The Update Message

The Update message, whose format is shown in Figure 2-45, is used to advertise a single feasible route to a peer, or to withdraw multiple unfeasible routes, or both. i Figure I he BGP Update Message Format Total Path Attribute Length (2 octets) Network Layer Reachability Information (variable) The BGP Update message contains the following fields Unfeasible Routes Length A 2-octet field indicating the total length of the following Withdrawn Routes field, in octets. A value of zero indicates that...

Neighbor Acquisition Protocol

Before EGP neighbors can exchange reachability information, they must establish that they are compatible. This function is performed by a simple two-way handshake in which one neighbor sends a Neighbor Acquisition Request message, and the other neighbor responds with a Neighbor Acquisition Confirm message. None of the RFCs specify how two EGP neighbors initially discover each other. In practice, an EGP gateway learns of its neighbor by manual configuration of the neighbor's IP address. The...

Duplicate Address Detection

All nodes perform duplicate address detection before assigning a unicast address to an interface. It is not performed for anycast addresses. This is performed regardless of whether the address is assigned via stateless, stateful, or manual configuration. It is performed before assigning an address to an interface and on an initializing interface. The address to be assigned to the interface is called tentative while the duplicate address detection process is taking place. Before sending a...

EGP Message Formats

EGP uses five different formats to encode the ten message types shown in Table 1-1. All the messages have a common header, as shown in Figure 1-4. The fields in the EGP message header are defined as follows Version Specifies the current EGP version number. If this number in a received message does not agree with the receiver's version number, the message is rejected. The version number of all current EGP implementations is 2. Type Specifies which of the five message formats follows the header....

CBT Message Formats

CBT messages are encapsulated in IP headers with a protocol number of 7 With the unicast exceptions documented earlier in this section, the packets are transmitted with a destination address of 224 0 0 15 and a TTL of 1 Figure 5-42 shows the format of the common header shared by all CBT messages Figure 5-42 The CBT Message Header Format The fields for the CBT message header are defined as follows Version specifies the CBT version number This section has dealt exclusively with version 2,...

PIMv2 Join Prune Message Format

Join Prune messages, the format of which is illustrated in Figure 5-66, are sent upstream to either RPs or sources and are used to join and prune both RPTs and SPTs. The message consists of a list of one more multicast groups. For each multicast address, there is a list of one or more source addresses. Together, these lists specify all (S, G) and (*, G) entries to be joined or pruned. Figure 5-66 The PIMv2 Join Prune Message Format Figure 5-66 The PIMv2 Join Prune Message Format Encoded Unicast...

Troubleshooting Exercises

1 What is the output of Example 6-63 telling you Example 6-63 The Output for Troubleshooting Exercise 1 IP multicast packets debugging is on R1 IP multicast packets debugging is on R1 Example 6-63 The Output for Troubleshooting Exercise 1 (Continued) IP s 192.168.14.35 (Serial 1.307) d 228.13.20.216 len 573, mrouting disabled IP s 192.168.14.35 (Serial0 1.307) d 228.13.20.216 len 573, mrouting disabled IP s 192.168.14.35 (Serial0 1.307) d 228.13.20.216 len 573, mrouting disabled 2 What is the...

PIMv2 Register Message Format

Register messages, the format of which is illustrated in Figure 5-64, used only by PIM-SM, are unicast from the source's DR to the RP, and they carry the initial multicast packets from the source. That is, Register messages are used to tunnel multicast traffic from the source to the RP when an SPT has not yet been established from the source's DR to the RP. Figure 5-64 The PIMv2 Register Message Format The fields for the PIMv2 Register message are defined as follows Checksum, in Register...

LSA Flooding Scope and Unknown LSA Types

The flooding scope of LSA packets has been generalized. The LSA type determines the scope of OSPFv2 flooding. Each type is associated with its flooding scope. In OSPFv3, the flooding scope is explicitly configured in the LSA header. An OSPFv3 router that does not recognize the LSA type still knows how to flood the packet. The scope could be local-link, Area, or AS. OSPFv3 allows routers to have differing capabilities. Routers are no longer required to drop received LSAs with unknown types....

DVMRP Message Header

Figure 5-25 shows the format of the DVMRP header, which begins every DVMRP message Figure 5-25 The DVMRP Message Header The fields for the DVMRP message are described as follows Type is the IGMP type number, which is set to 0x13 for all DVMRP messages RFC 1075 specifies a separate 4-bit Version field and 4-bit Type field in this position, in which the version is 0x1 and the type is 0x3 Note that the resulting 8 bits of the version 1 header is 0x13, the same as version 3, making version 3...

M

MAC (media access control) addresses functional, 403 GDA (Group Destination Address), 422-426 Token Ring, 403 USA (Unicast Source Address), 422-426 MAEs (Metropolitan Area Ethernets), 65 maintaining DVMRP neighbors, 442 loop-free multicast topologies, 437 Major Version field (DVMRP messages), 446 management processes, server design, 782 manager (SNMP), 731-732 managing BGP peer connections, 204-208 large-scale BGP 265 communities, 270-283 4 confederations, 287-300 peer groups, 265-266, 269...

DVMRP Prune Message Format

Figure 5-28 shows the format of the Prune message Figure 5-28 The DVMRP Prune Message Format The fields for the DVMRP Prune message are defined as follows Source Host Address is the IP address of the originating host Group Address is the IP address of the group to be pruned Prune Lifetime is the time, in seconds, that the upstream neighbor is to keep the prune This value is either the minimum remaining lifetime of all downstream prunes received for the group address or, if there are no...

