Change in the Network

A change in the network topology also necessitates information to be propagated through the network. For example, the rest of the network notices that a new router has joined their ranks. A particular router that notices a change floods the area with the update so that all routers can alter their routing tables to reflect the most current and accurate information. Different techniques are used for these different routing table requirements. Essentially, the difference between the two techniques...

Change in the Network Maintaining the Topological Database and the Routing Table

Now turn back to the 2500 router in Building A of the San Francisco campus in Figure 5-6. The router is now happily a member of the OSPF network. Now follow the process of hearing an update to the network in the form of an LSA. As soon as a router realizes that there has been a change in the network topology, the router is responsible for informing the rest of the routers in the area. Typically, it will identify a change in the state of one of its links for one of the following reasons The...

Confusion of Network Terminology

To appreciate fully the power and purpose of the Layer 3 address, it is important to understand the meaning of the term network (as defined by Layer 3). A network address has two parts the network and the host portions. The host portion of the address identifies the individual device within a group. The network portion identifies a group of individual devices. Unfortunately, the term network is used loosely although it is often defined, the term is seldom understood. In addition, the term...

Definition of Prefix RoutingCIDR

Prefix routing is just the means by which the Internet identifies the portion of the 32-bit TCP IP address that uniquely identifies the organization. In effect, this means that the Internet can allocate a group of class networks, which are represented by a single address. This allows for prefix routing and summarization within the routing tables of the Internet. Prefix masks represent a group of TCP IP network addresses using the method of address or subnet masks. This aggregation of Internet...

Router Joins the Network

The first way to build a routing table is for a new router to be added to the network and then for it to build a routing table by listening to the established routers with complete routing tables. Remember that every router within an area will have the same database and will know of every network within the area. The routing table built from this database is unique to the router because the decisions depend on the individual router's position within the area, relative to the remote destination...

Working Configuration of OSPF on a Multiarea Network

Example 6-8 is a working configuration tested for verification. It includes many of the commands explained earlier in this chapter. This is so that you see an entire working configuration rather than the relevant segment for configuring a particular networking nuance. The configuration should be used in conjunction with Figure 6-13. Figure 6-13 Diagram of the Example 6-8 Network Figure 6-13 Diagram of the Example 6-8 Network Example 6-8 Configuring OSPF in a Multiarea Network Router(config)...

Working Configuration of OSPF on a Single Router

Example 5-1 is a working configuration tested for verification. It should be used in conjunction with Figure 5-11. SanJose SanJose SanJose SanJose SanJose SanJose SanJose SanJose SanJose SanJose SanJose SanJose SanJose SanJose SanJose SanJose SanJose config) router) network 140.100.0.0 0.0.255.255 area 3 router) interface FastEthernet1 0 ip address 140.100.17.129 255.255.255.240 ip ospf priority 100 full-duplex no shutdown ip address 140.100.17.193 255.255.255.240 ip address 140.100.32.10...

Accessibility Security

Security is a popular topic and a major consideration, particularly as more companies connect to the Internet and thereby increase the chance of hackers, idly wandering into the network. Weighing the needs of users to access the network, particularly when remote access is required, against the need to secure company secrets is a difficult balance that requires careful consideration at the executive level. It is important to consider security as part of the initial design because it is very...

Adaptability Serviceability

It is difficult to anticipate every change that your company may make in terms of mergers and organizational structure. Therefore, building an adaptable network protects capital investment. It also increases the reliability of the network. Because network administrators are not issued crystal balls, it is essential that attention be given to the interoperability of both products and applications when designing the network. Thus, when the company makes a business change, network changes may be...

Adding a Network to the Topology Table

Imagine the router (Router A) that hears a new network. The administrator has plugged in another Ethernet cable to service a department that has moved into the building. As soon as Router A becomes aware of the new network, it starts to send Hello packets out the new interface. No one answers because this is an access router giving connectivity to the workstations and other end devices. There are no new entries in the neighbor table because no neighbors have responded to the Hello protocol....

Administrative Distance

The administrative distance will select one path to enter the routing table from several paths offered by multiple routing protocols. In Figure 4-2, for example, both RIP and EIGRP have paths to the network 140.100.6.0. RIP is offering a metric of 2 hops, and EIGRP is tendering a metric of 768. Without redistribution, no conversion or choice is possible because there is no similar criteria for distinguishing the two paths. Therefore, the metric is ignored, and the administrative distance is...

Advantages of Having Neighbors

There are obvious advantages to creating neighbor relationships. These advantages include the following There is another mechanism for determining that a router has gone down (obvious because its neighbor no longer sends Hello packets). Streamlined communication results because after the topological databases are synchronized, incremental updates will be sent to the neighbors as soon as a change is perceived, as well as every 30 minutes. Adjacencies created between neighbors control the...

Advantages of Prefix RoutingCIDR

CIDR offers several advantages, including the reduction of the size of the routing table. Prefix routing is used to reduce the size of Internet routing tables. As explained in the preceding example, the Internet gave away the equivalent of eight Class C networks, but just one network entry appeared in the Internet's routing table. In an environment that has more than 54,000 entries in the routing table at the time of this writing, the size of the routing table in many ISPs has peaked at 54,000...

