Routing TCP IP

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...

Route Reflectors

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...

Cnse Study Injecting IGP Routes into BGP

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 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...

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...

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...

Answers to Chapter 8 Review Questions

1 Which of the following are valid representations for the address 200A 0000 0000 0C00 0000 0000 0000 0000 with a 60 bit prefix Answer B C E A is not a complete address D is ambiguous with two sets of F doesn t expand to the correct address 2 For what is the address 0 0 0 0 0 0 0 0 used Answer This is the unspecified address It represents the absence of an address If this is the source address of a packet the interface has not yet been assigned an address It is attempting to discover whether...

Route Dampening

Route flaps are a leading contributor to instability on the Internet and, for that matter on any internetwork. Flaps occur when a valid route is declared invalid and then declared valid again. The problem is evident Every time the state of a route changes, the change must be advertised throughout the internetwork, and each router must make the appropriate recalculations. Both bandwidth and CPU resources are consumed. NOTE You might occasionally hear the term route oscillation used...

Caao Study Injecting BGP Routes into an IGP

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...

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...

PIMv2 Hello Message Format

PIMv2 Hello messages, the format of which is illustrated in Figure 5-63, are used for neighbor discovery and neighbor keepalives. The messages are sent every 30 seconds by default, and the period can be changed with the command ip pim query-interval. Figure 5-63 The PIMv2 Hello Message Format The fields for the PIMv2 Hello message are defined as follows Option Type specifies the type of option in the Option Value field. Presently, only Option Type 1 is used. This specifies that the Option Value...

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...

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...

NAT and IP Address Conservation

The original mission of NAT was to slow the depletion of IP addresses, and this is the focus of RFC 1631. The core assumption of the concept is that only some of an enterprise's host s will be connected to the Internet at any one time. Some devices (print servers and DHCP servers, for example) never require connectivity outside of the enterprise at all. As a result, the enterprise can be addressed out of the private RFC 1918 address space, and a significantly smaller number of uniquely assigned...

Case Study A Network Merger

NAT is useful for preventing possible address conflicts between internetworks. The previous two case studies demonstrate the connection of internetworks using private address space to an internetwork using public addresses. The publicly addressed internetwork might be some other enterprise, or it might be the Internet. The bottom line is that the private RFC 1918 addresses must be translated because they are not unique. Across the Internet, many enterprises use the same addresses in their...

Troubleshooting Exercises

1 Identify the mistake in the configuration in Example 4-33. Example 4-33 Configuration for Troubleshooting Exercise 1 ip nat pool EX1 192.168.1.1 192.168.1.254 netmask 255.255.255.0 type match-host ip nat pool EX1A netmask 255.255.255.240 address 172.21.1.33 172.21.1.38 address 172.21.1.40 172.21.1.46 ip nat inside source list 1 pool EX1 ip nat inside source static 10.18.53.210 192.168.1.1 ip nat outside source list 2 pool EX1A access-list 1 permit 10.0.0.0 0.255.255.255 access-list 2 permit...

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...

Answers to Chapter 3 Troubleshooting Exercises

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...

Network Robustness

A robust network can withstand outages and keep applications running smoothly. Router redundancy on LAN segments is required to maintain communication in the event of a router failure. Hosts on the IPv4 LAN, however, are very likely to rely on a single default router to communicate to hosts on a remote segment. If the default router fails, the host is not informed, and therefore sends traffic to a black hole. Router redundancy protocols, such as the Virtual Router Redundancy Protocol (VRRP) and...

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...

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...

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,...

Operation of NAT

NAT is described in RFC 1631.1 The original intention of NAT was, like classless interdomain routing (CIDR), to slow the depletion of available IP address space by allowing many private IP addresses to be represented by some smaller number of public IP addresses. Since that time, users have found NAT to be a useful tool for network migrations and mergers, server load sharing, and creating virtual servers. This section examines all these applications, but first describes the basics of NAT...

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...

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...