The Bootstrap Protocol

The bootstrap protocol, first supported in Cisco IOS Software Release 11 3T, is essentially the same protocol used by CBT to advertise core routers, with a few changes in message names and formats To run the bootstrap protocol, candidate bootstrap routers (C-BSRs) and candidate rendezvous points (C-RPs) are administratively designated in the internetwork Typically, the same set of routers is configured as both C-BSRs and C-RPs The C-BSRs and C-RPs identify themselves by means of an IP address,...

Overview of Syslog

Messages are sent from various services running on the UNIX server or from other network nodes The service that sends the message indicates its facility type The syslog daemon utilizes the indicated facility type when determining how to log the message Table 9 3 lists the various facility types A syslog daemon is configured by updating a file on the server in the etc directory called syslog conf The syslog daemon reads this file upon startup to determine how to handle incoming messages The file...

Developing IP Multicast Networks Volume I

Beau Williamson, CCIE 1-57870-077-9 AVAILABLE NOW This book provides a solid foundation of IP multicast concepts and explains how to design and deploy the networks that will support appplications such as audio and video conferencing, distance-learning, and data replication. Includes an in-depth discussion of the PIM protocol used in Cisco routers and detailed coverage of the rules that control the creation and maintenance of Cisco mroute state entries.

Case Study Administrative Weights

Frequently, a BGP router is presented with multiple routes to the same destination. Although BGP has default methods for choosing among these routes, you might need to override these defaults on occasion to implement a routing policy. Although the RFCs do not provide for methods to influence route preferences within a single router, Cisco's IOS Software does. The first of these Cisco-specific tools is administrative weight. Each route is assigned a weight, which is a number between 0 and...

Case Study Multicasting Across Non Multicast Domains

One challenge you will face is connecting diverse multicast domains across domains in which multicast is not supported. This may certainly be the case when multicasting is required in only certain areas of a large routing domain. You would not want to enable multicast on every router in the unicast domain just to provide connectivity to a relatively small number of multicast routers. A second and very common example is connecting multicast domains across the decidedly unicast Internet. In...

InterAS Multicasting

A challenge facing any multicast routing protocol (or any unicast routing protocol, for that matter) is scaling efficiently to the set of hosts requiring delivery of packets. You have seen how dense mode protocols such as PIM-DM and DVMRP do not scale well by definition, the protocols assume that most hosts in the multicast domain are group members. PIM-SM, being a sparse mode protocol, scales better because it assumes most hosts in the multicast domain are not group members. Yet the assumption...

Operation of Core Based Trees CBT

DVMRP and MOSPF have two limitations in common First, they are both dense-mode protocols and do not scale well in sparse topologies That is, when there are few group members relative to the total number of hosts in an internetwork, and the group members are widespread across the internetwork, both DVMRP and MOSPF consume an unacceptable amount of network resources to reach those group members Much of that resource consumption is in the overhead necessary to calculate and hold state for...

Sparse Versus Dense Topologies

A dense topology is one in which there are many multicast group members relative to the total number of hosts in an internetwork Sparse topologies have few group members relative to the total number of hosts Sparse does not mean that there are few hosts A sparse topology might mean there are 2,000 members of a group, for example, spread among 100,000 total hosts No specific numeric ratios delineate sparse and dense topologies It is safe to say, however, that dense topologies are usually found...

Source Based Trees Versus Shared Trees

Some multicast routing protocols construct separate multicast trees for every multicast source These trees are source-based trees, because they are rooted at the source The multicast trees that have been presented in previous sections have been source-based trees You have learned that multicast trees can change during the lifetime of a multicast session as members join and leave the group, and that it is the responsibility of the multicast routing protocol to dynamically adapt the tree to these...

Logging

When logging is enabled on a router, messages for certain events that occur on the router are created and stored. The log may reside on the router, or it may be an external log, residing on a server somewhere in the network. Routers send output from debug commands and system error messages to the logging process. The logging process distributes the messages to the various logging devices and files, depending on the router configuration. Messages are sent to the logging buffer, to terminal...

The BGP Finite State Machine

The stages of a BGP connection establishment and maintenance can be described in terms of a finite state machine. Figure 2-20 and Table 2-4 show the complete BGP finite state machine and the input events that can cause a state transition. Mgure 2-20 The BGP Finite State Machine Mgure 2-20 The BGP Finite State Machine Table 2-4 The Input Events (IE) of Figure 2-20 Table 2-4 The Input Events (IE) of Figure 2-20 BGP Transport connection open failed The following sections provide a brief...

Managing BGP Connections

Cisco IOS offers several features to help in the management of BGP peer connections. The first is the neighbor description statement. Like the description statement that can be entered under an interface configuration, this statement has no functional impact on the router. Rather, it adds an informational text string to the configuration. You already have encountered many configuration options that apply to a BGP neighbor, and you will encounter several more in the remaining sections of this...

Case Study Configuring Sparse Dense Mode

A slight cheat was used in the examples of the preceding case study. Examining Figure 6-5, notice that the C-RPs are directly connected to the mapping agent, and the mapping agent is directly connected to Bowler. In Figure 6-6, Homburg is now configured as the Auto-RP mapping agent. This topology gives rise to an interesting dilemma Homburg advertises the RPs to all routers in RP-Discovery messages, using the reserve address 224.0.1.40. All PIM-SM routers listen for this address. In a...

NAT and Multihomed Autonomous Systems

Nat Routing

Another shortcoming of CIDR is that multihoming to different service providers becomes more difficult. Figure 4-5 recaps the problem as discussed in Chapter 2. A subscriber is multihomed to ISP1 and ISP2 and has a CIDR block that is a subset of ISPl's block. To establish correct communication with the Internet, both ISP1 and ISP2 must advertise the subscriber's specific address space of 205.113.50.0 23. If ISP2 does not advertise this address, all the subscriber's incoming traffic passes...