Advertising Routes from an IGP into BGP4

First, consider the advertising routes into BGP-4. There are three ways of populating the BGP-4 table with IGP routes Using the network command This is used to advertise routes that are in the IP routing table. Redistributing static routes These are routes that have been summarized to a supernet, such as a Class C address with a prefix mask of 16 bits. This requires statically routing to null 0. This fools the system by creating a route that has no exit point from the router because the route...

Aggregating Routes

To summarize or aggregate routes within the BGP-4 domain, use the following command from the config-router mode aggregate-address ip address mask summary-only as-set If the parameter summary-only is used, then the specific routes are suppressed and the summary route is the only one propagated. If the parameter as-set is used, then all the autonomous systems that have been traversed will be recorded in the update message. This is the default configuration. Because it may be necessary to...

Allocating VLSM Addresses

Applying the addressing scheme designed in the preceding example is very simple after the design has been worked out. Taking the region of California as the example to address, you will now address the entire region. Figure 3-7 shows the bit allocation that was determined. San Francisco 01 San Jose 10 San Rafael 11 Buildings Building 1 001 Building 2 010 Building 3 011 Building 4 100 Floor Floor 1 001 Floor 2 010 Floor 3 011 Floor 4 100 Floor 5 101 Hosts 1-30 NOTE Remember that you will conform...

Alternative Methods of Connecting to an ISP

If BGP-4 is unnecessary in your network, consider the other possibilities, including the following A default route into the ISP and a static route from the ISP into the organization. A routing protocol into the ISP, making the ISP part of your autonomous system. The ISP will be using redistribution within its domain, and it is advisable for the organization to use some form of security, in the form of access lists or a firewall. Typically the ISP will give you a written sheet explaining the...

An Analogy for Understanding the Concept of a Network

Similar administrative lines are drawn between one city and another, between one state and another, and even between countries. These lines, or borders, serve the same purpose as the network portion of a Layer 3 address that is, they allow rules to be placed on a group of end systems (in the geographic analogy, humans). Traffic can now be specifically directed. Routing tables serve as maps and road signs. It is very important to remember to carefully plan the placement of these boundaries to...

An Autonomous System Defined

An autonomous system is a routing domain that shares routing information. Typically, an autonomous system is the same as an organization. Within the context of internal security, every network within the organization is available, and information about the networks is transmitted via an interior routing protocol such as EIGRP, OSPF, RIP, or IGRP. Although there may be more than one interior routing protocol running within the autonomous system, the probability is that they are sharing...

An Example of Bit Allocation in a Network Address

If 10 bits are allocated to the network portion of the address, 22 bits are left to the host portion of the address. In binary, 10 bits can be used to represent 1024 distinct entities (each being assigned a unique bit pattern or address). The 22 bits left to identify hosts can be used to represent four million hosts (actually, 4,194,304) on each network. The total number of devices that can be addressed is calculated by multiplying the number of hosts' addresses available on each network by the...

An Example of CIDR in Use in the Case Study

Imagine that the company discussed in the case study at the beginning of the chapter, Mental Merge, has applied for a Class B address from the Internet authorities. To everyone's surprise, Mental Merge has been awarded seven Class C networks. The company owners are delighted because they were expecting only one Class C address. Figure 3-3 shows the addresses awarded to Mental Merge, the use of CIDR addresses, and how prefix routing works at the binary level. Figure 3-3 Prefix Routing and the...

An Example of the Use of CIDR

It is easy to see how this works when the address and the mask are written in binary, as the router processes them. The Internet community has allocated a group of Class C addresses, although they are presented as a single network. Table 3-6 shows an example of an IP address in both decimal and binary format. Table 3-6 An IP Address and Mask Shown in Binary If it were a standard Class C address, the mask would be 255.255.255.0. By making the mask 255.255.248.0, the last three bits of the third...

An Example of VLSM

Consider that Mental Merge has been given a Class B address. The company has grown and now has some satellite offices that connect via point-to-point serial lines. The remote offices have eight workstations, three printers, and a router connecting them to the outside world. The main site has a building with ten floors, and each floor has approximately 25 workstations and four printers. A server farm in the basement has three servers and two routers. In this scenario, it is impossible to create...

An IP Address

TCP IP is unique in that the network portion of the address has not been allocated a fixed address space. The number of bits that the network portion may use depends on the number of networks that need to be identified. Although a governing body allocates an original address, the network portion of the address can be extended. To identify how many of the address bits have been extended into the network portion of the address, a subnet mask is used. Where do the unique addresses come from These...

ANDing Example

Figure 3-2 illustrates the ANDing logic. Figure 3-2 AND Logic and the Subnetwork IP address in binary 10010000.01100100.00010000. 00001000 IP subnet mask in binary 11111111.11111111.11111111. 00000000 The result of the logical AND 10010000.01100100.00010000. 00000000 Layer 3 can now make a decision on how to route the network number that has been revealed. The result is the removal of the host portion of the address, and the subnet address is left intact. Therefore, the host 144.100.16.8 is a...