PIMv2 Message Formats

PIMv2 messages are encapsulated in IP packets with a protocol number of 103. Except for the cases in which the messages are unicast, the IP destination address is the reserved multicast address 224.0.0.13, and the TTL is set to 1. Both the multicast address and the TTL ensure that the messages are forwarded only to neighboring routers. Although version 2 is the current version, PIMvl is still common. That version uses an IP protocol number of 2, making it a subset of the IGMP protocol. Version...

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 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...

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...

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....

The BGP Decision Process

The BGP Routing Information Database (RIB) consists of three parts Adj-RIBs-In Stores unprocessed routing information that has been learned from updates received from peers. The routes contained in Adj-RIBs-In are considered feasible routes. Loc-RIB Contains the routes that the BGP speaker has selected by applying its local routing policies to the routes contained in Adj-RIBs-In. Adj-RIBs-Out Contains the routes that the BGP speaker advertises to its peers. These three parts of the Routing...

The Atomicaggregate and Aggregator Attributes

A BGP-speaking router can transmit overlapping routes to another BGP speaker. Overlapping routes are nonidentical routes that point to the same destination. For example, the routes 206.25.192.0 19 and 206.25.128.0 17 are overlapping. The first route is included in the second route, although the second route also points to other more-specific routes besides 206.25.192.0 19. When making a best-path decision, a router always chooses the more-specific path. When advertising routes, however, the BGP...

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...

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...

Review Questions

1 What is the most important difference between BGP-4 and earlier versions of BGP 2 What two problems was CIDR developed to alleviate 3 What is the difference between classful and classless IP routers 4 What is the difference between classful and classless IP routing protocols 5 Given the addresses 172.17.208.0 23, 172.17.210.0 23, 172.17.212.0 23, and 172.17.214.0 23, summarize the addresses with a single aggregate, using the longest possible address mask. 7 The routing table in Example 2-16...

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...

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...

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...

MOSPF Basics

Figure 5-33 The Local Group Database Records Attached Groups and the Subnets on Which the Group Members Reside The DR then originates a Group Membership LSA for each attached group The LSA specifies the group address and the originating router ID and lists all the router's attached networks on which members of the group reside In some cases, the router itself may run multicast applications that make it a group member The LSA includes a Type field in which the router can indicate that it is...

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...

Answers to Chapter 1 Review Questions

2 What is an EGP interior neighbor An EGP 3 What is the primary difference between an EGP stub gateway and an EGP < 4 Why does EGP use the concept of a core, or backbone, AS' EGP has no i 6 What is the purpose of an EGP Poll message Answers to Chapter 2 Review Questions 2 What two problems was CIDR developed to alleviate CIDR was developed to alleviate the explosion of Internet routing tables 3 What is the difference between classful and classless IP 5 Given the addresses 172 17 208 0 23 172...

Internet Group Management Protocol IGMP

Regardless of which of the several routing protocols is used in a multicast internetwork, IGMP is always the language spoken between hosts and routers All hosts that want to join multicast groups, and all routers with interfaces on subnets containing multicast hosts, must implement IGMP It is a control protocol like ICMP, sharing some functional similarities Like ICMP, it is responsible for managing higher-level data exchanges IGMP messages are encapsulated in IP headers like ICMP (with a...

Classless Routing

Classless routing features two aspects Classlessness can be a characteristic of a routing protocol. Classlessness can be a characteristic of a router. Classless routing protocols carry, as part of the routing information, a description of the network portion of each advertised address. The network portion of a network address is commonly referred to as the address prefix. An address prefix can be described by including an address mask, a length field that indicates how many bits of the address...

Single Homed Autonomous System

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...

Using mrinfo

The mrinfo command enables you to observe a router's multicast connections and the details of those connections. The command is a part of the tools originally made available as part of mrouted for testing routers in the MBone. Therefore, the command is very useful in multivendor domains. Take, for example, the topology in Figure 6-10. Figure 6-10 This Topology Is Used Throughout the Troubleshooting Examples In Example 6-55, mrinfo is used at router Sombrero. The first line of the output shows...

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,...