BGP 4 Multiprotocol Extensions

Additions made to BGP 4 are not specific to IPv6 They also include support for other protocols such as IPX The multiprotocol additions to BGP 4 are discussed here as they relate to IPv6 Multiprotocol BGP (MBGP) is discussed in Chapter 7 Large Scale IP Multicast Routing Three pieces of BGP 4 information are IPv4 specific The network layer reachability information (NLRI) At the time of this writing it is assumed that every BGP 4 speaker will maintain at least one IPv4 address The AGGREGATOR...

Introduction to Protocol Independent Multicast PIM

If you are a CCIE candidate, studying the previous sections on protocols not supported or only partially supported (in the case of DVMRP) by Cisco may strike you as a poor investment of time Yet each protocol offers lessons in what is desirable about a multicast routing protocol and what is not DVMRP shares the characteristic of unicast distance vector protocols of being very simple to implement little more is required than to just turn it on But this simplicity comes at the expense of high...

Enhanced IP Services for Cisco Networks

Lee, CCIE 1-57870-106-6 AVAILABLE NOW This is a guide to improving your network's capabilities by understanding the new enabling and advanced Cisco IOS services that build more scalable, intelligent, and secure networks. Learn the technical details necessary to deploy Quality of Service, VPN technologies, IPsec, the IOS firewall and IOS Intrusion Detection. These services will allow you to extend the network to new frontiers securely, protect your network from attacks, and increase...

Case Study ISP Multihoming with NAT

The section NAT and Multihomed Autonomous Systems earlier in this chapter demonstrated ways you can employ NAT to overcome the problem of multihoming to different ISPs with different CIDR blocks. The subscriber in Figure 4-7 is multihomed, and each ISP sees packets with source addresses belonging to its own address space. Neither ISP receives packets from the subscriber with source addresses belonging to the other ISP's block of addresses. Based on the NAT case studies you have already seen,...

Multihoming to Multiple Autonomous Systems

Figure 2-14 shows a topology in which a subscriber has homed to more than one service provider. In addition to the advantages of multihoming already described, this subscriber is protected from losing Internet connectivity as the result of a single ISP failure. For a small corporation or a small ISP, there are substantial obstacles to multihoming to multiple service providers. You already have seen the problems involved if the subscriber's address space is a part of one of the service...

Summarization The Good the Bad and the Asymmetric

Summarization is a great tool for conserving network resources, from the amount of memory required to store the routing table to the amount of network bandwidth and router horsepower necessary to transmit and process routing information. Summarization also conserves network resources by hiding network instabilities. For example, the network in Figure 2-4 has a flapping route a route that, due to a bad physical connection or router interface, keeps transitioning down and up and down again....

Case Study IBGP Over an IGP

In Figure 3-6, the routers within AS 100 have been reconfigured. In this topology, OSPF is running as the autonomous system's IGP, and IBGP runs only between Vail and Telluride. Figure i f> OSPF Is Added to the Routers in AS 100 192.168.100.0 24 192.168.200.0 24 192.168.1.216 30 192.168.100.0 24 192.168.200.0 24 192.168.1.216 30 Example 3-35 shows the configurations of the three routers in AS 100. Example3-35 Configurations for Vail, Aspen, and Telluride in AS 100 In the BGP configurations,...

Case Study Multicast Load Sharing

At times, you may want to balance multicast traffic over parallel equal-cost paths, either to more fully utilize available bandwidth or to prevent a single path from becoming congested by heavy multicast traffic. But the RPF check prevents multicast load balancing directly over physical links. The problem is illustrated in Figure 6-8, where the same PIM topology used in the previous case studies is repeated, except that Bowler is removed and Homburg is both the Auto-RP mapping agent and the RP....

Case Study Configuring Protocol Independent Multicast Dense Mode PIMDM

After you have enabled IP multicast routing on a Cisco router, you can very simply enable PIM-DM by adding the command ip pim dense-mode to all the router's interfaces. Figure 6-1 shows a simple PIM-DM topology, and Example 6-2 shows the configuration of router Porkpie. The other router configurations are similar to that of Porkpie. Two important considerations when configuring PIM-DM are reflected in Example 6-2. The first and most obvious is that a unicast routing protocol in this case, OSPF...

Configuring IP Multicast Routing

Before you can configure a particular IP multicast routing protocol, you must set up the router for general, protocol-neutral multicast routing. NOTE Protocol-independent would be a better term than protocol-neutral, but it would cause confusion in light of PIM. Example 6-1 shows a configuration containing some of the commands you might use. Out of all the commands shown, ip multicast-routing is the only required one. Just as the default (and therefore hidden) ip routing enables unicast IP...

Boundaries Delineating Literals

Underscoie (J is used when you want to specify a string of liteials and must specify separation Suppose foi example that you want to match on the specific AS_PATH (5610, 148 284, 13) The filtei is as follows ip as path access list 30 permit 5610_148_284_13 The underscore matches a beginning of line an end of line a comma oi a space Notice the difference between the preceding filtei and this filter ip as path access list 31 permit _5610_148_284_13_ Because the first filter specified the...

Case Study Peering BGP Routers

A BGP session between routers is configured in two steps Step 1 Establish the BGP process and specify the local AS number with the router bgp command. Step 2 Specify a neighbor and the neighbor's AS number with the neighbor remote-as command. Figure 3-1 shows two routers in different autonomous systems. The structure of the BGP configuration for these routers differs from EGP configuration. Recall from Chapter 1, Exterior Gateway Protocol, that the router egp command specifies the remote AS,...

Operation of Protocol Independent Multicast Dense Mode PIMDM

As of this writing, no RFC describes PIM-DM It is, however, described in an Internet draft Beyond the common message formats, you are likely to find more similarities between PIM-DM and DVMRP than between PIM-DM and PIM-SM PIMv2 routers use Hello messages to discover neighbors When a PIMv2 router (either PIM-DM or PIM-SM) becomes active, it periodically sends a Hello message on every interface on which PIM is configured PIMvl routers have the same functionality, except that they use Query...