Answers

Figure 2-12 shows a diagram of the network. 2 Write the configuration commands to achieve the requirements stated previously. Make sure that this is reflected on your diagram. The following commands allow the IP clients to connect to the remote servers Router(config-if) ip helper-address 10.10.10.255 The IPX clients will need no additional configuration because they will find their servers using the normal method of GNS requests and SAP updates. 3 Which commands would you use to verify the...

Applications Time

The session layer of the OSI model is responsible for maintaining the communication flow between the two end devices. This includes assigning resources to incoming requests to connect to an application. To allocate resources adequately, idle timers disconnect sessions after a set time, releasing those resources for other requests. Note that although the OSI model assigns these duties to the session layer, many protocol stacks include the upper layers of the stack in the application. TCP IP is...

Authors Note About Exam Content

As Cisco's authorized external publishing company, Cisco Press is the only publisher that is partnered with Cisco. Cisco has shared other information with Cisco Press, part of which includes some details that are expected to be posted on Cisco's web site at a later date. At press time, Cisco had not finalized what other details about the exam will be posted on its web site, so I cannot list any of those details here. Fortunately, what does get posted by Cisco will be easily available to you I...

Automatic Redistribution Between Routing Protocols

Table 10-17 Automatic Redistribution Between Routing Protocols Table 10-17 Automatic Redistribution Between Routing Protocols Requires manual redistribution into other routing protocols. Unless included in the network command for the routing process, requires Will automatically redistribute between IGRP and EIGRP if the autonomous system number is the same. Otherwise, processes with different IGRP autonomous system numbers, or IGRP and EIGRP processes with different autonomous system numbers,...

Automatic Summarization

All routing protocols employ some level of summarization. The older protocols, such as RIP and IGRP, automatically summarize at the NIC or natural class boundary. They have no choice because the subnet mask is not sent in the routing updates. When a routing update is received, the router looks to see whether it has an interface in the same class network. If it has one, it applies the mask configured on the interface to the incoming routing update. With no interface configured in the same NIC...

Avoiding Routing Loops When Redistributing

Routing loops occur when a routing protocol is fed its own networks for example, networks that originated within that routing process, but that the routing protocol now learns from another routing protocol through redistribution. The routing protocol may now see a network that it owns as having a more favorable path although this will send the traffic in the opposite direction, into a different routing protocol domain. The confusion that can be caused is enormous, and it is very easy to create...

Backup Designated Routers

The previous paragraph is only a half truth because there has been a lot of effort put into ensuring that the network does not fail. It is a little nerve-racking for the network administrator to have the responsibility of the segment fall to one router. This poses the frightening situation of a single point of failure, a term that raises the blood pressure of any of those in charge of the network. Redundancy has been built into the network with the backup designated router. Just like an...

Bandwidth Control

A perennial concern of network administrators is the amount of bandwidth used for overhead traffic. Administrators want to minimize the amount of network control traffic sent through the network, to maximize the bandwidth available for user data. One of the major benefits of both EIGRP and OSPF is that they send as little network traffic as possible. This has the advantage of decreasing the convergence time of the network and ensuring that the network traffic that is sent arrives at the...

Becoming a Neighbor

The Hello protocol uses a multicast address of 224.0.0.10, and all routers periodically send hellos. On hearing hellos, the router creates a table of its neighbors. The continued receipt of these packets maintains the neighbor table. If a hello from a known neighbor is not heard within a predetermined amount of time, as stated in the holdtime, the router will decide that the neighbor is no longer operational and will take the appropriate action. The holdtime is set at the default of three times...

Benefits of the Synchronization Rule

The following list gives reasons for the synchronization rule It prevents traffic from being forwarded to unreachable destinations. It reduces unnecessary traffic. It ensures consistency within the autonomous system. On some occasions it is useful to turn off synchronization. This is rare, and, as with any default, it is unwise to turn off this option without a detailed understanding of the network. The occasions when it may be useful to turn off synchronization are as follows If all the...

BGP4 Attributes

Attributes in BGP-4 are used to determine the best path to be selected. In essence, they are the metric for BGP-4. However, they are more than a list of variables by which a route is selected. They also carry information that decisions are based on hence the name attributes. The variables describe characteristics or attributes of the path to the destination. These characteristics can be used to distinguish the paths, and this allows a choice to be made among the paths. Some of the information...

BGP4 Operation

BGP-4 is an extremely complex protocol used within the Internet and multinational organizations. Its main purpose is to connect very large networks that are mainly autonomous systems. Large companies may use BGP-4 as the glue between the countries for example, a government may use it as the glue between the divisions of the administration, and the military might use it between the Army, the Navy, and the Air Force. Lists of RFCs that define it, along with other references, are given at the end...

BGP4 Policy Based Routing

Policy-based routing gives the administrator the ability to define how traffic will be routed at the autonomous system level. This is a level of control above the dynamic routing protocol. Given that many variables in BGP-4 can influence dynamic routing (these are called variables), this is a very high level of control. This other dimension distinguishes BGP-4 from other routing protocols. Policy-based routing is a form of static routing enforced by specialized access lists called route maps.