Case Study Statically Configuring the RP

Figure 6-3 is the same internetwork you have been observing in this chapter, but now the routers are configured to run PIM-SM. Stetson has been chosen as the RP, and all routers are statically configured with that information. The illustration shows that Stetson's RP address is 10.224.1.1. This address can exist on any interface, as long as it is advertised by the unicast routing protocol so that the other routers know how to reach it. In practice, you should use the loopback interface. A minor...

Operation of Multicast Source Discovery Protocol MSDP

The purpose of MSDP is, as the name states, to discover multicast sources in other PIM domains. The advantage of running MSDP is that your own RPs exchange source information with RPs in other domains your group members do not have to be directly dependent on another domain's RP. NOTE You will see in some subsequent case studies how MSDP can prove useful for sharing source information within a single domain, too. MSDP uses TCP (port 639) for its peering connections. As with BGP, using...

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 -...

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...

Router Advertisements

Routers advertise their presence on a link and provide the information necessary for a node to configure itself. The RA is multicast to the link-scope all-nodes multicast group. An RA is an ICMP packet of type 134. Its source IP address is the link-local address of the sending router, and the destination address is either the address of a node that sent a Router Solicitation or the link-scope all-nodes multicast address. The hop limit must be set to 255. The hop limit is not used, in this case,...

MTU Path Discovery

The MTU is required to be at least 1280 bytes long on every link in an IPv6 network. However, the recommended size is 1500 bytes or larger. Any link that cannot handle a packet this large is required to provide link-level fragmentation. IP-level fragmentation is performed only by the source node, not by routers along the packet's path. Nodes are not required to implement MTU path discovery, but it is recommended. A node not implement ing MTU path discovery uses an MTU equal to the minimum IPv6...

Multigroup Hsrp With Secondary Ip

172.16.1.100 HSRP grp 1 172.16.1.201 HSRP grp 1 priority 120 Router Monet is configured with an interface IP address 172.16.1.100 and an HSRP group 1 IP address 172.16.1.201. Router Monet advertises an HSRP priority, for HSRP group 1, as 120. This is higher than the default priority of Picasso. Monet, therefore, is the active router for group 1. When Wks 1 wants to send a packet toward its default gateway, it ARPs for the HSRP group 1 address. Monet responds with the HSRP group 1 MAC address....

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,...

Case Study Private AS Numbers

You are familiar with private IP addresses, as specified by RFC 1918. These addresses, in the range 10.0.0.0-10.255.255.255, 172.16.0.0-172.31.255.255, and 192.168.0.0-192.168.255.255, are designed to help alleviate the depletion of IP addresses. When an internetwork has a need for IP addresses, but the addresses do not need to be known publicly (that is, they do not need to be reachable from the Internet), you can use private addresses rather than public addresses. Because anyone can use any...

Simple Network Management Protocol

Network management software, such as CiscoWorks, uses the Simple Network Manage ment Protocol (SNMP) to manage network devices. SNMP is the workhorse behind all those nice network diagrams, charts, and graphs. It queries the devices, collecting the data necessary to build the diagrams, charts, and graphs. SNMPvl1 is supported on all Cisco routers. SNMPv2C is supported on all Cisco routers with IOS 11.3 or later. SNMPv2C supports bulk data transfers and more detailed error reporting than SNMPvl....

The PIMv2 Assert Message Format

The PIMv2 Assert message, the format of which is illustrated in Figure 5-68, is used to elect a designated forwarder on multiaccess networks. When a PIM router receives a multicast packet on an interface that is on the outgoing interface for the packet's group, the router assumes that there must be another router connected to that data link forwarding for the group. The router sends an Assert so that other routers sharing the multiaccess network can decide which of them will forward packets for...

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 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...

Why Two Name Servers

One of the core functions of any TCP IP internetwork, and especially of the Internet, is the Domain Name System (DNS). If systems cannot get DNS queries and responses across a NAT, DNS can become complicated. Figure 4-11 shows ways you can implement DNS servers around a NAT that cannot translate DNS packets. The NAT in Figure 4-11 translates in both directions outside hosts are made to appear to the inside as if they are on the 10.0.0.0 network, and inside hosts are made to appear to the...