Multicast IP Addresses

The IANA has set aside Class D IP addresses for use as multicast addresses According to the first octet rule, as described in Chapter 2, TCP IP Review, of Volume , the first four bits of a Class D address are always 1110, as shown in Figure 5-4 Finding the minimum and maximum 32-bit numbers within this constraint, the range of Class D addresses is 224 0 0 0-239 255 255 255 Figure 5-4 Class D Addresses Are in the Range 224 0 0 0-239 255 255 255 Unlike the Class A, B, and C address ranges, the...

Configuration Exercises

Table 3-4 shows the routers and addresses used for configuration exercises 1 through 13. Table 3-4 Routers Addresses for Configuration Exercises 1-13 Table 3-4 Routers Addresses for Configuration Exercises 1-13 (Continued) Table 3-4 Routers Addresses for Configuration Exercises 1-13 (Continued) Table 3-4 lists the autonomous systems, routers, interfaces, and addresses used in configuration exercises 1 through 13. All interfaces of the routers are shown. For each exercise, if the table indicates...

Advanced IP Network Design

Alvaro Retana, CCIE Don Slice, CCIE and Russ White, CCIE 1-57870-097-3 AVAILABLE NOW Network engineers and managers can use these case studies, which highlight various network design goals, to explore issues including protocol choice, network stability, and growth. This book also includes theoretical discussion on advanced design topics. Large-Scale IP Network Solutions Khalid Raza, CCIE and Mark Turner Network engineers can find solutions as their IP networks grow in size and complexity....

NAT and ISP Migration

One of the drawbacks of CIDR, as discussed in Chapter 2, Introduction to Border Gateway Protocol 4, is that it can increase the difficulty of changing Internet service providers. If you have been assigned an address block that belongs to ISP1, and you want to change to ISP2, you almost always have to return ISPl's addresses and acquire a new address range from ISP2. This return can mean a painful and costly re-addressing project within your enterprise. TIP It cannot be overemphasized that the...

IBGP and IGP Synchronization

With very few exceptions, interior BGP BGP between peers in the same AS is used only in multihomed scenarios. IBGP allows edge routers to share NLRI and associated attributes, to enforce a systemwide routing policy. IBGP also is the means by which an edge router in a transit AS passes routes learned from an external peer to other edge routers for advertisement to their external peers. You might be tempted to think that in some cases IBGP could be used as an IGP. For instance, an ISP's AS is...

Case Study Filtering with Route Maps

You also can implement route filters with route maps. The route map can use either access lists, to filter by NLRI, or AS_PATH access lists, to filter by the ASJPATH attribute. Example 3-88 shows a possible configuration for Zermatt in Figure 3-15. Example 3-88 Configuring Zermatt to Filter Routes with a Route Map router bgp 30 redistribute isis level-2 neighbor 10.100.83.1 remote-as 100 neighbor 10.100.83.1 ebgp-multihop 2 neighbor 10.100.83.1 update-source LoopbackO neighbor 10.100.83.1...

Advertising Aggregate and Selected More Specifics

The previous scenarios send the more-specific routes of AS 100 to AS 200 so that AS 200 can implement routing policy. That is, AS 200 uses the routes to set routing preferences for sending traffic to AS 100. AS 100 also can influence its incoming traffic by manipulating its outgoing advertisements. For example, advertising 192.168.193.0 24 over the Stowe Sugarbush link and not over the Mammoth Diamond link causes incoming traffic to use the Stowe Sugarbush link. An administrator might want to...

Case Study BGP Communities

Whereas peer groups enable you to apply common policies to a group of neighbors, communities enable you to apply policies to a group of routes. A community is a route attribute and therefore is communicated from one BGP speaker to another. You follow three steps to configure a community attribute Step 1 Use a route map to identify the routes in which the attribute is to be set. Step 2 Use the set community command to set the attribute. Step 3 Use the neighbor send-community command to specify...

Case Study Route Dampening

Route dampening, as discussed in Chapter 2, is a process that can assign a penalty to a flapping route. If the route accumulates enough penalties, the route is suppressed that is, it is not advertised for a certain period of time. By default, a route is assigned a penalty value of 1000 for each flap. If the value of the route's accumulated penalties exceeds 2000, the route is suppressed until the penalty value drops below 750. These upper and lower thresholds are the suppress limit and the...

CIDR Reducing Routing Table Explosion

Given the somewhat hierarchical structure of the Internet, you can see how the structure lends itself to an address summarization scheme. At the top layers, large blocks of contiguous Class C addresses are assigned by the Internet Assigned Numbers Authority (IANA) to the various addressing authorities around the globe, known as the regional IP registries. Currently, there are three regional registries. The regional registry for North and South America, the Caribbean, and sub-Saharan Africa is...

The Localpref Attribute

LOCAL_PREF is short for local preference. This well-known discretionary attribute is used only in updates between internal BGP peers it is not passed to other autonomous systems. The attribute is used to communicate a BGP router's degree of preference for an advertised route. If an internal BGP speaker receives multiple routes to the same destination, it compares the LOCAL_PREF attributes of the routes. The route with the highest LOCAL_PREF is selected. Figure 2-26 demonstrates how the LOCAL...

PIMv2 Message Header Format

All PIM messages have a standard header, shown in Figure 5-62. Figure 5-62 The PIMv2 Message Header The fields for the PIMv2 message header are defined as follows Version specifies the version number. The current version number is 2, although PIMvl is still in common usage. Type specifies the type of PIM message encapsulated behind the header. Table 5-12 lists the PIMv2 message types. 8 Candidate-RP-Advertisement (used in PIM-SM only) Checksum is a standard IP-style checksum, using a 16-bit...