BGP4 show Commands

Example 9-9 shows a list of all the show commands available to the BGP-4 administrator. Example 9-9 BGP-4 show Command Options A.B.C.D Network in the BGP routing table to display cidr-only Display only routes with non-natural netmasks community Display routes matching the communities community-list Display routes matching the community-list dampened-paths Display paths suppressed due to dampening filter-list Display routes conforming to the filter-list flap-statistics Display flap statistics of...

Bit Allocation the Subnet Rule and VLSM

In truth, the subnet does have to conform to the rule as described. With VLSM, however, it is often forgotten that the entire subnet area is considered one subnet. Therefore, the rule must be obeyed once, not on each instance of variable subnetting. In the preceding example, you would choose to obey the rule either in the bits allocated to the region, campus, or building, but not in each hierarchical layer. It would make most sense to adhere to the rule using the least-significant bits. In this...

Building the Routing Table on a New OSPF Router

Five packets are used to build the routing table for the first time Hello protocol Is used to find neighbors and to determine the designated and backup designated router. The continued propagation of the Hello protocol maintains the transmitting router in the topology database of those that hear the message. Database descriptor Is used to send summary information to neighbors to synchronize topology databases. Link-state request Works as a request for more detailed information, which is sent...

Case Study

A small startup company, Mental Merge, has grown in the last year. No longer three recent college graduates, working out of a parent's guestroom, the founders now have their own premises and 100 employees. The company has a flat, switched network providing print and file services using NT over IP. The owners have been simply adding devices as needed, without much heed to the future requirements of the company. Having run out of IP addresses once, and with an increase in delays, they realize...

Case Study Addressing the Network

For a complicated example, use a Class B address and create an addressing scheme for Mental Merge. If the Internet assigns the address 140.100.0.0, how might you address the network shown in the diagram The first task is to determine the number of regions, campuses, buildings, floors, and hosts on each floor. You also need to consider any anticipated growth or change in the network. For this example, the network is comprised of the following Four regions exist, but the company has plans to...

Changing the Default Metric Using the cost Command

Another command that may be useful is the cost command. This command manually overrides the default cost that the router assigns to the interface. The default cost is calculated based on the speed of the outgoing interface. The cost command syntax is as follows ip ospf cost cost A lower cost increases the likelihood that the interface will be selected as the best or shortest path. The range of values configurable for the cost of a link is 1 to 65535. In general, the path cost in Cisco routers...

Chapter Glossary

This glossary provides an official Cisco definition for key words and terms introduced in this chapter. I have supplied my own definition for terms that the Cisco glossary does not contain. The words listed here are identified in the text by italics. A complete glossary, including all the chapter terms and additional terms, can be found in Appendix C, Glossary. connection-orientated Software on two end nodes guarantees the transmission of network traffic because a circuit setup is established...

Characteristics of BGP4

The main characteristics of BGP-4 may be distilled into a few short points. The interaction of these components and their relative importance to the operation or design of a BGP-4 network is discussed in the rest of the chapter. The key features of BGP-4 include these It is a path vector protocol. Full routing updates are sent at the start of the session, trigger updates are sent subsequently. It creates and maintains connections between peers, using TCP port 179. The connection is maintained...

Characteristics of Policy Routing

The rules that define policy routing are as follows Traffic can be directed on either the source address or both the source and destination addresses. Policy routing affects only the routing of the router on which it is configured in determining the next hop in the path to the destination. Policy routing does not affect the destination of the packet. Policy routing does not allow traffic sent into another autonomous system to take a different path from the one that would have been chosen by...

Characteristics of Route Maps

A route map has certain characteristics A route map has a list of criteria, stated with the match statement. A route map can change packets or routes that are matched by using the set statement. A collection of route map statements that have the same route map name are considered one route map. Within a route map, each route map statement is numbered with sequence numbers and, therefore, can be edited individually. The statements in a route map correspond to the lines of an access list....

Characteristics of Route Reflectors

The following are the chief characteristics of route reflectors A route reflector is a router that forwards updates to its clients. When a client sends an update to the route reflector, it is forwarded or reflected to the other clients. The route reflector is the only router that is configured or that has the remotest idea that it is anything other than a peer. A client is a router that receives updates from a route reflector. Both a route reflector and a client, therefore, form a unit that...

Checking the Configuration of OSPF on a Multiarea Network

The show commands shown here are in addition to the commands described in Chapter 5, in the section, Checking the Configuration of OSPF on a Single Router. The single router commands are also extremely useful in a multiarea configuration. They are all invaluable in both the configuration and maintenance of a live network. They are particularly useful in troubleshooting the network. The following are the additional commands that you can use in conjunction with single router commands when...

Choosing a Successor

To determine whether a path to a remote network is feasible, EIGRP considers the FC of the route. Essentially, each router holds a routing table that is a list of the available networks and the best or most efficient path to each of them. The term used to describe this is the feasible distance of the successor, otherwise known as the metric for the route. The router also holds the routing table of its neighbors, referred to as the AD. If the AD is within scope, this route may be identified as...