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...

The Internet Still Hierarchical After All These Years

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...

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...

Resetting BGP Connections

When a BGP speaker's configuration is changed, it is often necessary to reset the connections to the affected neighbors for the change to take effect. The Cisco IOS Software Command Summary lists the following circumstances under which you must reset a BGP connection Additions or changes to BGP-related access lists Changes to BGP-related weights Changes to BGP-related distribution lists Changes in the BGP-related timer's specifications Changes to the BGP administrative distance Changes to...

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...

Case Study Default Router Failure and Communication Recovery

A host transfers a file from a remote server using FTP. The host sends the traffic to its on-link default router. The host continues to receive ACKs for data sent, so the host knows that its default router must be reachable. In mid-session, the router fails. The host stops receiving ACKs. The host can no longer verify forward-direction communication through hints from the TCP layer, so it changes the router's state to STALE. It still attempts to send packets, so the state changes to DELAY....

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....

Troubleshooting IP Multicast Routing

Your primary weapon when attacking problems in IP multicast internetworks is a solid understanding of the IP multicast protocols. Without that, no number of troubleshooting tools will help you wend your way through the often confusing, sometimes complex behaviors of IP multicast to the root of a particular problem. And understanding a single protocol is not enough. You also must understand how PIM, IGMP, and unicast routing all interact. If you have been closely following the troubleshooting...

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...

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...

IGMPv2 Host Functions

Hosts running IGMPv2 use three types of messages Version 1 Membership Report messages Membership Report messages are sent to indicate that a host wants to join a group The messages are sent when a host first joins a group, and sometimes in response to a Membership Query from a local router When a host first learns of a group and wants to join, it does not wait for the local router to send a query As you will learn in the sections on the various multicast routing protocols, the router may not in...

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...

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...

Changing the Attributes of the Aggregate

Yet another option that you can use with the aggregate-address command is the attribnu map option. This option enables you to change the attributes of the aggregate route. Nof u in Example 3-57, for instance, that all the more-specific routes have an ORIGIN attribui of Incomplete, because the routes are redistributed into BGP from EIGRP. The aggregair have an origin of IGP, however, because they originated within the BGP processes of Sum. and Mammoth. Suppose the administrator wants AS 200 to...

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...

Troubleshooting Exercises Got

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...

Case Study EBGP Multihop

Just as you can establish an IBGP session between loopback interfaces, as demonstrated in the preceding case study, you also can establish EBGP sessions between loopback interfaces. Figure 3-9 shows such a session. Here, the end points of the EBGP session between Telluride and Alta are loopback interfaces. Figure 3-9 An EBGP Session Is Established Between Telluride's and Alta s Loopback Interfaces Figure 3-9 An EBGP Session Is Established Between Telluride's and Alta s Loopback Interfaces...

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...

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...

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,...

The Aspath Attribute

AS_PATH is a well-known mandatory attribute that uses a sequence of AS numbers to describe the inter-AS path, or route, to the destination specified by the NLRI. When a BGP speaker originates a route when it advertises NLRI about a destination within its own AS it adds its AS number to the AS_PATH. As subsequent BGP speakers advertise the route to external peers, they prepend their own AS numbers to the AS JPATH (see Figure 2-21). The result is that the AS_PATH describes all the autonomous...

Port Address Translation

The many-to-one applications of NAT discussed so far have involved a statistical multiplexing of a large range of addresses into a smaller pool of addresses. However, there is a one-to-one mapping of individual addresses. When an address from an inside global pool is mapped to an inside local address, for instance, that IG address cannot be mapped to any other address until the first mapping is cleared. However, there is a specialized function of NAT that allows many addresses to be mapped to a...

Anycast Addresses

Anycast routing is a mechanism for addressing multiple interfaces, usually on different nodes, with the same IP address. Traffic destined to the address gets routed to the nearest node. The anycast functionality is discussed later, in the section The Anycast Process. Tin anycast address has the same format as a unicast address. No special FP defines an anycasi address. Anycast addresses are assigned from the unicast address space. In fact, the addresses are taken from the Interface ID field....