Implicit Joins Versus Explicit Joins

As was previously observed, members may join or leave a group at any time during the lifetime of a multicast session, and as a result, the multicast tree can change dynamically It is the job of the multicast routing protocol to manage this changing tree, adding branches as members join and pruning branches as members leave The multicast routing protocol may accomplish this task by using either an implicit or explicit join strategy Implicit joins are sender-initiated, whereas explicit joins are...

Single Homed Autonomous System

Autonomous System Cisco

Figure 2-10 shows a subscriber attached by a single connection to an ISP. BGP, or any other type of routing protocol, is unnecessary in this topology. If the single link fails, no routing decision needs to be made, because no alternative route exists. A routing protocol accomplishes nothing. In this topology, the subscriber adds a static default route to the border router and redistributes the route into his AS. Figure 2-10 Static Routes Are All That Is Needed in This Single-Homed Topology...

Operation of Multicast Ospf Mospf

Multicast OSPF (MOSPF) offers an improvement over DVMRP in two aspects First, it is a link-state protocol, whereas DVMRP is distance vector That difference carries with it all the usual advantages of link state over distance vector better convergence properties, better loop avoidance, and less periodic control traffic The second improvement is that MOSPF is more scalable in a dense environment This is partly due to its link-state algorithms, but also to the fact that MOSPF uses explicit joins...

New LSAs and LSA Changes

Although most of the functionality has remained unchanged some OSPFv2 LSA fields have been modified and LSAs have been renamed in OSPFv3 New LSAs have been added to OSPF to carry IPv6 addresses and next hop information The OSPFv2 LSA header contained these fields Age Options Type Link State ID Advertising Router Sequence Number Checksum and Length The OSPFv3 LSA removed the Options field from the header expanded it from 8 to 24 bits and moved it to the body of Router LSAs Network LSAs Inter...

Multihoming to a Single Autonomous System

Figure 2-12 shows an improved topology, with redundant links to the same provider. How the incoming and outgoing traffic is manipulated across these links depends on how the two links are used. For example, a typical setup when multihoming to a single provider is for one of the links to be a primary, dedicated Internet access link say, a T1 and for the other link to be used only for backup. In such a scenario, the backup link is likely to be some lower-speed connection. Figure 2-12 Multihoming...

Route Reflectors

Route Reflector Cisco

Route reflectors are useful when an AS contains a large number of IBGP peers. (For more information, see RFC 1966 at www.isuedu in-notes rfc 1771.txt.) Unless EBGP routes are redistributed into the autonomous system's IGP, all IBGP peers must be fully meshed. For every n routers, there will be n(n - l) 2 IBGP connections in the AS. For example, Figure 2-35 shows six fully meshed IBGP routers, hardly a large number of routers even here, however, 15 IBGP connections are needed. Route reflectors...

DVMRP Neighbors 2 Message Format

A DVMRP router in response to an Ask Neighbors 2 message sends the Neighbors 2 message, shown in Figure 5-32 The message is unicast to the originator of the Ask Neighbors 2 message The message indicates the sender's DVMRP capabilities and lists the addresses of the sender's logical interfaces For each interface listed, the DVMRP parameters for the interface are specified, and the DVMRP neighbors known on that interface are listed Figure 5-32 The DVMRP Neighbors 2 Message Format The fields for...

Answers to Chapter 6 Configuration Exercises

1 What global Cisco IOS Software command is required to enable IP multicast routing Answer ip multicast routing 2 Show the commands that enable PIM on an interface in dense mode sparse mode and sparse dense mode xp pim dense-mode lp pirn sparse-mode lp pim sparse-dense mode 3 Show the command to statically specify an pP with an address of 172.18.20.4. Answer* lp pim rp-address 172.18.20.4 4 Write the configuration statements necessary to statically map groups 239.1.2.3 and 228.1.8.0 -...

Neighbor Reachability Protocol

After a gateway has acquired a neighbor, it maintains the neighbor relationship by sending periodic Hello messages. The neighbor responds to each Hello with an I-H-U message. RFC 904 does not specify a standard period between Helios Cisco uses a default period of 60 seconds, which can be changed with the command timers egp. When three Hello I-H-U message pairs have been exchanged, the neighbor state changes from Down to Up (see Example 1-2). The neighbors can then exchange network reachability...

Cnse Study Injecting IGP Routes into BGP

Picture Case Routing

Chapter 2 emphasizes that at an AS border, outgoing route advertisements affect incoming traffic, and incoming route advertisements affect outgoing traffic. As a result, outgoing and incoming advertisements should be considered separately. This section begins the discussion of BGP route advertisements by examining basic methods of injecting routes into BGP. Figure 3-3 shows that AS 200 uses EIGRP as its IGP. Taos must advertise three addresses to its EBGP peer 192.168.200.0 24 is learned via...

Case Study An EGP Core Gateway

By definition, an EGP core gateway can peer with multiple neighbors within multiple far autonomous systems and can pass network information from one FAS to another FAS. Because of this, the configuration of a core gateway differs slightly. Figure 1-11 shows a core router, Stan, which is peered with a router in a FAS (Buster) and a router within its LAS (Ollie). Figure 1-11 Core Router Stan Must Peer with Both Remote Neighbor Buster and Local Neighbor Ollie Figure 1-11 Core Router Stan Must Peer...

Case Study Configuring the Bootstrap Protocol

When PIMv2 was first described in RFC 2117, the bootstrap protocol was specified as the mechanism for automatic RP discovery. Cisco first supported PIMv2 in Cisco IOS Software Release 11.3T, and the bootstrap protocol is included in that support. The two steps to configure bootstrap are very similar to the two steps for configuring Auto-RP 1 All candidate RPs must be configured. 2 All candidate bootstrap routers (C-BSRs) must be configured. Figure 6-7 shows the same PIM topology used in the...