CIDR and Route Aggregation

As explained in Chapter 3, IP Addressing, there is a shortage of IP addresses in the Internet. There is also a problem with the size of the routing tables, in terms of the memory, bandwidth, and CPU that they consume when updating the routing tables. Classless interdomain routing (CIDR) is one of the main solutions implemented in recent years. This is a method of consolidating addresses into a few summary addresses. Instead of a subnet having a subnet mask to identify the network portion of the...

CIDR as a Solution

An organization requiring multiple Class C addresses is allocated consecutive Class C addresses but issues only one address for the Internet routing entry (representing the multiple addresses). This is achieved by pulling the network mask to the left. The shorter the prefix, the more generally the network is defined the longer the prefix, the more specific the identification is. Table 3-5 visually demonstrates the use of the prefix. The Internet IP addressing group ARIN, at www.arin.net,...

Cisco Express Forwarding

This is a very high-end solution and is available on 7500 routers with Versatile Interface Processors (VIPs) and the 8510. It is extremely fast and is used for high volumes of traffic. On some Cisco platforms, the Cisco processor automatically turns on this feature if the appropriate hardware and software are available. NOTE A real performance gain can be realized with the reduction of CPU cycles needed when NetFlow or fast switching are used. In fact, the risk of losing packets or buffer...

Cisco Proprietary Solutions

With the assumption that connections are between Cisco routers, Cisco has optimized many network operations. This sometimes requires a proprietary solution. While defying some of the standards to streamline network traffic, Cisco has been conscientious by providing both the standard solution and a proprietary Cisco solution. To ensure the capability to integrate Cisco equipment into all networks, Cisco offers sophisticated methods of translation between the standard and the proprietary Cisco...

Cisco Routing Exam Outline File Excerpts From CCO

Given your experience, this outline and guide will help you with the best methods of preparation for the Cisco Career Certifications exam. The BSCN course is the recommended method of preparation for the Routing exam. The topic areas listed in this outline are general guidelines for the type of content that is likely to appear on the exam. However, please be advised that other relevant or related topic areas may also appear. The Routing (640-503) exam will contain a combination of the following...

Ciscos Hierarchical Design

To achieve these networks requirements that have been described and to keep local traffic local, preventing network congestion Cisco suggests a network design structure that allows for growth. The key to the design is that it is hierarchical. There is a division of functionality between the layers of the hierarchy, allowing only certain traffic based on clear criteria to be forwarded through to the upper levels. A filtering operation restricts unnecessary traffic from traversing the network....

Classful Routing

Classful IP routing protocols include RIPv1 and IGRP. The characteristics of a classful routing protocol are listed here Summarization occurs at the network boundary. Routes exchanged between foreign networks are summarized to the NIC number network boundary. Within the same network (NIC number), subnet routes are exchanged by routers, without the mask. The subnet mask is assumed to be consistent for a NIC number used within a network, so all router interfaces must share the subnet mask for...

Classless Routing

Classless routing protocols were designed to overcome the constraints listed previously. The routing protocols that can do this are OSPF, EIGRP, RIPv2, IS-IS, and BGP. The characteristics of a classless routing protocol are listed here Router interfaces within the same network can have different subnet masks (VLSM). Some of the classless routing protocols, including BGP-4 and RIP v2, support the use of classless interdomain routing (CIDR). Some routes can be summarized within the major NIC...

Clients Cannot Connect to Network Resources

The client server environment is one in which applications take advantage of networks. Servers provide services to many clients that access them across the network. Both data and application software may be stored centrally on a server that allows many clients to share them. If all users depend on the client server interaction, this can create excessive traffic across the network. In a client server environment, the available resources are communicated throughout the network. The dynamic nature...

Command Summaries

The following is a list of the commands explained in this chapter. This list is not intended to teach the use of the commands, but to remind the reader of the options available. The commands in Table 5-16 are configuration commands. Table 5-16 OSPF Configuration Command Summary Turns on the OSPF process and identifies it with a Identifies which networks and, thus, interfaces belong Creates the loopback interface that may now be used to Sets the cost or metric for the outgoing interface. Allows...

Command Syntax Conventions

The conventions used to present command syntax in this book are the same conventions used in the Cisco IOS Command Reference, as follows Boldface indicates commands and keywords that are entered literally as shown. In examples (not syntax), boldface indicates user input (for example, a show command). Italics indicates arguments for which you supply values. Square brackets ( and ) indicate optional elements. Braces ( and ) contain a choice of required keywords. Vertical bars (I) separate...

Commands to Tune BGP4Using the Local Preference and Weight Attributes

To configure the weight attribute, use the following command neighbor ip-address peer-group-name weight weight Table 9-6 explains the meaning of the preceding syntax. Table 9-6 An Explanation of the Command to Configure the Weight Attribute This identifies that the rest of the command is directed at a BGP-4 peer. This is the IP address of the neighboring router. This identifies the BGP-4 peer group, if there is one. weight weight This is proprietary to Cisco and is used in route selection. It...