Ftwi jmssing More Specific Routes

In a simple topology such as the one in Figure 3-10, this first method normally suffices. As the topology and the routing policies grow more complex, however, the options available with the aggregate-address command make that method more useful. The remainder of this case study examines the aggregate-address command and its options. For the aggregate specified by the aggregate-address command to be advertised, at least one of the more-specific addresses belonging to the aggregate must be...

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...

The Site Local Address

A site is an organization or part of an organization. It could be a certain topological location, or it could be multiple topological locations interconnected in some way. A network configured with a site-local address is not reachable from locations outside the site. The site's edge routers must be able to keep site-local traffic within the site and are responsible for controlling the route propagation. In addition to the site-local FP and Interface ID, site-local addresses have a subnet...

Cisco Group Membership Protocol CGMP

I Building C, 2nd floor 48 ports I Building C, 2nd floor 48 ports Just as broadcast frames are forwarded to every port within a broadcast domain, so too are frames carrying IP multicast packets After all, a broadcast domain is nothing more than a multicast group to which all hosts belong Figure 5-14 illustrates the problem Three group members are attached to a 24-port switch An IGMP Membership Report is sent to the router, and the router begins forwarding the appropriate multicast session onto...

The Anycast Process

Anycast is a mechanism used to route packets to one of many identically addressed nodes The identically addressed nodes might be a group of servers offering a well known service to clients or a group of routers belonging to an ISP which requires that traffic pass through one of its anycast addressed routers A node addresses the IP packet to the single anycast address of the group The node learns the next hop for the address just as it would for a unicast address If the anycast address is on...

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...

Operation of the Distance Vector Multicast Routing Protocol DVMRP

DVMRP uses the broadcast-and-prune method to build a separate source-based tree for every multicast source It uses a variant of RIP to discover the shortest path to the source hence the name Distance Vector Multicast Routing Protocol Each multicast tree is maintained dynamically by pruning and grafting branches as group members leave and join the group DVMRP Graft Acknowledgement All the packets have a destination address of 224 0 0 4, the reserved All DVMRP Routers address (see Table 5-1) The...

IGMP Message Format

IGMPv2 uses a single message format, as shown in Figure 5-11 The IP header encapsulating the message indicates a protocol number of 2 Because the IGMP message must not leave the local subnet on which it was originated, the TTL is always set to 1 Additionally, IGMPv2 messages carry the IP Router Alert option that informs routers to examine this packet more closely6 Figure 5-11 The IGMPv2 Message Format The fields for the IGMPv2 message are defined as follows Type describes one of four message...

Shortcomings of EGP

The fundamental problem with EGP is its inability to detect routing loops. Because there is an upper boundary on the distance EGP uses (255), you might be tempted to say that counting to infinity is at least a rudimentary loop-detection mechanism. It is, but the high limit combined with the typical Poll interval makes counting to infinity useless. Given a default Poll interval of 180 seconds, EGP peers could take almost 13 hours to count to infinity. As a result, EGP must be run on an...

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...

Basing an Aggregate on Selected More Specific Routes

In some situations, you might want to advertise an aggregate with the AS_SET but do noi want the aggregate to inherit all the attributes of all the aggregated routes. In Figure 3-14, Sugarbush receives all the routes from AS 100 and AS 500 and advertises an aggregate to Burke. Figure * 14 lor Sugarbush to Advertise the Aggregate with an AS_SET, the Aggregate Must Not Inherit the NO_EXPORT COMMUNITY Attribute from 192.168.197.0 24 Figure * 14 lor Sugarbush to Advertise the Aggregate with an...

InterAs Mospf

RFC 1584 provides for the routing of multicast packets into and out of an MOSPF domain You know from Chapter 9 of Volume I that a router redistributing routes into an OSPF domain from some other routing protocol is called an Autonomous System Boundary Router (ASBR) An ASBR uses AS-External (type 5) LSAs to advertise destinations outside of the OSPF domain and ASBR Summary (type 4) LSAs to advertise their own location These LSAs are flooded into all areas of the OSPF domain, with the exception...