: 192.168.10.0
• The Perth router has to support 60 hosts. That means a minimum of six bits are needed in the host portion of the address. Six bits will yield 2⁶ – 2, or 62 possible host addresses. The LAN connection for the Perth router is assigned the 192.168.10.0/26 subnet.
• The Sydney and Singapore routers have to support 12 hosts each. That means a minimum of four bits are needed in the host portion of the address. Four bits will yield 2⁴ – 2, or 14 possible host addresses. The LAN connection for the Sydney router is assigned the 192.168.10.96/28 subnet and the LAN connection for the Singapore router is assigned the 192.168.10.112/28 subnet.
• The KL router has to support 28 hosts. That means a minimum of five bits are needed in the host portion of the address. Five bits will yield 2⁵ – 2, or 30 possible host addresses. The LAN connection for the KL router is assigned the 192.168.10.64/27 subnet.

The following are VLSM calculations for the point-to-point connections in Figure :

• Perth to KL

The connection from Perth to KL requires only two host addresses. That means a minimum of two bits are needed in the host portion of the address. Two bits will yield 2² – 2, or 2 possible host addresses. The Perth to KL connection is assigned the 192.168.10.128/30 subnet.

• Sydney to KL

The connection from Sydney to KL requires only two host addresses. That means a minimum of two bits are needed in the host portion of the address. Two bits will yield 2² – 2, or 2 possible host addresses. The Sydney to KL connection is assigned the 192.168.10.132/30 subnet.

• Singapore to KL

The connection from Singapore to KL requires only two host addresses. That means a minimum of two bits are needed in the host portion of the address. Two bits will yield 2² – 2, or 2 possible host addresses. The Singapore to KL connection is assigned the 192.168.10.136/30 subnet.

The following configuration is for the Singapore to KL point-to-point connection: 

Singapore(config)#interface serial 0

Singapore(config-if)#ip address 192.168.10.137 255.255.255.252

KualaLumpur(config)#interface serial 1

KualaLumpur(config-if)#ip address 192.168.10.138 255.255.255.252

This page concludes this lesson. The next lesson will discuss RIP. The first page describes RIP v1.

Route aggregation with VLSM

Route aggregation with VLSM

1.1.5 This page will explain the benefits of route aggregation with VLSM.
When VLSM is used, it is important to keep the subnetwork numbers grouped together in the network to allow for aggregation. For example, networks like 172.16.14.0 and 172.16.15.0 should be near one another so that the routers only carry a route for 172.16.14.0/23. 
The use of classless interdomain routing (CIDR) and VLSM prevents address waste and promotes route aggregation, or summarization. Without route summarization, Internet backbone routing would likely have collapsed sometime before 1997. 
Figure illustrates how route summarization reduces the burden on upstream routers. This complex hierarchy of variable-sized networks and subnetworks is summarized at various points with a prefix address, until the entire network is advertised as a single aggregate route of 200.199.48.0/20. Route summarization, or supernetting, is only possible if the routers of a network use a classless routing protocol, such as OSPF or EIGRP. Classless routing protocols carry a prefix that consists of a 32-bit IP address and bit mask in the routing updates. In Figure , the summary route that eventually reaches the provider contains a 20-bit prefix common to all of the addresses in the organization. That address is 200.199.48.0/22 or 11001000.11000111.0011. For summarization to work, addresses should be carefully assigned in a hierarchical fashion so that summarized addresses will share the same high-order bits.
The following are important rules to remember:

• A router must know in detail the subnet numbers attached to it.
• A router does not need to inform other routers about each subnet if the router can send one aggregate route for a set of routes.
• A router that uses aggregate routes has fewer entries in its routing table.

VLSM increases route summarization flexibility because it uses the higher-order bits shared on the left, even if the networks are not contiguous. 
Figure shows that the addresses share the first 20 bits. These bits are colored red. The 21^st bit is not the same for all the routes. Therefore the prefix for the summary route will be 20 bits long. This is used to calculate the network number of the summary route.
Figure shows that the addresses share the first 21 bits. These bits are colored red. The 22^nd bit is not the same for all the routes. Therefore the prefix for the summary route will be 21 bits long. This is used to calculate the network number of the summary route.
The next page will teach students how to configure VLSM.

Calculating subnets with VLSM

Calculating subnets with VLSM

1.1.4 VLSM helps to manage IP addresses. This page will explain how to use VLSM to set subnet masks that fit the link or 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. 
The example in Figure shows a network that requires an address scheme.
The example contains a Class B address of 172.16.0.0 and two LANs that require at least 250 hosts each. If the routers use a classful routing protocol, the WAN link must be a subnet of the same Class B network. Classful routing protocols such as RIP v1, IGRP, and EGP do not support VLSM. Without VLSM, the WAN link would need the same subnet mask as the LAN segments. A 24-bit mask of 255.255.255.0 can support 250 hosts.  
The WAN link only needs two addresses, one for each router. That means that 252 addresses would be wasted.
If VLSM was used, a 24-bit mask would still be applied on the LAN segments for the 250 hosts. A 30-bit mask could be used for the WAN link because only two host addresses are needed.
Figure shows where the subnet addresses can be applied based on the number of host requirements. The WAN links use subnet addresses with a prefix of /30. This prefix allows for only two host addresses which is just enough for a point-to-point connection between a pair of routers.
In Figure , the subnet addresses used are generated when the 172.16.32.0/20 subnet is divided into /26 subnets.
To calculate the subnet addresses used on the WAN links, further subnet one of the unused /26 subnets. In this example, 172.16.33.0/26 is further subnetted with a prefix of /30. This provides four more subnet bits and therefore 16 (2⁴) subnets for the WANs. Figure illustrates how to work through a VLSM system.
VLSM can be used to subnet an already subnetted address. For example, consider the subnet address 172.16.32.0/20 and a network that needs ten host addresses. With this subnet address, there are 2¹² – 2, or 4094 host addresses, most of which will be wasted. With VLSM it is possible to subnet 172.16.32.0/20 to create more network addresses with fewer hosts per network. When 172.16.32.0/20 is subnetted to 172.16.32.0/26, there is a gain of 2⁶, or 64 subnets. Each subnet can support 2⁶ – 2, or 62 hosts.
Use the following steps to apply VLSM to 172.16.32.0/20:

1. Write 172.16.32.0 in binary form.
2. Draw a vertical line between the 20^th and 21^st bits, as shown in Figure . The original subnet boundary was /20.
3. Draw a vertical line between the 26^th and 27^th bits, as shown in Figure . The original /20 subnet boundary is extended six bits to the right, which becomes /26.
4. Calculate the 64 subnet addresses with the bits between the two vertical lines, from lowest to highest in value. The figure shows the first five subnets available.

It is important to remember that only unused subnets can be further subnetted. If any address from a subnet is used, that subnet cannot be further subnetted. In Figure , four subnet numbers are used on the LANs. The unused 172.16.33.0/26 subnet is further subnetted for use on the WAN links.
The Lab Activity will help students calculate VLSM subnets.
The next page will describe route aggregation.