Conclusion

EIGRP is an IP routing protocol that attempts to solve many of the problems experienced by standards-based solutions. As a proprietary protocol, it has the freedom to create a very specific product that works well with the technology that the company produces. After a troubled childhood, EIGRP has proved itself an excellent solution for large corporations that need a routing protocol that will scale. With the use of redistribution, which will be dealt with in a future chapter, it can be...

Configuration Commands for a Multiarea OSPF Network

The following commands are necessary to configure a multiarea OSPF network The OSPF area range command for an ABR The OSPF summarization command for an ASBR The OSPF command for a stub area The OSPF command for a totally stubby area The OSPF command for the cost of a default route propagated into the area The command for configuring a virtual link

Configuration Examples

The following examples show how one command can cover all router interfaces and also how each individual interface can be specified. Given a router with six interfaces, three with addresses in the 10.0.0.0 class and three with addresses in the 172.16.0.0 class, the following would configure all interfaces to participate in OSPF Area 0 network 0.0.0.0 255.255.255.255 area 0 The following would have only the interfaces addressed from 10.0.0.0 participating in OSPF Area 0 network 10.0.0.0...

Configuration for EIGRP or IGRP

To configure the default metric for IGRP or EIGRP, use the following command syntax default-metric bandwidth delay reliability loading mtu Typically, you should take the values shown on one of the outgoing interfaces of the router being configured, by issuing this exec command The significance of the metric values is shown in Table 10-5. Table 10-5 The Parameters of the default metric Command The minimum bandwidth seen on route to the destination. It is presented The delay experienced on the...

Configuration Notes

There are many things to be aware of in configuring a router that is directing the network traffic. These devices are responsible largely for the end users being able to accomplish their work and return home in the evening to loved ones. When configuring policy routing or route maps, pay very careful attention to the logic and rules by which they operate. Refer to the guidelines for writing access lists in Chapter 2, Managing Scalable Network Growth. The following are a couple warnings to...

Configuration of IP Helper Address

The syntax for the ip helper-address command is as follows ip helper-address address The IP helper address forwards broadcasts for the following UDP ports NetBIOS datagram service (138) In addition to the helper address, the IP forward protocol instructs the router to forward broadcasts. By stating the port number, particular types of broadcast may be identified. This is very useful when used in conjunction with the helper address because it identifies those broadcasts to be readdressed to...

Configuration of Tunneling into IP

The following syntax shows the configuration for the tunnel interface command interface tunnel interface-number The following shows syntax for the configuration of the tunnel associated with the tunnel tunnel source interface-number ip-address tunnel destination hostname ip-address Table 2-3 explains the previous syntax. Table 2-3 Tunneling Configuration Explanation Table 2-3 explains the previous syntax. Table 2-3 Tunneling Configuration Explanation Identifies the interface number to be...

Configuring a Virtual Link

When it is not possible to connect an area to Area 0 directly, a solution is to create an IP tunnel called a virtual link. This is remarkably easy to configure. As with many things in OSPF, of course, this ease of configuration belies the complexity of the technology being used. Many things can go wrong. The most common problem is in the address of the other end of the virtual link. The command is given between ABRs, at least one of which must be in Area 0. The command, issued at both ABRs,...

Configuring Bandwidth over a Hybrid Multipoint Network

If the multipoint network has differing speeds allocated to the VCs, a more complex solution is needed. There are two main approaches. To take the lowest CIR and to simply multiply this by the number of circuits. This is applied to the physical interface. The problem with this configuration is that the higher-bandwidth links will be underutilized for some things. If possible, it is much easier to configure and manage an environment that has used subinterfaces, where a VC is logically treated as...

Configuring Bandwidth over a Multipoint Network

The configuration of the bandwidth command in an NBMA cloud depends on the design of the VCs. If the serial line has many VCs in a multipoint configuration, then EIGRP will evenly distribute its overhead between the VCs, without the use of subinterfaces. The bandwidth command should therefore reflect the access link speed into the Frame Relay cloud. If the serial interface is accessing an NBMA environment such as Frame Relay, the situation is straightforward. Your company may have five VCs from...

Configuring EIGRP

The commands for EIGRP are consistent with the other IP routing protocols. Although IP routing is on automatically, the chosen routing protocol must be configured and the participating interfaces must be identified. EIGRP allows for VLSM and, therefore, summarization because the mask is sent in the update packets. Although summarization is automatic, EIGRP summarizes at the NIC or major network boundary. To summarize within the NIC number, it must be manually configured. Unlike OSPF that can...

Configuring EIGRP for Apple Talk

NOTE This section is included to place EIGRP in context. The exam will test only on topics pertaining to EIGRP using IP. Therefore, this section should be read only for interest and should not be studied in depth in preparation for the Routing exam. EIGRP also supports the client server protocol AppleTalk. Conceptually, the use of EIGRP is the same, although the configuration details differ. The main difference in configuration is that, whereas in configuring IP or IPX the autonomous system...

Configuring OSPF in a Single Area

When configuring any device, it is important to establish why you are configuring the system and what you are trying to achieve. This section examines the configuration of a Cisco router for OSPF within a single area. The commands are few and extremely simple the implications are somewhat more difficult. What each configuration command achieves How the configuration command achieves its goal An example of a working configuration that uses the commands discussed in this section illustrates the...