PIM Forwarder Election

In Figure 5-52, both Mercury and Copper have a route to source 172 16 1 1 They also have downstream interfaces to a member of group 239 70 49 238 that are connected to a common multiaccess network Both Meicury and Copper are receiving copies of the same multicast packets from the source, but it would obviously be inefficient for both routers to forward the packets onto the same network Figure 5-52 Both Copper and Mercury Are Receiving Copies of the Multicast Packets Sent by Source 172 161 1,...

Case Study Using the Localpref Attribute

The LOCAL_PREF attribute is used to set preferences among multiple routes to the same destination. Unlike administrative weight, the LOCAL_PREF is not limited to a single router. Rather, it is communicated to IBGP peers. The attribute is not communicated to EBGP peers hence the name local preference. A route's LOCAL_PREF attribute can be any number between 0 and 4,294,967,295 the higher the number, the more preferable the route. By default, all routes advertised to IBGP peers have a LOCAL_PREF...

The Internet Still Hierarchical After All These Years

Cdn Hierarchy Isp

Although the Internet has grown away from the single-backbone architecture of the ARPANET,described in Chapter 1, it retains a certain hierarchical structure. At the lowest level, Internet subscribers connect to an Internet service provider (ISP). In many cases, that ISP is one of many small providers in the local geographic area (called local ISPs). For example, there are presently almost 200 ISPs in Colorado's 303 area code. These local ISPs in turn are the customers of larger ISPs that cover...

Cnse Study Filtering Routes by NLRI

Route filters are at the heart of almost any routing policy. After all, if you have an inbound and an outbound routing policy, what you are most likely defining is which routes a router accepts and which routes a router advertises. The first and simplest of the route filters available to BGP are defined by the distribute-lisi command. This route filter is defined for each neighbor or peer group and points to an access list that defines the prefixes, or NLRI, on which the filter will act. The...

Interpreting the Neighbor Table

An examination of the EGP neighbor table using show ip egp will tell you about the state and configuration of a gateway's neighbors. Example 1-18 displayed the output of this command Example 1-22 shows some additional output from the show ip egp command that examines Stan's neighbor table. Example 1-22 show ip egp Command Output Displays Information Useful for Troubleshooting EGP Peers Example 1-22 show ip egp Command Output Displays Information Useful for Troubleshooting EGP Peers You can see...

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Example 3-51 Configuring Stowe to Advertise Both the Aggregate and the More-Specific Routes router eigrp 100 network 192.168.199.0 router bgp 100 aggregate-address 192.168.192.0 255.255.248.0 redistribute eigrp 100 neighbor 192.168.1.253 remote-as 200 neighbor 192.168.1.253 send-community neighbor 192.168.1.253 route-map COMMUNITY out access-list 101 permit ip host 192.168.192.0 host 255.255.248.0 i route-map COMMUNITY permit 10 match ip address 101 set community none route-map COMMUNITY permit...

Difficulties with CIDR

Although CIDR has proven successful in slowing both the growth of Internet routing tables and the depletion of Class B addresses, it also has presented some problems for the users of CIDR blocks. The first problem is one of portability. If you have been given a CIDR block, the addresses are most likely part of a larger block assigned to your ISP. Suppose, however, that your ISP is not living up to your expectations or contractual agreements, or you have just gotten a more attractive offer from...

Case Study BGP Confederations

BGP confederations make large transit autonomous systems more manageable by enabling the administrator to break the AS into subautonomous systems. The subdivided AS itself becomes the confederation, and the subautonomous systems are the member autonomous systems. Autonomous systems outside of the confederation see the entire confederation as a single AS and do not see the member autonomous systems. Because the member autonomous systems are hidden from the outside, they may use either public or...

Operation of CGMP

Although both Cisco routers and Cisco switches must be configured to run CGMP, only the routers produce CGMP packets The CGMP process on switches only reads the packets There are two types of CGMP packets Join packets are sent by the router to tell the switch to add one or more members to a multicast group Leave packets are sent by the router to tell the switch to remove one or more members from a multicast group, or to delete the group altogether These two packet types have an identical...

Multicast Addresses

Multicast addresses identify groups of interfaces, each of which can contain multiple multicast addresses. Multicast addresses are distinguishable from unicast addresses because they always begin with OxFF. There is no such thing as broadcasting at the network layer in IPv6. Broadcasting induced a lot of extra overhead on nodes that were not necessarily interested in the broadcast packet. All IP interfaces that received a broadcast packet had to process the packet to see whether it might be the...

BGP Basics

Like EGP, BGP forms a unique, unicast-based connection to each of its BGP-speaking peers. To increase the reliability of the peer connection, BGP uses TCP (port 179) as its underlying delivery mechanism. The update mechanisms of BGP are also somewhat simplified by allowing the TCP layer to handle such duties as acknowledgment, retransmission, and sequencing. Because BGP rides on TCP, a separate point-to-point connection to each peer must be established. BGP is a distance vector protocol in that...

Answers to Chapter 6 Troubleshooting Exercises

1 What is the output of Example 6 63 telling you Example 6 63 The Output for Troubleshooting Exercise 1 IP multicast packets debugging is on R1 IP multicast packets debugging is on R1 Answer Multicast packets are being dropped because multicast routing is not enabled on the router. 2 What is the output of Example 6 64 telling you Example 6 64 The Output for Troubleshooting Exercise 2 Answer Packets for group 227 134 14 26 sourced by 192 168 13 5 are being received on interface EO This interface...