Configuring OSPF in Broadcast Mode

The use of the broadcast mode is to avoid the use of the neighbor command and all the attendant configurations. This broadcast mode works best with a fully meshed network. Example 5-4 shows a typical configuration of OSPF in broadcast mode. Example 5-4 Configuring a Broadcast Network ROUTER(config-if) ip address 10.1.1.1 255.255.255.0 ROUTER(config-if) encapsulation frame-relay ROUTER(config-if) ip ospf network broadcast ROUTER(config) router ospf 1 ROUTER(config-router) network 10.1.1.0...

Configuring OSPF in NBMA Mode

In NBMA mode, the design considerations are imperative because the selection of the DR and BDR need to have physical connectivity to all routers in the NBMA cloud. Remember that this is a nonbroadcast environment, so the DR and BDR must be configured with a static list of the other routers attached to the cloud so that they can become neighbors and create adjacencies. This is achieved with the use of the neighbor command. Other technologies have superceded the need to manually configure the...

Configuring OSPF in Pointto Multipoint Mode

An OSPF point-to-multipoint interface is seen as one or more numbered point-to-point interfaces. The cloud is configured as one subnet. A host route will be added for each router involved in the OSPF cloud. NOTE The point-to-multipoint non-broadcast command option is a new feature related to point-to-multipoint networks with Cisco IOS Release 11.3a. You can find more information on the subject by searching at www.cisco.com with the keywords OSPF point-to-multipoint network with separate costs...

Configuring OSPF in Pointto Point Mode on a Frame Relay Subinterface

In this mode, the adjacency created between the routers is automatic because each subinterface behaves as a physical point-to-point network. Therefore, the communication is direct and automatic. The following steps explain how to configure OSPF point-to-point mode on subinterfaces Step 1 At the interface level, create a subinterface. Step 2 It is recommended that you remove any network layer address assigned to the physical interface using the no IP address command. The Layer 3 address should...

Configuring OSPF over an NBMA Topology

The design considerations of running OSPF over an NBMA topology are outside the scope of this book, but it is important to research this area because it speaks to the configuration choices that will be made. However, one of the common choices on a partially meshed network illustrates the kind of decisions that need to be made. If the network is partially meshed, then the choice to use only point-to-point subnets can waste addresses. If a point-to-multipoint configuration is chosen, the network...

Configuring Summarization

In the newer routing protocols, summarization must be manually configured this manual configuration lends greatly to its subtlety and strength. Each routing protocol deals with summarization in a slightly different way, and how summarization works or is configured depends on the routing protocol used. This is discussed in detail in Chapter 4. NOTE Although Border Gateway Protocol (BGP) and Enhanced IGRP (EIGRP) perform automatic summarization, the summarization is done at the NIC boundary,...

Configuring the Administrative Distance

To ensure that the optimal path is chosen, it is sometimes necessary to change the administrative distance, to make it less favorable. The command structure is protocol-dependent, in that EIGRP requires a separate command. The following command syntax is used for EIGRP distance eigrp internal-distance external-distance The distance command, as used to configure the EIGRP administrative distance, is explained in Table 10-6. Table 10-6 Configuring Administrative Distance for EIGRP Command...

Configuring the Default Metric

The default metric can be configured in several ways. The first is to include the metric in the redistribute command, as shown in the preceding command syntax and as illustrated in Example 10-1. Example 10-1 Including the Metric in the redistribute Command router eigrp 100 redistribute rip metric 10000 100 255 1 1500 network 140.100.0.0 passive interface e1 This configuration shows the following The use of the redistribute command The routing process from which the routes are being accepted The...

Configuring the Default Metric for Ospf Rip Egp or BGP4

Alternatively, it is possible to redistribute the routing protocol and then, with a separate command, to state the default metric. The advantage of this is it is a simpler configuration visually, which is helpful in troubleshooting. Also, if more than one protocol is being redistributed into the routing protocol, the default metric applies to all the protocols being redistributed. To configure the default metric for OSPF, RIP, EGP, or BGP-4, use the following command syntax The default-metric...

Connecting to Other Autonomous Systems The Basics of BGP4

This chapter details Border Gateway Protocol 4 (BGP-4). Although BGP-4 is covered briefly in the ACRC exam, it is given far greater attention in the CCNP CCDP Routing exam. Even so, the exam barely scratches the surface of the detail available to the protocol. This chapter deals with the basic concepts and configuration commands of BGP-4 and leads into greater complexity in the next chapter. This chapter builds on the understanding of routing within large enterprise networks that was dealt with...

Connecting to the Outside World

When connecting to the outside world, some filtering and address translation may be necessary. Unless an address has been obtained from the Internet or from an ISP, it is necessary to perform address translation. The RFC that defines NAT is RFC 1631. NAT is the method of translating an address on one network into a different address for another network. It is used when a packet is traversing from one network to another and when the source address on the transmitting network is not legal or...

Considerations for Summarization with Discontiguous Networks

If there are discontiguous networks in the organization, it is important that summarization is turned off or not configured. Summarization may not provide enough information to the routing table on the other side of the intervening NIC number to be capable of appropriately routing to the destination subnets. This is especially true of EIGRP, which automatically summarizes at the NIC boundary, which would be disastrous in this situation. In OSPF and EIGRP, manual configuration is required for...

Contents at a Glance

Chapter 1 Cisco Certifications, the Routing Exam, and This Book's Features Chapter 2 Managing Scalable Network Growth Chapter 4 IP Routing Principles Chapter 5 Using OSPF in a Single Area Chapter 6 Using OSPF Across Multiple Areas Chapter 7 Using EIGRP in Enterprise Networks Chapter 8 Connecting to Other Autonomous Systems The Basics of BGP-4 Chapter 9 Implementing and Tuning BGP for Use in Large Networks Chapter 10 Controlling Routing Updates Across the Network Chapter 11 Scenarios for Final...

Controlling BGP4 Traffic

It is often advantageous to limit the way that the BGP-4 routing updates are propagated, for the same reasons that any routing protocol is best limited to those updates that are required. This not only streamlines the traffic flow on the network, but it also simplifies the network and thus its maintenance. Designing how the routing information should be forwarded through the network forms a basic level of security and can reduce the possibility of routing loops....

Controlling Routing Updates

Various methods enable you to control the routing information sent between routers. These methods include the following Passive interfaces An interface that does not participate in the routing process. In RIP and IGRP, the process listens but will not send updates. In OSPF and EIGRP, the process neither listens nor sends updates because no neighbor relationship can form. The interfaces that participate in the interior routing process are controlled by the interface configuration. During...

Controlling Routing Updates Across the Network

The topics in this chapter deal with the traffic generated by the routing updates in terms of both the network resources that they use and the information contained within them. This covers two different but related areas. The network overhead involved in routing updates has already been dealt with in other chapters, and it keeps recurring as a theme because all network traffic directly influences the network's capability to scale or grow. The issue is very complex and deals with the design of...

Controlling Routing Updates with Filtering

Despite all the mechanisms for controlling and reducing the routing updates on your network, it is sometimes necessary to wield greater and more flexible power. This comes in the form of access lists, which when applied to routing updates are referred to as distribute lists. The logic used in the distribute lists is similar to that of an access list. It is summarized in the flowchart in Figure 10-11 and the process listed in the following text. Figure 10-11 Distribute List Logic on an Incoming...

Controlling Terminal Access

Access lists filter traffic traversing the router they do not prevent traffic generated by the router. To control Telnet traffic in which the router is the end station, an access list can be placed on the virtual terminal line (vty). Five terminal sessions are available vty 0 through vty 4. Because anticipating which session will be assigned to which terminal is difficult, control is generally placed uniformly on all virtual terminals. Although this is the default configuration, some platforms...

Cost Command Design Considerations

Considerations in using the cost command include the following Never change defaults unless you can explain why the change is necessary. Reasons for using the cost option in OSPF include the following You want to maintain interoperability among different vendors running OSPF. There is a design reason to choose a different path than the one selected by the Cisco default metric. You want to allow greater granularity in the application of the cost metric. If the default is to be overridden by the...

Creating a Network That Meets the Key Requirements

Although is important to know how to reduce network congestion when it occurs, it is crucial to build a network that can grow with the user needs, thus preventing a network that reels from crisis to crisis. Designing the network so that it can scale, therefore, is crucial. Cisco provides a hierarchical design that should simplify network management and also allow the network to grow. This growth may be a physical growth as well as a capacity growth. Creating a Network That Meets the Key...

Default Redistribution Between Routing Protocols

EIGRP is a routing protocol that carries updates for multiple protocols. The key to how this works is the separate routing tables held for each protocol, using the routing protocol as the mechanism for the forwarding of updates and path selection. EIGRP supports AppleTalk's RTMP, IPX's RIP and NLSP, as well as IP. Automatic redistribution is performed between RTMP and EIGRP, and IPX RIP and EIGRP. EIGRP must be manually redistributed into NLSP. There is also automatic redistribution between...

Default Routes

A default route is a route that is used when the routing table has no entry for the destination network in a packet that it is attempting to forward. In larger networks, there may well be many static routes to be configured. Not only is this a chore for the administrator, but it also requires vigilance so that changes in the routing table can be reconfigured. It may be that turning on a routing protocol is advised, or, alternatively you can configure a specialized static route, called a static...

Defining the Networks to Be Advertised

To define the network that is to be advertised for this autonomous system, the following command is used (each network requires a separate command) network network-number mask network-mask The network command determines the networks that are originated by this router. This is a different use of the command that you are accustomed to configuring with EIGRP, OSPF, and RIP. This command is not identifying the interfaces upon which to run BGP instead, it is stating the networks that are to be...

Design Considerations in Multiple Area OSPF

The major design consideration in OSPF is how to divide the areas. This is of interest because it impacts the addressing scheme for IP within the network. An OSPF network works best with a hierarchical design, in which the movement of data from one area to another comprises only a subset of the traffic within the area itself. It is important to remember that with all the interarea traffic disseminated by the backbone, any reduction of overhead through a solid hierarchical design and...