Connecting to Dvmrp Networks

You might, on occasion, have to connect your PIM router to a DVMRP router. This is not necessarily a large-scale multicast issue routers that can speak only DVMRP can be encountered in an internetwork of any size. However, the most likely circumstance is when you are connecting to the MB one. When you configure an interface on a Cisco router to run PIM, it listens for DVMRP Probe messages. When Probes are heard, as demonstrated in the output in Example 7-4, Cisco IOS Software automatically...

Case Study Dynamic NAT

The problem with the configurations of the preceding case study is one of scalability. What if, instead of the four inside devices shown in Figure 4-18, there are 60 or 6000 Maintaining static NAT mappings, like maintaining static route entries, quickly becomes an administrative burden as the network grows. The inside network in Figure 4-19 uses 10.1.1.0-10.1.2.255 for its IL address space and has been assigned the public address space 204.15.86.0 23 by its ISP. This public address space is...

Router Configuration for RMON

The command to define an alarm table entry and the variable for which the alarm is being set is as follows rmon alarm number variable interval delta absolute rising-threshold value event number falling-threshold value event-number owner string The number uniquely identifies the entry in the alarm table. The variable is a MIB OID. The interval is the time between subsequent monitors of the MIB object. The delta or absolute keywords specify whether the alarm will test the change in MIB values...

Answers to Chapter 3 Troubleshooting Exercises

Internetwork Diagram

Figure 3-37 shows the internetwork diagram for Troubleshooting Exercises 1 through 6. Figure 3 37 The Internetwork for Troubleshooting Exercises 1 through 6 Figure 3 37 The Internetwork for Troubleshooting Exercises 1 through 6 1 Example 3 165 shows the BGP configuration of router R2 in Figure 3 37 Example 3 165 BGP Configuration of Router R2 Example 3 166 shows the BGP table and routing table for R2 Although there are routes to the destinations in the autonomous systems shown in Figure 3 37...

Case Study MSDP Mesh Groups

In the preceding case study, routers Rodan and Megalon are RPs in the same AS. Large multicast domains can frequently have many RPs to share the workload or to localize multicast trees. Although MSPD has been presented so far as a tool for sharing inter-AS source information, it also proves useful when there are multiple RPs in a single domain, and sources always register to certain RPs but members throughout the domain must find any source. Every RP in the domain commonly has an MSDP peering...

Ftl Png

RIPng (ng stands for next generation) is based on RIP version 2 (RIP-2). None of the operational procedures, timers, or stability functions have been changed. RIPng is RIP-2, modified to support the larger IP addresses and multiple addresses on each interface of IPv6. The UDP port number for RIPng is 521. RIPng does not support both IPv4 and IPv6 and is therefore not backward-compatible with RIP-2. NOTE Chapter 7, Routing Information Protocol Version 2, of Routing TCP IP, Volume , discusses RIP...

PIMv2 Bootstrap Message Format

Bootstrap messages, the format of which is illustrated in Figure 5-67, are originated by bootstrap routers (BSRs) every 60 seconds and are flooded throughout a PIM-SM domain to ensure that all routers determine the same RPs for the same groups. The message contains a list of one or more multicast group addresses. For each of these group addresses, there is a list of Candidate RPs (C-RPs) and their priorities. This list is the RP-Set for that group. Receiving routers use a common algorithm to...

Network Reachability Protocol

When the neighbor state is Up, the EGP neighbors can begin exchanging reachability information. Each gateway periodically sends a Poll message to its neighbor, containing some sequence number. The neighbor responds with an Update message that contains the same sequence number and a list of reachable networks. Example 1-5 shows how Cisco's IOS Software uses the sequence numbers. Example 1 -5 EGP Neighbors Poll Each Other Periodically for Network Reachability Updates EGP from 192.168.16.1 to...

Case Study Configuring MBGP

Figure 7-14 depicts three autonomous systems. AS 200 is advertising unicast prefixes 172.16.226.0 24 and 172.16.227.0 24 to transit AS 100 and is used for normal inter-AS routing. AS 200 also has several multicast sources. These are hosts at 172.16.224.1 and 172.16.225.50. Additionally, several multicast sources are on subnet 172.16.227.0 24, and that prefix is advertised not only as a unicast prefix but also as a multicast source prefix. Figure 7-14 AS 200 Is Advertising Several Prefixes and...

Answers to Chapter 4 Configuration Exercises

Refer to Figure 4 28 for Configuration Exercises 1-5 Figure 4 28 The Internetwork for Configuration Exercises 1-5 Figure 4 28 The Internetwork for Configuration Exercises 1-5 1 ISP1 in Figure 4 28 has assigned the address block 201 50 13 0 24 to AS 3 ISP2 has assigned the address block 200 100 30 0 24 to AS 3 RTR1 and RTR2 are accepting full BGP routes from the ISP routers but do not transmit any routes to the ISPs They run IBGP between them and OSPF on all Ethernet interfaces No routes are...

PIMSM and Shortest Path Trees

In Figure 5-61, router Lead has been added to the PIM-SM domain, and Lead has a group member attached. Under basic shared-tree rules, Lead would join the shared tree rooted at Brass. It is obvious in the illustration, however, that the direct link to Aluminum is a more efficient path for the multicast packets from the source to Lead's group member. You already have seen how PIM-SM can build an SPT between the RP and the source DR. The protocol also allows SPTs to be built all the way from a...

Case Study IBGP without an IGP

In Figure 3-5, another router is added to AS 100 it connects to another AS via EBGP. AS 100 is now a transit AS, carrying traffic that neither originates nor terminates in AS 100. Figure 3-5 AS 100 Is Running IBGP to Carry the Transit Traffic Between AS 400 and the Other Two Autonomous Systems Figure 3-5 AS 100 Is Running IBGP to Carry the Transit Traffic Between AS 400 and the Other Two Autonomous Systems 192.168.100.0 24 192.168.200.0 24 192.168.1.216 30 192.168.100.0 24 192.168.200.0 24...

Answers to Chapter 5 Review Questions

1 Give several reasons why replicated unicast is not a practical substitution for true multicast in a large network Answer Replicated unicast places a processing burden on the source and can cause severe bottlenecks at the source interface data link and connected router. The source also must hold state to remember what addresses to send the replicated packets and there must be some potentially complex mechanism for members to signal joins and leaves to the source Finally replicated unicast can...

Case Study Route Reflectors

Route reflectors are another way to reduce the number of IBGP peer connections in a large AS. The use of route reflectors has two advantages over confederations All routers in a confederation must understand and support confederations. But only the route reflectors themselves must understand route reflection the client routers see their connection to the RR as just another IBGP connection. Route reflection is simpler to implement, both in terms of the commands needed and in terms of topology...

Neighbor Unreachability Detection

If a node to which another is communicating fails, it is not very beneficial to detect the failure before the upper layers do. If a router in the path to the destination fails, however, there may be an alternative router to use, and it would be extremely helpful to be able to detect that failure before the upper-layer protocol does. Neighbor reachability is verified in one of two ways from hints from the upper-layer protocols or from responses to Neighbor Solicitations. Forward-direction...

Router Configuration for NTP

When configuring NTP first create an association Use the following commands to initiate the creation of the associations ntp server ip_address version number key keyid source interface prefer ntp peer ip_address version number key keyid source interface prefer Create a server association if this router is going to synchronize its clock to another NTP clock source Create a peer association if this router is willing to synchronize to another device or allow another device to synchronize to it The...

Case Study Filtering Routes by ASPATH

In the face of a large number of advertised addresses, filtering by NLRI can quickly become unwieldy or completely impractical. Only a few addresses are being advertised in Figure 3-15, yet the access lists shown in the previous section are already somewhat lengthy. A common factor in the examples in the preceding section is that in each case, the access lists are used to identify all the addresses within a single AS. In such situations, it is easier to filter on the AS number instead of...

Case Study Administrative Distances and Backdoor Routes

The other Cisco-specific tool for manipulating preferences on a single router is administrative distance. Whereas administrative weight influences preference among multiple routes to the same destination that have been learned from different BGP peers, administrative distance influences preference among multiple routes to the same destination that have been learned from different routing protocols. This means that whereas the effects of administrative weights are seen in the BGP table, the...

Router Configuration for Syslog

Cisco routers use the local7 facility by default when sending messages to a syslog server. If this facility is being used by another process sending messages to the syslog server, you can change the facility type on the Cisco router using the following configuration command The router configuration in Example 9-17 enables syslog logging to the specified host. Messages with level notifications and above are logged. Example 9-17 Enabling Syslog Logging to a Specified Host logging 172.16.1.2...

Caao Study Injecting BGP Routes into an IGP

192 168 222

Prefixes that are learned from an EBGP neighbor are automatically added to the routing table. In Figure 3-4, for instance, AS 300 is advertising two routes 192.168.250.0 24 and 192.168.1.212 30. AS 300's IGP, and the configuration of router Tahoe, are unimportant to this example. The important observations are that the prefixes advertised by Tahoe to its external BGP peer are displayed in the Taos routing table as reachable and that pings to a destination in AS 300 are successful (see Example...

Ease Study Configuring MSDP

Figure 7-15 again shows the routers from the preceding case study. Here, the four routers are also RPs for their respective autonomous systems, and the illustration shows their RP addresses. igure 7-15 MSDP Sessions Are Configured Between the Four RPs Figure 7-15 again shows the routers from the preceding case study. Here, the four routers are also RPs for their respective autonomous systems, and the illustration shows their RP addresses. igure 7-15 MSDP Sessions Are Configured Between the Four...

Case Study An EGP Stub Gateway

Figure 1-10 shows an EGP stub gateway in AS 65502, connected to a core gateway in AS 65501. The IGP of the stub AS is RIP. Figure 1-10 EGP Stub Gateway Advertises the Interior Networks of AS 65502 to the Core Gateway Figure 1-10 EGP Stub Gateway Advertises the Interior Networks of AS 65502 to the Core Gateway Example 1-7 shows the initial configuration of the stub gateway. Example 1-7 Stub Gateway Configuration for Figure 1-10 router rip redistribute connected redistribute egp 65501 metric 5...

Router Configuration for SNMP

Various global SNMP commands enable the router to be managed by CiscoWorks. All the global snmp commands begin with snmp-server. No specific one enables SNMP The first snmp-server command entered enables both versions of SNMP on the router. The router must be configured to use the same SNMP version supported by the management station. The command to create the management community is as follows no snmp-server community string view view-name ro rw access list number The community string acts as...

Ileum ASSET with Aggregates

Figure 3-12 shows a modified version of the internetwork shown in Figure 3-11, including a change in the source of the aggregate address. Here, both AS 100 and AS 200 advertise the full routes of AS 100 to AS 300 and AS 400, without an aggregate. Figure 3-12 Burke Is Creating an Aggregate and Advertising It to Sun 192.168.192.0 24 192.168.193.0 24 192.168.194.0 24 192.168.195.0 24 192.168.196.0 24 192.168.197.0 24 192.168.198.0 24 192.168.199.0 24 Router Burke, in AS 300, suppresses the...

Answers to Chapter 1 Configuration Exercises

1 Autonomous System 65531 m Figure 1-14 is a core AS. Figure 1-14 The Internetwork for Configuration Exercise 1 Figure 1-14 The Internetwork for Configuration Exercise 1 Configure EGP on RTA and RTB, with the following constraints The data link interior to the AS is not advertised to any exterior neighbor. RTA advertises the network attached to its SI interface to RTB with this exception, no other inter-AS link is advertised between RTA and RTB. RTA and RTB advertise a default route to their...